ΜΕΤΑΠΤΥΧΙΑΚΩΝ ΣΠΟΥΔΩΝ ΣΤΗΝ ΙΑΤΡΙΚΗ ΦΥΣΙΚΗ. Αρ.. Μητρώου: 1287

ΠΑΝΕΠΙΙ ΙΣΤΗΜΙΙΟ ΠΑΤΡΩΝ ΤΜΗΜΑ ΙΑΤΡΙΚΗΣ ΤΜΗΜΑ ΦΥΣΙΚΗΣ ΔΙΑΤΜΗΜΑΤΙΚΟ ΠΡΟΓΡΑΜΜΑ ΜΕΤΑΠΤΥΧΙΑΚΩΝ ΣΠΟΥΔΩΝ ΣΤΗΝ ΙΑΤΡΙΚΗ ΦΥΣΙΚΗ ΕΛΕΓΧΟΣ ΠΟΙΟΤΗΤΑΣ ΣΥΣΤΗΜΑ ΑΤΩΝ Ο ΔΟΝΤΙΑΤΡΙΚΗΣ ΠΑΝΟΡΑΜΙΚΗΣ ΑΚΤΙΝΟΓΡΑΦΗΣΗΣ ΔΙΠΛΩΜΑΤΙΚΗ ΕΡΓΑΣΙΑ του Νιώτη Δημήτρη Αρ.. Μητρώου: 1287 ΕΠΙΒΛΕΠΩΝ: ΠΑΝΑΓΙΩΤΑΚΗΣ ΓΕΩΡΓΙΟΣ ΚΑΘΗΓΗΤΗΣ ΠΑΝΕΠΙΣΤΗΜΙΟΥ ΠΑΤΡΩΝ ΠΑΤΡΑ ΔΕΚΕΜΒΡΙΟΣ, 2010

UNIIVERSIITY OF PATRAS DEPARTMENT OF MEDICINE DEPARTMENT OF PHYSICS INTERDEPARTMENTAL PROGRAM OF POSTGRADUATE STUDIES IN MEDICAL PHYSICS QUALITY CONTROL ON DENTAL PANORAMIC RADIOGRAPHY UNITS MSc Thesis Niotis Dimitris I.D. No: 1287 SUPERVISOR: PANAYIOTAKIS GEORGE PROFESSOR of Patra s University PATRAS DECEMBER, 2010

ACKNOWLEDGMENTS I would like to thank Professor George Panayiotakis for his inspiring, encouraging and helpful guidance and consistent supervision on my work, as also for the extensive and sincere discussions on several subjects that we shared. I would also like give special thanks to Medical Radiation Physicist Harry Delis for his enormous and direct help concerning various questions and clarifications aroused on my research as well as on the experimental procedures. Last but not least, I would like to thank all friends, closed ones and colleagues who showed a remarkably inexhaustible, encouraging yet hopefully not inexplicablepatience concerning the making of this work. iii

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PREFACE Over the last decades, medical x rays have remarkably expanded their necessity on medical diagnosis, treatment planning and evaluation of therapy. In this way, the parallel ongoing development as well as the evolution of the x ray techniques has brought a vast range of potential usage, which needs to be fairly acknowledged for the optimization of each technique and the overall control of the radiation dosage, not to mention the permanently questioned necessity and high rates of the examinations. Dental radiography represents the most frequent diagnostic x ray examination undertaken in the industrialized countries of the world. On this basis, the relatively low dosage of these techniques can not underestimate the questions of radioprotection fields, as for example the overall dosage in each country from these techniques is more than negligible. Panoramic radiography is a simplified dental extraoral procedure which depicts the entire maxillomandibular region on a single film. The development of the principles of dental panoramic radiology represented a major innovation in dental imaging. Prior to this, dental radiographic examinations were limited to intraoral and oblique lateral projections of the jaws taken using a dental x ray set. For the first time practitioners were able to produce an image of both jaws and their respective dentitions on a single radiographic film by a quick and relatively simple procedure. The simplicity of operation, the broadened scope of examination, the ability to project anatomic structures in their normal relationship with reduced superimposition of intervening parts, and the low radiation dosage are reasons for its widely growing popularity. The latter has raised the necessity of the formation of a regulatory basis in each country, according to the demographic and practitioners standards, which will provide a safe and proper quality assurance guide, allowing each practitioner to optimize the technique, both in terms of image quality and patient dosimetry, according to the subjective grounds of every laboratory, rather than restricting the practice into a single and solid mode. Quality Control (QC) protocols, Diagnostic Reference Levels (DRLs) and guidance programs for proper usage of the panoramic unit as well as the processing procedures are some of the fields that need to researched and well established. In Greece, where the number of both panoramic units and examinations follow a remarkably yet undefined increasing curve during the last two decades, there has been v

initialized a researching and scientific debate about these fields although it is still in a primary stage. The following thesis, taking into consideration the difficulties and the complexity of this technique, as well as the questions and problems that rise upon its general practice in Greek laboratories, and trying to provide a more aggregate view of panoramic imaging, focuses on the QC of panoramic radiography, including the determination of DRLs in this technique, with a respective presentation of the principles of function and the necessary radioprotection information. vi

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Contents SECTION I GENERAL PART CHAPTER 1 : The included anatomical structures on the panoramic radiograph Introduction 3 The Skull 3 The Mandible 4 The Maxilla (Upper Jaw) 5 The Temperomandibular Joint 5 The Foramina 6 The Salivary Glands 7 The Cervical Vertebrae 8 General Overview of the Oral, Neck and Lower Face Anatomy 8 The Tongue 9 Oral Anatomy Elements 9 The Tooth 11 Tooth Development 12 CHAPTER 2 : Panoramic Radiography: a clinical overview Introduction 19 Diagnostic Regions in PR 19 The Normal Panoramic Radiograph 21 CHAPTER 3 : Panoramic Radiography: principles of function Introduction 30 Broad Beam Linear Tomography 31 Slit or Narrow Beam Linear Tomography 31 Narrow Beam Rotational Tomography 32 Dental Panoramic Tomography 33 Focal Trough 35 Formation of the Image Layer 38 Geometric Distortion 41 Screen Film and Intensifying Screens 43 The Screen Film 43 Intensifying Screens 45 Digital Panoramic Tomography 48 viii

Solid state systems using CCD 50 Storage Phosphor Plates Technology 53 Interoperability 55 Radiation Dosage 56 Comparison Between Film and Digital PR 56 Equipment 58 Patient Positioning 61 Field Limitation Techniques 62 CHAPTER 4: Radiation Effects, Doses and Protection concerning Quality Controlling of panoramic radiography Introduction 65 Sources of Radiation 66 Classification of Biological Effects 67 Somatic Deterministic Effects 67 Somatic Stochastic Effects 67 Genetic Stochastic Effects 68 Harmful Effects Important in Dental Radiology 69 Estimating the Dose and Risk of PR 69 Main Methods of Monitoring and Measuring Radiation Dose 73 Film Badges 73 TLDs 74 Ionization Chambers 75 Measurements using Phantoms 75 Patient Dosimetry 75 List of Equipment 76 Methods 76 Worksheets 83 Dose Area Product 86 Diagnostic Reference Levels (DRLs) deriving from DAP measurements 87 Dose Width Product 88 DAP and Effective Dose 89 CHAPTER 5: Quality Control Protocols Codes of Practice Legislation Greek Atomic Energy Commission Protocol of Periodical Quality Control Checks on Orthopantograph 92 Requirements for Dental Panoramic and Cephalometric Examinations, by GAEC 93 Conference of Radiation Control Program Directors, Inc Quality Control Recommendations for Diagnostic Radiography European Commission Radiation Protection 136 European Guidelines Radiation Protection in Dental Radiology, 2004 International Atomic Energy Agency Dosimetry in Diagnostic Radiology: An International Code of Practice 94 95 96 ix

Health Canada Radiation Protection in Dentistry, Recommended Safety Procedures for the Use of Dental x ray Equipment 96 Care Quality Commission The Ionizing Radiation (Medical Exposure) Regulations 97 Greek Ministry of Health, Greek Regulations for Radiation Protection, 2001. 97 SECTION II EXPERIMENTAL PART CHAPTER 6: Calculation of the Effective Dose (E), Using the DRLs of Tierris et al. (2004) 101 CHAPTER 7: Quality Control of a Panoramic Unit A.1 LABORATORY DESCRIPTION EQUIPMENT RECORD A.1.1 Equipment Description 104 A.1.2 Ventilation Air Condition Illumination 106 A.2 CONTROLS A.2.1 General Apparatus Controls 107 A.2.1.1 Inspectional Control of the Unit Components 107 A.2.1.3 Presence of Technical Manuals and Log Book 107 A.2.2 Radioprotection Control 108 A.2.2.1 Spatial Characterization Chamber Signage 108 A.2.2.2 Verification of Radioprotection Report Shield Control 108 A.2.2.3 Record and Control of Physical Condition of Radioprotection 109 Apparatus A.2.2.4 Tube Head Escape 109 A.2.3 Beam Geometry Control 110 A.2.3.1 Conjunction of Radiation Field with the Alignment Slit of the Digital Detector 110 A.2.3.2 Measurement of Minimum Distance Focus Examinee 110 A.2.3.3 FFD Control 110 A.2.4 Beam Quality Control 111 A.2.4.1 Accuracy of High Voltage Values 111 A.2.4.2 Repeatability of High Voltage Values 112 A.2.4.3 Half Value Layer (HVL) of the beam Tube Total Filtering 113 A.2.5 Beam Quantity Control 114 x

A.2.5.1 Timer Accuracy 114 A.2.5.2 Timer Repeatability 115 A.2.5.3 Tube Supply Linearity and Repeatability 115 A.2.6 Automatic Exposure Selection System 116 A.2.7 Typical Patient Doses 118 A.2.8 Image Quality Control 118 REFERENCES 120 Appendix I: The Diagnostic Value of the Panoramic Radiograph The Popularity of Panoramic Imaging 128 The Quality of Panoramic Images 129 Technical and Processing Faults Affecting Image Quality 130 Film Fault Frequency within Panoramic Radiographs taken in General Dental Practice 134 The Questionable Necessity of the Panoramic Radiograph 138 The Panoramic X ray Equipment and the Operating Personnel 142 xi

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Lists of Figures CHAPTER 1 Figure 1: Figure 2: Figure 3: Figure 4: Figure 5: Figure 6: Figure 7: Figure 8: Figure 9: Figure 10: Figure 11: Figure 12: Figure 13: Figure 14: Figure 15: Figure 16: Figure 17: Figure 18: Figure 19: Figure 20: Figure 21: Figure 22: Figure 23: http://www.3dmouth.org/4/4_1.cfm http://www.3dmouth.org/4/4_2_1.cfm http://www.3dmouth.org/4/4_2_2.cfm http://www.3dmouth.org/4/4_2_3.cfm http://www.3dmouth.org/4/4_2_4.cfm http://www.3dmouth.org/4/4_3.cfm http://www.en.wikipedia.org/wiki/file:illu_vertebral_column.jpg http://www.doctorspiller.com/oral%20anatomy.htm http://www.med.mun.ca/anatomys/head/head.htm http://www.doctorspiller.com/oral%20anatomy.htm http://www6.ufrgs.br/favet/imunovet/molecular_immunology/tooth1.jpg http://www.3dmouth.org/6/6_2_1.cfm http://dentdoctor.tripod.com/oral_anatomy/index2.html http://dentdoctor.tripod.com/oral_anatomy/index2.html http://www.3dmouth.org/6/6_2_3.cfm http://www.nytimes.com/imagepages/2007/08/01/health/adam/9445de ntalanatomy.html http://dentdoctor.tripod.com/oral_anatomy/index2.html Pasler A Friedrich. Color Atlas of Dental Medicine, p. 62 Radiology, Thieme, 1993 http://en.wikipedia.org/wiki/file:teeth_diagram.png http://dentdoctor.tripod.com/oral_anatomy/index2.html Pasler A Friedrich, Visser Heiko. Pocket Atlas of Dental Radiology, Panoramic Radiography, Tooth and Jaw Development as Depicted in Panoramic Radiographs, page 37, Thieme, 2007. Pasler A Friedrich, Visser Heiko. Pocket Atlas of Dental Radiology, Panoramic Radiography, Tooth and Jaw Development as Depicted in Panoramic Radiographs, page 37, Thieme, 2007. Pasler A Friedrich, Visser Heiko. Pocket Atlas of Dental Radiology, Panoramic Radiography, Tooth and Jaw Development as Depicted in Panoramic Radiographs, page 39, Thieme, 2007. xiii

Figure 24: Figure 25: Figure 26: Pasler A Friedrich, Visser Heiko. Pocket Atlas of Dental Radiology, Panoramic Radiography, Tooth and Jaw Development as Depicted in Panoramic Radiographs, page 37, Thieme, 2007. Pasler A Friedrich, Visser Heiko. Pocket Atlas of Dental Radiology, Panoramic Radiography, Tooth and Jaw Development as Depicted in Panoramic Radiographs, page 37, Thieme, 2007. http://media 2.web.britannica.com/eb media/91/74891 004 345232AC.jpg CHAPTER 2 Figure 1: Figure 2: Figure 3: Figure 4: Figure 5: Figure 6: Figure 7: Figure 8: Figure 9: Pasler A Friedrich. Color Atlas of Dental Medicine, p. 5 Radiology, Thieme, 1993 Pasler A Friedrich. Color Atlas of Dental Medicine, p. 5 Radiology, Thieme, 1993 Pasler A Friedrich. Color Atlas of Dental Medicine, p. 5 Radiology, Thieme, 1993 Pasler A Friedrich. Color Atlas of Dental Medicine, p. 5 Radiology, Thieme, 1993 Farman G Allan. Panoramic Radiology Seminars on Maxillofacial Imaging and Interpretation, Chapter 1, p. 4, Springer, 2007 Pasler A Friedrich. Color Atlas of Dental Medicine, p. 26 Radiology, Thieme, 1993 William S. Moore. Kodak Successful Panoramic Radiography, p.2 Murray Diane, Whyte Andy. Dental Panoramic Tomography: What the General Radiologist Needs to Know. Clinical Radiology 57: 1 7, 2002. Murray Diane, Whyte Andy. Dental Panoramic Tomography: What the General Radiologist Needs to Know. Clinical Radiology 57: 1 7, 2002. CHAPTER 3 Figure 1: Whaites E. Essentials of Dental Radiography and Radiology, Chapter 15, p. 162. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. Figure 2: Whaites E. Essentials of Dental Radiography and Radiology, Chapter 15, p. 163. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. Figure 3: Whaites E. Essentials of Dental Radiography and Radiology, Chapter 15, p. 163. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. Figure 4: Whaites E. Essentials of Dental Radiography and Radiology, Chapter 15, p. 164. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. Figure 5: Whaites E. Essentials of Dental Radiography and Radiology, Chapter 15, p. 164. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. Figure 6: White S C, Pharoah M J. Oral Radiology: Principles and Interpretation, Chapter 11, p. 208, 4 th ed. (St.Louis: Mosby Inc.) 2000. Figure 7: Whaites, E. Essentials of Dental Radiography and Radiology, Chapter 15, p. 165. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. Figure 8: X ray Phantoms, Panoramic Dental Test Object, TO PAN, Leeds Test Objects xiv

Figure 9: Figure 10: Figure 11: Figure 12: Figure 13: X ray Phantoms, Panoramic Dental Test Object, TO PAN, Leeds Test Objects William S. Moore. Successful Panoramic Radiography, Kodak Dental Radiography Series. http://www.kodak.com/us/plugins/acrobat/en/motion/support/h1/h1_2 3 27.pdf http://www.medcyclopaedia.com/library/topics/volume_i/f/film_screen_ radiography.aspx http://www.medcyclopaedia.com/upload/medcyc/volumes/volume_i/din tensifying_screen_fig1.jpg Figure 14: Farman G Allan. Panoramic Radiology Seminars on Maxillofacial Imaging and Interpretation, Chapter 3, Springer, 2007 Figure 15: Farman G Allan. Panoramic Radiology Seminars on Maxillofacial Imaging and Interpretation, Chapter 3, Springer, 2007 Figure 16: Farman G Allan. Panoramic Radiology Seminars on Maxillofacial Imaging and Interpretation, Chapter 3, Springer, 2007 Figure 17: Farman G Allan. Panoramic Radiology Seminars on Maxillofacial Imaging and Interpretation, Chapter 3, Springer, 2007 Farman G Allan. Panoramic Radiology Seminars on Maxillofacial Imaging Figure 18: and Interpretation, Chapter 3, Springer, 2007 Figure 19: Farman G Allan. Panoramic Radiology Seminars on Maxillofacial Imaging and Interpretation, Chapter 3, Springer, 2007 Figure 20: Farman G Allan. Panoramic Radiology Seminars on Maxillofacial Imaging and Interpretation, Chapter 3, Springer, 2007 Figure 21: Farman G Allan. Panoramic Radiology Seminars on Maxillofacial Imaging and Interpretation, Chapter 3, Springer, 2007 Figure 22: Whaites E. Essentials of Dental Radiography and Radiology, Chapter 15, p. 168. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. Figure 23: Whaites E. Essentials of Dental Radiography and Radiology, Chapter 15, p. 168. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. Figure 24: Whaites E. Essentials of Dental Radiography and Radiology, Chapter 15, p. 166. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. Figure 25: Whaites E. Essentials of Dental Radiography and Radiology, Chapter 15, p. 167. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. CHAPTER 4 Figure 1: White S C, Pharoah M J. Oral Radiology: Principles and Interpretation, Chapter 3, p. 44, 4 th ed. (St. Louis: Mosby Inc.) 2000. Figure 2: Whaites E. Essentials of Dental Radiography and Radiology, Chapter 4, p. 31. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. Figure 3: Whaites E. Essentials of Dental Radiography and Radiology, Chapter 6, p. 60. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. Figure 4: Whaites E. Essentials of Dental Radiography and Radiology, Chapter 6, p. 64. 2 nd ed. (Edinburgh: Churchill Livingstone) 1996. International Atomic Energy Agency. Dosimetry in Diagnostic Radiology: Figure 5: An International Code of Practice (Technical Reports Series No. 457). Vienna, 2007 Figure 6: Williams JR, Montgomery A. Measurement of dose in panoramic dental radiology. Br J Radiol;73(873):1002 6, 2000. xv

Figure 7: Figure 8: http://www.gehealthcare.com/usen/xr/edu/products/dose.html http://www.e radiography.net/radtech/d/dose_ge/dose.htm APPENDIX I Figure 1 6: Akarslan ZZ, Erten H, Güngör K, et. al. Common Errors on Panoramic Radiographs Taken in a Dental School. J Contemporary Dental Practice;(4)2:024 034, May 2003 xvi

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SECTION I GENERAL PART 1

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CHAPTER 1 THE INCLUDED ANATOMICAL STRUCTURES ON THE PANORAMIC RADIOGRAPH INTRODUCTION On this section, a short anatomical reference of the dental region is presented, as well as all the additional structures that appear or may appear in a panoramic radiograph. Due to this project s goals, which are not such of a pure medical interest, this section consists a basic and very brief anatomical reference. The knowledge of the dental structures, their surrounding tissues and their relations is an essential tool for a proper interpretation of the panoramic radiography and the discussion of its quality control. THE SKULL The skull is a hollow and rigid structure made of bone tissue. It is attached on the top of the human spine providing a protective shell to the brain and the eyes. Upon the skull, all the head muscless are attached, enabling all the functions of the head (chewing, air circulation, facial expressions etc.). The skull is divided into two parts: 1) The cranial part or cranium. It is the top part that covers the brain. 2) The facial part which is consisted of the facial bones at the front region. Figure 1: The Human Skull 3

THE MANDIBLE (LOWER JAW) The lower jaw has its own separate bone which is called thee mandible. It is U shaped and it stretches from one ear, down to the chin area and then back up again to the other ear. It is joined to the upper part of the head around the ear region by two jaw joints, the temporo mandibularr joints TMJs. The mandible is divided into the following parts: The body f the mandible the middle section of the U shape which supports the lower teeth. The condyle the rounded end of bone that fits into the movable joint between the mandible and the cranium. There is one for each side of the mandible. The coronoid process triangular projection Figure 2: The Human Mandible from the mandible which joins one of the chewing muscles to the cranium. There is one for each side of the mandible. The ascending ramus the flatter, straighter part on the sides of the lower jaw which joins the body of the mandible to the coronoid processes and the condyles. 4

THE MAXILLAA (UPPER JAW) The maxilla, or the upper jaw, is made up of several bones stuck (or fused) together, and sits in front of and just below the cranium. It is attached to the cranium and forms the cheeks, the nose and the roof of the mouth. The maxilla is divided into: The maxillary antrum or maxillary sinus the air filled space that sits just under the cheekbone and just above the roof of the mouth. There is one for each side of the face, either side of the nose. The anterior nasal spine a bit of bone which protrudes from the maxillaa at the lower end of the nose. The zygomatic process a curved piece of bone which extends outwards from the maxilla and forms a part of the cheekbone. Figure 3: The Human Maxilla The palate the roof of the mouth, separating the nose and the mouth. The hard part is called the hard palate and it is towards the front of the mouth. The softer part, the soft palate, is at the back near the throat. THE TEMPEROMANDIBULAR JOINT (TMJ) The temperomandibular joint (TMJ) is the movable joint between the mandible and a part of the cranium called the temporal bone. It is a complicated joint and the two hard bone surfacess are separated by a circular piecee of softer cartilage which acts like a cushion. 5

The TMJ moves up and down, sideways and forwards. It is in constant use during chewing, swallowing, talking or laughing, while some of these movements involve rotation of the joint and some others are sliding movements. Figure 4: The Human TMJ in action, causing movements of the lower jaw THE FORAMINA A foramen is an opening or hole whichh lets nerves and blood vessels pass through bone. The foramina are divided into the following parts: Figure 5: The Human Foramina The mental foramen on the body of the mandible. Nerves and blood vessels which travel to the lower lip pass through this. The mandibular foramen or inferior dental foramen on the ascending ramus. A nerve called the inferior dental nerve and blood vessels which go to the lower teeth pass through this. The incisive foramen at the front of the palate. Nerves called incisive nerves and the blood which supply the front f the palate pass through this. 6

THE SALIVARY GLANDS The salivary glands produce the clear liquid that is released into the mouth (saliva). There are three pairs of major salivary glands and many minor glands. Saliva lubricates the mouth and start the breakdown of chewed food. It is made up of water, enzyme, mucin and protein. The salivary gland pairs are the following: The parotid the largest salivary gland, situated below the ear. The saliva is released through an opening called the parotid duct whichh enters the mouth on the inside of the cheek next to the upper molar teeth. There is one on each side. The submandibular salivary gland situated under the mandible. It releases saliva just underneath the front of the tongue, behind the front teeth. There is one on each side. The sublingual salivary glands situated under the tongue. They release saliva from many small Figure 6: The Human Salivary Glands openings (ducts) under the tongue. This pair of salivary glands sits next to each other under the tongue. 7

THE CERVICAL VERTEBRAE The cervical vertebrae are positioned immediately posterior to the skull. They are the smallest among all vertebrae and can be readily distinguished from those of the thoracic and lumbar regions due to the presence of a foramen in each transverse process. Through each foramen the vertebral artery passes. There are seven vertebrae, numbered from C1 to C7, which form the cervical column, with C1 or atlas connecting the skull to the spine. Figure 7: The Cervical Vertebrae on the Vertebral Column and a single cervical vertebra, wiki GENERAL OVERVIEW OF THE ORAL, NECK AND LOWER FACE ANATOMY The following figure represents a sagittal section of the face and the neck. Among the different structures (nasopharynx, adenoids, oropharynx, tongue, esophagus, larynx, trachea) the proximity between the back of the tongue, the soft palate and the epiglottis is clear. The epiglottis is the valve that Figure 8: Sagittal section of the face and the neck determines into which tube the air or food flows (trachea or esophagus respectively). The pharynx is divided into three parts according to the anatomical regions in whichh it extends: the oropharynx (behind the oral cavity), the nasopharynx (behind the nasal cavity) and the laryngopharynx. 8

THE TONGUE The tongue is made mostly of skeletal muscle. The tongue, much as is commonly believed, extends from the posterior border of the mouth and into the oropharynx. The dorsum, or the upper surface, is divided into two parts: an oral, whichh lies mostly in the mouth, and a pharyngeal, which faces backward to the oropharynx. Figure 9: The tongue from a rear view The two parts are separated by a V shaped groove, which marks the Terminal Sulcus (tongue) ORAL ANATOMY ELEMENTS Moreover, the following anatomical structures of the oral cavity are presented as shown in the respective figure. The uvula is a valve which keeps food and drink from regurgitating up into the nasal cavity. The labial frenum is a little tag of tissue in the center of the upper and the lower lip that attaches the lip to the gums. The gingiva are what is more commonly named gums. The hamulii are hard little bumps in the corners of the soft palate, just where the soft palate meets the very back of the tuberosities. 9

Figure 10: Anatomy of the Oral Cavity The maxillary tuberosities are the tough, hard pumps behind the top back teeth on both sides of the dental arch. These humps have underlying bone and hard gum tissue covering them, and they are persistent, permanent parts of the mouth even if all the upper teeth are extracted. The tonsils are at the border between the mouth and the throat. The retromolar pad is similar to the maxillary tuberosities, except that it is behind the last lower molars, and it is not underlain by corresponding hump of bone. The vestibule is the curvature of the tissue where the lining of the inside of the lips (labial mucosa) or cheeks (buccal mucosa) meet the gingiva. The vermillion border is the junction of the dry, pink part of the lip with the skin of the face. 10

THE TOOTH Crown The crown of a tooth is its portion that is covered with enamel. It commonly lies nearly entirely exposed above the gum line. In children the gum may partially cover the cervical (lower) part of the enamel. Enamel The enamen is the substance that covers the crown of the tooth. It is the hardest and most highly mineralized substance of the body and is one of the four major tissues which make up the tooth, along with dentin, cementum and dental pulp. Dentin Dentin is the hard, yellow bone like material that underlie s the enamel and surrounds the entire nerve. It is a mineralize d connective tissue with an or ganic matrix of collagenous proteins. Cementum Cementum is the root of a tooth as enamel is to the crown. It is a relatively soft tissue that covers the root surface in a thin layer. Figure 11: The Human Tooth Dental pulp The dental pulp is the central part of the tooth filled with soft connective tissue. This tissue contains blood vessels and nerves that enter the tooth from a hole at the apex of the root, the apical foramen. The dental pump is commonly called as the nerve of the tooth. 11

TOOTH DEVELOPMENT Infants and Children: The milk or deciduous teeth are the first set of teeth of humans. Between the ages of 6 and 13, all milk teeth are pushed, extracted and replaced by new and bigger adult teeth. By the time most people are teenagers, they should have 28 adult teeth although some of these may still be coming through. The only teeth left to come through after this are the wisdom teeth, which usually erupt in late teens. Figure 12: The Deciduous Teeth (milk teeth) Milk or deciduous teeth start coming through when babies are between 5 and 8 months old. They are smaller than adults teeth because children s jaws are smaller. As the jaws grow, more teeth come through. There are 20 deciduous teeth altogether and they finish coming through by about age 2 to 2.5 years. The deciduous teeth are: Deciduous Incisors: These are the front teeth and there are 8 of them altogether (four at the top and four at the bottom). They are usually the first teeth to come through between the 5 th and the 8 th month. They have a flat biting edge and only one root. Deciduous Canines: Also known as eye teeth, these front teeth have a more pointed shape. There are four altogether and they come through the age 16 to 23 months and they also have a single root. Figure 13: The Primary Dentition Sequence Deciduous Molars: These are the larger back teeth, easy to tell because they look bigger than the front teeth and have bumpy or irregular surfaces for chewing with. They also have more than one root and the roots are quite splayed. There are 8 molars altogether 12

which come through between the ages of 1 to 2.5. Later on these teeth are replaced by premolars in adults. Figure 14: The Timelinee of the Primary Dentition Teenagers and Adults 28 Adult Teeth: Adult or permanent teeth start coming through at around the age of 6. They are larger than milk teeth. As the jaw grows, there is more room for the new adult teeth to erupt. There are 32 adult teeth altogether but sometimes the wisdom teeth have trouble coming through if the jaw is not big enough. Figure 15: The 28 Adult Teeth The Adult Teeth are: Permanent Incisors. These are the front teeth and there are 8 of them altogether (four at the top and four at the bottom). They are usually the first permanent teeth to come through. They are distinguished due to their flat biting edge and the one and only root that they have. Permanent Canines: Also known Figure 16: The Permanent Dentition 13

as eye teeth, these front teeth have a more pointed shape. Being 4 in number, they also have one root. Figure 17: The Timeline of the Permanent Dentition Premolars: These are the new adult back teeth which replace the first and second deciduous molars. The four 1 st premolars erupt first followed by the four 2 nd premolars. The deciduous teeth do not have premolars as back teeth, but just molars. Permanent Molars: These are the larger back teeth and they usually have more than one root. They also have irregular or bumpy surfaces with grooves called fissures. There are first, second and third permanent molars which come through that order. 1 st Permanent Molars: The four first permanent molars are similar in size to the second molars and come through age 6 to 8 years. 2 nd Permanent Molars: The four second permanent molars come through at around age 12 years. 3 rd Permanent Molars: Commonly known as the wisdom teeth. They do not erupt until late teens if they come through at all, because sometimes there is not enough space for them to erupt properly and they get impacted often against the tooth in the front. The latter can sometimes be damaged, or the gums around the wisdom teeth can become infected and quite painful. Figure 12: A Wisdom Tooth erupting against its neighbor tooth 14

Figure 19: Deciduous and Adult Teeth Figure 20: A panoramic x ray of a 7 years old child. One can notice the complex mix of the permanent and the primary teeth at these ages. The developing permanent teeth up to the 2nd premolar are also called succedaneous teeth because they succeed their corresponding primary teeth. Permanent molars are not considered succedaneous teeth. 15

Figure 21: Schematicc diagram of development and eruption of the primary dentition Figure 22: Panoramic radiograph of a 9 year old female. All of the permanent incisors have erupted. Their apical foramina, especially in the maxilla, have not yet assumed the normal diameter, indicating incomplete maturation. Tooth 12 is rotated about its axis. Note in the bifurcation of the first permanent molars a radiographic addition effect caused by superimposition of the root trunks. This is not due to enamel pearls. Figure 23: Schematic depiction of development and eruption of the permanent dentition 16

Figure 24: Panoramic radiograph of a 15 year old female. Root formation of the erupted teeth is complete. Teeth 35 and 45 have developed into taurodonts with a coronoapically expanded pulp chamber. During the pubertal growth spurt, dorsally and cranially directed growth of the maxilla and the mandible created space for the buds of the wisdom teeth and their root development has begun. The neck and condyle of the mandible are not yet fully developed. Figure 25: Panoramic radiograph of a 20 year old female. Development of the dentition is complete. The apical foramina and the root canals exhibit normal diameter of adult age. The third molars are completely erupted but exhibit a long axis that is oriented slightly dorsally. 17

Figure 26: A complete view of the Oral Anatomy 18

CHAPTER 2 PANORAMIC RADIOGRAPHY: A CLINICAL OVERVIEW INTRODUCTION Millions of dental panoramic radiographs are performed every year in a global basis. Being a simple, quick and convenient technique as also as providing a respectful image of the dentition and its related structures on a single film, with a relatively low dose in comparison to other radiographic techniques, it is easy to understand why its popularity has become so grown; and it is still growing. A panoramic radiograph contains a substantial amount of diagnostic information. This information, being sometimes difficult to be detected, finds itself upon the four basic diagnostic regions in panoramic radiography: 1) The Dentoalveolar region Figure 1: The Dentoalveolar region 19

2) The Maxillary region Figure 2: The Maxillary region 3) The Mandibular region Figure 3: The Mandibular region 20

4) The Temporomandibular joint region (abbreviated as TMJ), including the retromaxillary and cervical regions. Figure 4: The Temporomandibular Joint region (TMJ) THE NORMAL PANORAMIC RADIOGRAPH The normal panoramic radiograph contains a broad range of information that covers the entire dentition and its surrounding structures, the facial bones and condyles and parts of the maxillary sinus and nasal complexes. Various interpretations of the normal panoramic radiograph exist in current literature, so that the practitioner, the radiologist and the physician will be able to have an appropriate comprehensive pattern of the technique and its imaging and diagnostic potentials. However, it is crucial that each individual develops his/her own method of interpretation according to: a) the modality that is being used and its imaging programs and abilities, b) the clinical cases that are taken place in general and c) the basic anatomic characteristics of the population. The development of a consistent approach that ensures that all diagnostic information in the panoramic radiograph is indeed read is absolutely essential. Indicatively, four suggestions of interpreting the normal panoramic radiograph are following: 21

A) Allan G. Farman (2007) remarks 50 distinct soft tissues, bony and dental landmarks on a normal panoramic radiograph. His approach on reading and evaluating the radiograph follows roughly the numerical sequence of the figure below. It starts with the bony landmarks from the midline of the upper jaw and nasal cavity, then back in the maxilla and zygomatic complex on each side. The soft tissue shadows of the tongue and soft palate follow, and then the cervical spine and its associated structures. Afterwards, the focus is on the contents of the mandible starting from the midline and then progressing posterior on each side. Any examination would be incomplete without a thorough evaluation of the soft tissues anterior to the spine and inferior to the mandible. Lastly, there is the evaluation of the area of chief complaint and the dental arches. While these regions draw the reader s attention automatically, the other features within the radiograph can be missed without careful sequencing. Figure 5: Interpreting a Normal Dental Panoramic Radiograph A 22

1. nasal septum 26. mandibular canal 2. anterior nasal spine 27. mental foramen 3. inferior turbinate 28. inferior border of mandible 4. middle turbinate 29. hyoid 5. superior turbinate 30. pharyngeal airspace 6. soft tissue shadow of the nose 31. epiglottis 7. airspace between soft tissue shadow of the upper border of tongue and hard palate 32. coronoid process of mandible 8. lateral wall of nasal passage 33. inferior orbital rim 9. maxillary sinus (antrum) 34. mastoid process 10. nasolacrimal canal orifice 35. middle cranial fossa 11. orbit 36. bite block for patient positioning during panoramic radiography 12. infraorbital canal 37. chin holder (cephalostat) 13. zygomatic process of the maxilla 38. shadow of cervical spine 14. pterygomaxillary fissure 39. ethmoid sinus 15. maxillary tuberosity with developing third permanent molar tooth 40. angle of mandible 16. zygoma 41. crypt of developing mandibular third permanent molar tooth 17. zygomatico temporal structure 42. developing mandibular second premolar tooth 18. articular eminence of temporal bone 43. primary second molar tooth showing physiological root resorption 19. mandibular condyle 44. maxillary permanent central incisor tooth 20. external auditory meatus 45. maxillary permanent lateral incisor tooth 21. first cervical vertebra (atlas) 46. maxillary permanent canine tooth 22. second cervical vertebra (axis) 47. maxillary first premolar tooth 23. third cervical vertebra 48. maxillary permanent first molar tooth 24. fourth cervical vertebra 49. ramus of mandible 25. mandibular foramen & lingual 50. pterygoid plates 23

B) Friedrich A. Pasler (2007) suggests the following reading of the panoramic radiograph: Figure 6: Interpreting a Normal Dental Panoramic Radiograph B 1. orbit 15. zygomatic arch, articular tubercle 2. intraoral canal 16. coronoid process 3. nasal cavity 17. condyle 4. nasal septum 18. external ear with external auditory meatus 5. inferior nasal concha 19. cervical vertebrae 6. incisive foramen, superiorly located anterior nasal spine, nasopalatine canal 7. maxillary sinus 21. oblique line 20. temporal crest of the mandible 8. palatal roof and floor of the nose 22. mandibular canal 9. soft palate 23. mental foramen 10. maxillary tuberosity 24. dorsum of the tongue 11. pterygoid processes (lateral and medial lamina) and the pyramidal process of the palatal bone 12. pterygopalatine fossa 26. hyoid bone 25. compact bone of the inferior border of the mandible 13. zygomatic bone 27. superimposition of the contralateral 14. zygomaticotemporal suture jaw 24

C) William S. Moore presents his landmarks of interest for the interpretation of the normal panoramic radiography: Figure 7: Interpreting a Normal Dental Panoramic Radiograph C 1. coronoid process 19. infraorbital canal 2. sigmoid notch 20. nasal septum 3. mandibular condyle 21. inferior turbinate 4. condylar neck 22. medial wall of maxillary sinus 5. mandibular ramus 23. inferior border of maxillary sinus 6. angle of mandible 24. posterolateral wall of max. sinus 7. inferior border of mandible 25. malar process 8. lingual 26. hyoid bone 9. mandibular canal 27. cervical vertebrae 1 4 10. mastoid process 28. epiglottis 11. external auditory meatus 29. soft tissues of neck (look vertically for carotid artery calcifications here) 12. glenoid fossa 30. auricle 13. articular eminence 31. styloid process 14. zygomatic arch 32. oropharyngeal airspace 15. pterygoid plates 33. nasal air space 16. pterygomaxillary fissure 34. mental foramen 17. orbit 35. hard palate 18. inferior orbital rim 25

D) Having in mind the figure of a normal panoramic radiography, another suggestion for its interpretation follows, by Eric Whaits (2006): A. General Overview of the Entire Panoramic Film 1. Note the chronological and development age of the patient 2. Trace the outline of all normal anatomical shadows and compare their shape and radiodensity B. The Teeth 3. Note particularly: i) the number of teeth present, ii) stage development, iii) position, iv) condition of the crowns (caries, restorations), v) condition of the roots (length, fillings, resorption, crown/root ratio) C. The Apical Tissues 4. Note particularly: i) the integrity of lamina dura, ii) any radiolucencies or opacities associated with the apices D. The Periodontal Tissues 5. Note particularly: i) the width of the periodontal ligament, ii) the lavel and quality of crestal bone, iii) any vertical or horizontal bone loss, iv) any furcation involvements, v) any calculus deposits E. The Body and Ramus of the Mandible 6. Note: i) shape, ii) outline, iii) thickness of the lower border, iv) trabeculae pattern, v) any radiolucent or radiopaque areas, vi) shape of the condylar heads F. Other Structures 7. These include: i) the antra (note the outline of the floor, the anterior and posterior walls and the radiodensity), ii) nasal cavity, iii) styloid processes. 26

As mentioned above, the strategy for the interpretation of a panoramic radiograph is varies according to various and subjective factors. Ideally, one should create his/her own permanent and unique method of reading a panoramic image, so that all included diagnostic information can be detectable. Apart from the methods of interpreting the whole normal panoramic radiograph, there are also similar methods of interpretation specific regions, tissues or structures such as: facial skeleton, maxillary sinus, retromaxillary space, external ear and TMJ region, palatal bone, chin region and many others. The presentation of these methods does not fit to the goals of this paper. However, the reference bibliography includes such sections, for those interested in these subjects. Errors in interpretation are commonly made in the maxillary and mandibular incisor region. It is important to be aware of the normal appearances of the dental panoramic image when assessing the mandibular bone density: Lucency: Normal symmetrical lucency is common in the mandibular body inferior to the premolar and molar apices and may be mistaken for a lytic lesion. The lucency is due to the submandibular fossa on the lingual aspect of the mandible. The appearance is exacerbated in middle aged and elderly women by reduced bone density. Artifactual lucency over the mandibular angle is also produced by incorrect tongue positioning. Figure 8: Coned panoramic tomograph demonstrating normal radiolucencies. The pseudolucency (air space) between the soft palate and tongue (curved arrow), the normal mandibul ar lucency (straight arrows), the nasopharynx ( np) and oropharynx (op) are shown. 27

Areas of increased density: The mandible may appear sclerotic in the midline owing to superimposition of the density of cervical spine. However, intra oral densities may also appear as an area of mandibular sclerosis. Figure 9: There is an apparent area of sclerosis within the left lower mandible (arrows) A very significant point concerning radiographic anatomy must be taken into consideration. Representing the basis for radiographic interpretation, it follows its own rules and demands understanding and knowledge of how x ray work, as well as the normal anatomy of the irradiated spaces, depending on the radiographic technique used. Analogous to this essential knowledge, the following basic rules must be obeyed for every type of radiograph, including the panoramic technique: - The tangential effect of x rays renders clearly visible in the irradiated space only those hard tissues with either high density or significant thickness; thin lamella which, at the moment of the exposure, are parallel or nearly parallel to the central ray simulate hard tissue of significant thickness and therefore appear in the radiograph as densely opaque. On the other hand, similar structures which, at the moment of exposure, are perpendicular to the central ray or nearly so may, even though they are relatively thick, appear transparent in the radiograph because of the exposure data necessary to penetrate the tissue. - The summation effect of x rays may lead to hard and soft tissue structures in the field being exhibited more clearly, or they may disappear entirely depending upon the selection of exposure data. For example, if soft tissues are projected 28

upon one section of the bone, it will appear more dense than adjacent areas because the x ray beam is already weakened when it hits the bone. On the other hand, if an air containing space is projected onto a section of bone, the situation is on in which the x ray beam is not weakened before it encounters the bone, penetrates I readily and therefore eliminates the typical radiograph features of bone. The first example is referred to as addition effect, and the second example as subtraction effect. The situation in such cases has absolutely nothing to do with radiographic signs of sclerosis or resorption. Panoramic radiography is not an exception; it depicts in focus layers of various thickness (but always thicker than 5mm), and thus may be classified as a type of zonography. In the panoramic radiograph, the picture of the irradiated tissues is determined by the tangential effect and the summation effect; however, in keeping with the principle of tomography, all of the structures within the in focus layer are shown relatively distinctly and somewhat enlarged, while all structures outside of the layer are depicted as blurred and reduced in size or as blurred, broadened and enlarged superimpositions; such appearance will depend upon whether the superimposed structures are between the infocus layer and the film or between the in focus layer and the focal spot. 29

CHAPTER 3 PRINCIPLES OF FUNCTION INTRODUCTION The theory of dental panoramic tomography is complicated. Nevertheless, an understanding is necessary of how the resultant radiographic image is produced and which structures are in fact being imaged, so that a critical evaluation and for the interpretation of this type of radiograph. The difficulty in panoramic tomography arises from the need to produce a final shape of focal trough which approximates to the shape of the dental arches. An explanation of how this final horseshoe shaped focal trough is achieved is given below. Before that, other types of tomography which form the basis of panoramic tomography are described, showing how the result in different shapes of focal trough. These include: Linear tomography using a wide or broad x ray beam Linear tomography using a narrow or slit x ray beam Rotational tomography using a slit x ray beam 30

Broad Beam Linear Tomography The synchronized movement of the tubehead and film, in the vertical plane, results in a straight linear focal trough. The broad x ray beam exposes the entire film throughout the exposure. Figure 1: Diagram showing the theory of broad beam linear tomography to produce a vertical coronal section with the synchronized movement of the x ray tubehead and the film in the vertical plane. Using a broad beam, there will be multiple centers of rotation (three are indicated), all of which will lie in the shaded zone. As all these centers of rotation will be in focus, this zone represents the focal plane or section that will appear in focus on the resultant tomography. Note, the broad x ray beam exposes the entire film throughout the exposure. Slit or Narrow Beam Linear Tomography A similar straight linear tomograph can also be produced by modifying the equipment and using a narrow or slit x ray beam. The equipment is designed so that the narrow beam traverses the film during the tomographic movement. Only by the end of the tomographic movement has the entire film been exposed. The following equipment modifications are necessary: The x ray beam has to be collimated from a broad beam to a narrow beam. The film cassette has to be placed behind a protective metal shield. A narrow opening in this shield is required to allow a small part of the film to be exposed to the x ray beam at any one instant. A cassette carrier, incorporating the metal shield, has to be linked to the x ray tubehead to ensure that they move in the opposite direction to one another during the exposure. This produces the synchronized tomographic movement in the vertical plane. 31

Within this carrier, the film cassette itself has to be moved in the same direction as the tubehead. This ensures that a different part of the film is exposed to the x ray beam throughout the exposure. Figure 2: Diagram showing the theory of narrow beam linear tomography to produce a vertical coronal section. The tomographic movement is produced by the synchronized movement of the x ray tubehead and the cassette carrier, in the vertical plane. The film, placed behind the metal protective front of the cassette carrier, also moves during the exposure, in the same direction as the x ray tubehead. The narrow x ray beam traverses the patient and film, exposing a different part of the film throughout the cycle. Narrow beam rotational tomography In this type of tomography, narrow beam equipment is again used, but the synchronized movement of the x ray tubehead and the cassette carrier are designed to rotate in the horizontal plane, in a circular path around the head, with a single center of rotation. The resultant focal trough is curved and forms the arc of a circle, as shown below. Some important points to note are the following: The x ray tubehead orbits around the back of the head while the cassette carrier with the film orbits around the front of the face. The x ray tubehead and the cassette carrier move in opposite directions to one another. The film moves in the same direction as the x ray tubehead, behind the protective metal shield of the cassette carrier. A different part of the film is exposed to the x ray beam at any one instant, as the equipment orbits the head. 32

The simple circular rotational movement with a single center of rotation produces a curved circular focal trough. As in conventional tomography, shadows of structures not within the focal trough will be out of focus and blurred owing to the tomographic movement. Figure 3: Diagrams showing the theory of narrow beam rotational tomography. The tomographic movement is provided by the circular synchronized movement of the x ray tubehead in one direction and the cassette carrier in the horizontal plane. The equipment has a single center of rotation. The film also moves inside the cassette carrier so that a different part of the film is exposed to the narrow beam during the cycle, thus by the end the entire film has been exposed. The focal plane or trough (shaded) is curved and forms the arc of a circle. Dental Panoramic Tomography The dental arch, though curved, is not the shape of an arc of a circle. To produce the required elliptical, horseshoe shaped focal trough, panoramic tomographic equipment employs the principle of narrow beam rotational tomography, but uses two or more centers rotation. There are several dental panoramic units available. They all work on the same principle but differ in how the rotational movement is modified to mage the elliptical dental arch. Four main methods have been used including: 33

Two stationary centers of rotation, using two separate circular arcs Three stationary centers of rotation, using three separate circular arcs A continually moving center of rotation using multiple circular arcs combined to form a final elliptical shape A combination of three stationary centers of rotation and a moving center of rotation Figure 4: The main methods that have been used to produce a focal trough that approximates to the elliptical shape of the dental arch using different centers of rotation. A: 2 stationary, B: 3 stationary, C: continually moving, D: combination of 3 stationary and moving centers. However, the focal troughs are produced, it should be remembered that they are 3 dimentional. The focal trough is thus sometimes described as a focal corridor. All structures within the corridor, including the mandibular and maxillary teeth, will be in focus on the final radiograph. The vertical height of the corridor is determined by the shape and height of the x ray beam and the size as shown below. 34

As in other forms of narrow beam tomography, a different part of the focal trough is imaged throughout the exposure. The final radiograph is thus built up of sections, each created separately, as the equipment orbits around the patient s head. Figure 5: The figure shows how the height of the three dimensional focal corridor is determined. The height (x) of the x ray beam is collimated to just cover the height (f) of the film. The separation of the focal trough and the film (d) coupled with the 8⁰ upward angulation of the x ray beam results in the final image being slightly magnified. FOCAL TROUGH (Image Layer) The focal trough or the image layer of a panoramic radiograph is a 3 dimensional curved zone. Objects inside this zone are reasonably well defined. The anatomical structures which lie inside the image layer are depicted with the minimum of unsharpness and distortion. Going outside this layer, unsharpness is growing. Objects outside the focal trough are blurred, magnified or reduced in size and sometimes they are distorted to the extent of not being visible or recognizable. As it can be easily concluded, the focal trough and all its diagnostic information is all in all the very essence of a panoramic radiograph. 35

The exact shape of the focal trough (horseshoe curve) varies with the brand of the equipment used. The factors that affect its size are variables that influence image definition: Arc path Velocity of the film and the x ray tube head Alignment of the x ray beam Collimator width The location of the focal trough can change with extensive machine use. In this way, recalibration may be necessary if consistently suboptimal images are produced. Figure 6 The Focal Trough. The closer to the center of the trough (dark zone) an anatomic structure is positioned. The more clearly it is imaged on the resulting radiograph. 36

Figure 7: Gradual formation of a panoramic tomograph over an 18 second cycle, illustrating how a different part of the patient is imaged at different stages in the cycle. 37

FORMATION OF THE IMAGE LAYER As the x ray tube and the cassette holder are mounted at opposite ends of a gantry, during the exposure the x ray beam and the cassette rotate around the patient. The x ray beam is collimated both at the tube head and immediately in front of the film screen cassette. The effect of the collimation is to create a beam that is vertically long although horizontally narrow. Control of the rotational movement and film velocity is achieved by computer software and mechanical gearing. The projection technique employed in rotational panoramic radiography is unique, as there are two foci of projection working simultaneously. In the vertical plane the situation is analogous to conventional radiography where the x ray tube focus acts as the focus of projection. Consequently, the magnification in the vertical plane (M V ) is as follows: where FFD is the focus to film distance and FOD is the focus to object distance. In the horizontal plane, the effect of the narrow collimated beam combined with its motion creates the appearance that the point of divergence for the x rays is the rotation centre of the beam. Therefore, this point acts as an effective focus for the x rays in the horizontal plane and the distance between this point and the object is termed the effective projection radius (R). The magnification in the horizontal plane (M H ) is therefore: As the effective rotation centre is nearer the object than the conventional focus, the effective projection distance is always smaller than the conventional focus to object distance. Consequently, the magnification in the horizontal plane is greater than the magnification in the vertical plane, leading to geometric distortion. 38

Figure 8: Creation of an effective focus of projection The rotational movement of the x ray beam also means that every ray in the beam will project the image of a discrete object point onto the film plane at a different position. The image of a discrete object point would therefore be portrayed as a horizontal line at the film plane if the film remained stationary during the exposure producing motion unsharpness. However, by moving the film in the same direction as the beam and selecting an appropriate velocity of movement, the film can be made to match the projected path of an object plane within object. Object points within this plane are therefore depicted with minimum motion unsharpness at the film plane. The velocity of the film relative to the beam also affects the horizontal magnification of points within the object. It has been shown that an object will be sharply depicted at the film plane where the following equation is standing: where VB is the velocity of the beam, VF is the velocity of the film, FFD is the focus to film distance and R is the effective projection radius. The horizontal magnification therefore depends not only on the geometric properties of projection but also on the relative velocity of the film to the beam and consequently the horizontal magnification is a non linear function with object depth. 39

The position of the focal trough within the object is not constant but depends on the relative velocity of the film to the beam. As the FFD is generally constant, increased acceleration of the film velocity relative to the beam shifts the position of the focal trough away from the rotation centre and towards the film cassette, with a corresponding increase in the effective projection radius. Decreasing the film velocity relative to the beam moves the position of the focal trough towards the rotation centre and away from the film cassette, thereby reducing the effective projection radius. As the width of the focal trough is proportional to the effective projection radius, it is possible to alter the size and position of the focal layer by adjusting the relative velocity of the film to the beam and thereby create a focal layer that corresponds to the idealized shape of the jaws. In many modern systems the projection of the jaw is achieved by using an effective rotation centre that is continuously moving throughout the exposure. In these units, the x ray beam is always directed perpendicularly to the path of the effective rotation centre throughout the exposure. Consequently, the effective projection radius varies continuously throughout the exposure, being wider in the lateral aspects of the jaw than in the anterior regions. As a result, the focal layer is narrower in the anterior region of Figure 9: Diagram of beam projection in modern dental panoramic x ray units. 40

the jaw where the effective projection is smaller. Furthermore, as the size of the effective projection radius also influences the horizontal magnification, the horizontal magnification of object points outside the focal trough is more marked in the anterior regions of the jaw compared to the lateral regions. The path of the beam also determines the angle at which the central ray of the beam traverses the object. Ideally, to aid interpretation and measurement, the central ray of the beam should be projected perpendicularly to the object throughout the exposure. However, the projection angle between the central ray and the object varies throughout the exposure, only approaching perpendicularity in the anterior regions of the jaw. Distortion effects are therefore produced by the oblique projection angle between the beam and some regions of the jaw. As in conventional radiography, this effect leads to compression of objects in the resulting image. It should be noted however, that this effect is independent of both geometric distortion and motion unsharpness and therefore distortion effects due to oblique projection occur even at the focal trough position. GEOMETRIC DISTORTION For object points located outside the focal layer, the greatest source of distortion is the geometric distortion caused by the discrepancy between the horizontal and vertical magnification factors. The degree of distortion can be assessed using a relationship termed the Distortion Index: At the focal trough, where an object s vertical and horizontal magnification are equal, the DI is one and geometric distortion is minimized. A distortion index greater than one, indicates that the horizontal magnification of the image is greater than the vertical magnification. This effect occurs if the velocity of the film is greater than the velocity of the beam. This occurs if the object is displaced away from the focal trough position towards the centre of rotation (i.e. towards the beam). 41

Conversely, a distortion index less than one indicates that the vertical magnification is larger than the horizontal one in the image and it occurs if the velocity of the beam is greater than the velocity of the film. This occurs when the object is displaced away from the focal trough position towards the film. Figure 10: Magnification and x ray tube focal spot size. Displaced towards the rotation centre of the beam Placed at the focal trough Displaced towards the film Table 1: Distortion effects. Relationship between type of distortion and displacement from the focal trough. 42

Screen Film and Intensifying Screens A beam of x ray photons that passes through the dental arches is reduced in intensity (attenuated) by absorption and/or scattering effects of photons out of the primary beam. The pattern of the photons that exists the subject, the remnant beam, conveys information about the structure and the composition of the absorber. This is why the remnant beam has to be written on an image receptor, in order to be diagnostically useful. Dental panoramic x ray units use a combination of screen film and intensifying screens. The advantages of this combination are the increased sensitivity and better contrast. The screens have much higher x ray absorption efficiency (510 times) than a photographic film and will also produce a great number of light photons per x ray photon absorbed, thus yielding a more efficient film exposure. The x ray exposure can often be reduced by factor of 10 50, depending on the screen characteristics. The sensitivity can vary depending on the screen thickness. The thicker the screen, the higher the absorption efficiency. However, a thicker screen will also result in an impairment o spatial resolution. The light produced in the screen will have a longer distance before it hits the film and it will therefore be more diffused than for a thinner screen. More sensitive screens can also be produced using larger phosphor crystals in the screen material. This will lower the spatial resolution and will also increase the noise level. Due to the higher atomic number of the screen material compared to the silver halide grains, the film screen combination is always relatively more sensitive to higher energy x rays than film alone. This extends the use of film screen combinations to techniques which use higher energy beams, compared to film alone. Thus, apart from the panoramic technique, screen film combinations have been one of the most important components in all modern radiology. The Screen Film Screen film is different than dental intraoral films in that it is designed to be particularly sensitive to visible light rather than to x radiation because this film is placed between two intensifying screens when an exposure is made. The intensifying screens absorb x rays and emit visible light, which exposes the screen film. Silver halide crystals are inherently sensitive to ultraviolet (UV) and blue light (300 to 500 nm) and thus are sensitive to screens that emit UV and blue light. When film is 43

used with screens that emit green light, the silver halide crystals are coated with sensitizing dyes to increase absorption. Because the properties of intensifying screens vary, the dentist should use the appropriate screen film combination recommended by the screen and film manufacturer so that the emission characteristics of the screen match the absorption characteristics of the film. In panoramic radiography fast films that require less radiation exposure are mainly used, as fine image detail is not available because of the movement of the x ray tube head during the exposure period. The design of screen films changes constantly to optimize imaging characteristics. As an example, Kodak has introduced T Mat films, which have tabular shaped (flat) grains of silver halide. The tabular (T) grains are oriented with their relatively large, flat surfaces facing the radiation source, providing a larger cross section (target) and resulting in increased speed without loss of sharpness. In addition, green sensitizing dyes are added to the surface of the tabular grains, increasing their light gathering capability and reducing the crossover of light from the phosphor layer on one side of the intensifying screen to the film emulsion on the other. Kodak s Ektavision system also coats the film base with an absorbing dye to prevent crossover of light from one screen to the other emulsion. These properties increase both the speed of the film and the sharpness of the image. Sterling uses tabular grains in its Cronex I OT film, and Imation coats its XDA+ and XLA+ film base with an anti crossover agent as well. Figure 11: T grains of silver halide in an emulsion of T Mat film (A) are larger and flatter that he smaller, thicker crystals in an emulsion of conventional film (B). Note that he flat surfaces of the T grains are oriented parallel with the film surface, facing the radiation source. (Courtesy Eastman Kodak, Rochester, N.Y) 44

INTENSIFYING SCREENS Wilhelm Conrad Roentgen, along with the x rays, discovered the intensifying screens. He was experimenting with high energy cathode ray tube and had enclosed the tube in black cardboard. When passing a high voltage discharge through the tube, he noticed a faint light from a piece of paper left on a work bench. The paper was covered with a thin layer of barium platinocyanide. This first intensifying screen sent out fluorescent light caused by the x rays. An intensifying screen is a sheet of crystals of inorganic salts (phosphors) which emit fluorescent light when excited by x ray radiation. The sheets are used to intensify the effect of x rays during exposure of x ray film. The intensifying screen or a pair of screens is nearly always used with x ray film in radiography. Film may be used as the only radiation detector but having a relatively low atomic number (that of silver halide), film is relatively radiolucent. At direct exposure of film, only about 5% of the x ray photons will be absorbed by the film and react with the emulsion. For comparison, a high speed calcium tungstate screen will absorb approximately 40% of the x ray photons. Furthermore, each absorbed x ray photon will be converted into many light photons. The efficacy of the screen in converting x rays into light photons is called the intrinsic conversion efficiency. The efficiency of calcium tungstate is about 5%. A 50 kev x ray photon when absorbed by calcium tungstate (by photoelectric absorption), will be converted into about 17,000 light photons of 3 ev energy at 100% efficiency. Since the efficiency is onl y 5%, the actual number of light photons emitted is 850. Approximately, half of these will escape from the screen to expose the emulsion. About 100 light photons may be sufficient to form one latent image centre. There are two major types of phosphors and therefore intensifying screens : calcium tungstate screen (CaWO 4 ) and rare earth screens (La 2 O 2 S:Tb, Gd 2 O 2 S:Tb, Y 2 O 2 S:Tb), where terbium (Tb) is often used as an impurity or activating substance. During the 70 s, the principal material in intensifying screens was calcium tungstate (CaWO 4 ). Nowadays, the principle materials are based on gadolinium and lanthanum substances. Calcium tungstate suffers from the drawback that the K edge for W in the absorption curve is situated at 69.5 kev. This means that the CaWO 4 screen is quite insensitive to the part of the x ray spectrum with photons of energies between 50 and 69 kev when compared to gadolinium or lanthanum screens, which have their Kedges lower at 50.2 and 38.9 kev respectively, due to their lower atomic number. Since this part of the x ray spectrum often contains a significant fraction of the x rays that exit 45

from the patient, the sensitivity is increased using these newer materials. This is furthermore amplified by the fact that the new materials have a higher light conversion efficiency, which means that more light photons are generated per x ray photon absorbed. Compared to CaWO 4, the newer screens produce about 3.5 to 4 times more light. Figure 12: X ray absorption spectra for calcium tungstate (W) and gadolinium (Gd) based intensifying screens. The spectral output of the phosphor must be matched to the response of the film. Calcium tungstate screens emit blue light of continuous spectrum with a peak wavelength at about 430 nm. The term blue screen refers both to the screen itself and to the blue sensitive film used together with the Ca WO screen. Rare earth screens emit light in narrow lines with strong peak(s) in the green part of the spectrum but smaller ones also in the blue, blue green and yellow regions. The term green screen may be used. It is absolutely necessary to use green sensitive film with these screens to make sure that useful transmitted radiation is not lost. 46

Figure 13: Spectrum of light emitted from calcium tungstate (CaWO) and rare earth screen (GdOS), respectively, in comparison with the light sensitivity of blue sensitive and green sensitive film. There are many factors affecting the speed of a screen. The phosphor type determines: The x ray radiation absorption efficiency The radiation to light conversion efficiency The thickness of the phosphor The fraction of x rays absorbed by a pair of calcium tungstate screens is about 20 40% depending on the speed (determined mainly by screen thickness), while rare earth screens absorb about 60%. The radiation to light conversion efficiency of calcium tungstate is about 1/3 or 1/4 of that of the film screen cassette. The relative speed of film screen combinations is normalized to 100 which corresponds the basic calcium tungstate screen with the basic film. Speed values vary from 20 and 50 (slow screens) through 100, 200, 400 and 800 to 1,600. An essential basic feature of the two screen types is related to the position of the K edge on the energy axis. Tungsten (W) being a heavy element has its K edge at 69.5 kev, while that for rare earth elements is in the vicinity of 50 kev. Most x ray spectra used in conventional radiography have their mean energy between 40 50 kev, signifying that also for this reason rare earth screens are more effective than CaWO 4 in absorbing x ray quanta. 47

Compared to film only, the contrast of the same film together with screens is always higher. The reason for this is not precisely known. DIGITAL PANORAMIC RADIOGRAPHY An image is said to be a digital one when it is composed of separate (distinct) elements. Each element is called a picture element or a pixel. If an image is displayed on the computer monitor and the pixel is smaller than the smallest detail the viewer s eye can see, it is hard to determine that the image is indeed a digital one. If this is not the case, that is the individual pixels can be spotted, the eye views the image as a mosaic of pixels. Each pixel can only take on a limited number of gray shades. The number of possible gray shades depends on the number of bits (binary digits) that are used to store a pixel. A 1 bit pixel can only take two values (0 or 1 that is black or white). An 8 bit pixel can take any one of 256 (2 8 ) values. A 16 bit pixel can take more than sixty five thousand grayscale values (216). The total number of bits that are used to store an image is the number of pixel times the number of bits per pixel. Most systems of digital panoramic radiographs use on the final depiction mostly 8 bits (thus, 256 different grey levels). Newer systems deposit the image as data of 10, 12 or 16 bits (thus 1024, 4096 or 65536 different grey levels). However, these systems lay the final image out using 8 bit data and 256 different grey levels. It is generally accepted that the human eye can only distinguish about 20 magnitudes of light intensity, and is probably unable to discern all 256 gray levels that a standard computer monitor can display. Thus, a human eye cannot distinguish totally a little more than 100 different grey levels. Indeed, in cases where these levels of grey find themselves on the same radiograph, then the number of the distinguished levels is limited between 30 and 40. For example, for the panoramic system Dimax I (Planmeca Oy, Helsinki, Finland) the functional and true size of the pixel that presents the final image is 132 μm. This pixel size delivers a maximum theoretical resolution, which is the Nyquist frequency, of 3.78 cycles per mm. There are three methods available to produce digital panoramic images. First, it is possible to digitize conventional analog film radiographs through secondary capture using transparency scanners or specialized digital cameras. Film scanners and digitals cameras, though, can be used to produce digital images only from an analog film radiograph. 48

In general, secondary capture is best achieved with a good quality scanner having a radiography adaptor (i.e. scanning light in the lid to pass light through the radiograph. A sharp black and white photograph setting is preferred. Excellent scanners for this purpose for a sufficient high quality system have a cost varying from 600$ to 1,500$. Scanners are preferred to digital cameras as they practically eliminate optical distortion and the reflection from the surface of the radiograph that would otherwise reduce image quality. Film scanners do not change the need to continue making radiographs with x ray film. They introduce additional time consuming activity to scan images but that is the price for continuing the use of analog film radiographs while digitally storing images. No matter the quality of the film scanner, scanned images can only be as good as the priginal film radiographs. The advantage is that the user can scan and archive the existing film files over time and determine if digital panoramic imaging is needed without spending a lot of money in purchasing sophisticated equipment. While Schultz et al. (2002) found the sensitivity for detection of low contrast simulated bone lesions was greater with film than after digitization, the absolute differences were small. Figure 14: Nikon CoolPix scanner with transparency adaptor in lid sufficient for extraoral radiograph duplication. Figure 15: Panoramic radiograph placed for digitization in an alternative flat bed scanner, the Epson FinePix Z2 with transparency adaptor. The result of this method is apparently substandard in comparison to the following two methods. However, these two technologies have the same principles of function with those that are applied in endodentulous radiographies. 49

Solid state systems using the technology of Charged Coupled Device (CCD) or complementary metal oxide semiconductor (CMOS) comparable to the computer chip found in a digital photographic camera McDavid et al. (1995) presented and evaluated the first experimental system for panoramic radiography with the use of CCD technology. This original system was a modification of the panoramic machine OP10 (Instrumentarium Imaging, Finland). The rotation time around the patient s head remained stable and the same with the equivalent conventional system, but the combination of film and rare earth screens had been replaced by a narrow linear sensor CCD. The height of this sensor was approximately 15 cm and its width a few millimeters. During the last years, different systems of digital panoramic imaging using CCD technology have been introduced into the market. The procedure of the production of the image remains the same. Solid state digital x ray detectors are based on a silicon chip that permits the acquisition of an image. Such a chip consists of a myriad of pixels and each pixel captures a small quantity of energy (usually light from a scintillator) and converts this radiant energy into electricity. For panoramic radiology, this generally involves a charged coupled device (CCD) or complementary metal oxide semiconductor (CMOS) of sufficient dimensions to cover the secondary slit of the panoramic machine (i.e. tall and narrow). The solid state chip (CCD or CMOS) converts radiant light photons into electrons when a scintillator is used. The ability of detectors to capture radiant energy is no longer limited to visible photon as cadmium telluride can produce electrons directly on impact of x ray photons. Most systems, however, still use a scintillator layer, similar to the scintillators that are used as intensifying screens in analog film panoramic radiography. An example of one of the earliest commercialized digital panoramic systems was that of the Trophy Digipan adaptor for the Instrumentarium OP 100. Figure 16: The Trophy Digipan adaptor used with Instrumentarium OP 100 panoramic system in place of the film cassette. A variety of add on systems from several different vendor sources are now available for most panoramic systems. 50

Figure 17: Schematic representation of a solid state detector. Unlike analog x ray film radiography, the receptor is stationary and the image for each segment is read out in appropriate sequence. Solid state systems are available both to retrofit an existing panoramic system and as integrated units dedicated to a specific panoramic x ray generator. A potential concern with retrofitting a unit is that if something does go wrong, the owner may be in the position of working together with the manufacturer of the panoramic system, the manufacturer of the retrofit system and the installer. As with analog film, the panoramic image is pieced together during the scan. Unlike analog film radiography, the receptor is stationary and the image for each segment is read out in appropriate sequence. As it was mentioned above, in conventional panoramic imaging, the position of the image layer is determined by the velocity of the film in relation to the x ray beam. In digital panoramic radiography using CCD technology, it is not the film that is moving in order to form the image properly but the charges themselves as they are read out. Thus, the velocity by which the pixels give their data for the final imaging is exactly the same with the velocity of the movement, in the scintillator s plane, of the projections of the anatomical structures and elements inside the focal trough. In this way, the image is formed partially and almost simultaneously with the patient s radiation, thus in real time. 51

A direct practical consequence of this fact is that the user can control at any time of the radiation the quality of the final imaging. If the user sees that the patient is not properly positioned during the radiation, he/she is able to interrupt the procedure and reposition the patient, so that a more correct imaging will be achieved. Researchers report that the width of the x ray beam which scans every moment the patient to form the image, is much more narrow in digital panoramic imaging using CCD technology, in relation to conventional panoramic radiography. This happens due to the high sensitivity of the CCD and to the attempt of bounding the x ray beam, using special collimators, exactly at the width of the slit linear sensor. This offers an important advantage, which is a significant reduce of the radiation dose of the patient. Farman et al. (2000), as well as Dula et al. (2008), confirmed this experimentally. Additionally, MacDavid and Dove (1995) report two direct practical consequences. The zone in which the depiction s sharpness is maximum is larger, thus the positioning of the patient is not as much important as in conventional procedures and in this way some small positioning errors may not be of such significant importance. However, many times the over projection of other anatomical regions cannot be avoided, resulting in the black out of elements of diagnostic interest. For example, the spine can break the depiction of the front regions, or the depiction of the lateral regions of the mandible being blocked by the respective regions of the other side. Moreover, as the CCD sensor is much more sensitive to the combination of film and rare earth screens, a reduction of the ma of the machine (less quantity of radiation) may be achieved with the same kv, as in conventional methods, or even higher. In this way, a reduction in the patient s dose is possible. Higher photon energy is less harmful for the patient. The contrast of the final image will be lower in this case, with higher voltage. This happens due to the different absorption of the radiation, depending on the beam energy when the beam enters through matter (an object). However, this is something that can be overcome, as there is the possibility of editing the image, adjusting the contrast. The percentage of dose reduction in digital panoramic radiography using CCD technology is not always the same. It is a fact that a reduction of the radiation dose similar to intraoral radiography is not possible, due to the fact that in conventional panoramic radiography the dose is already very low, as a consequence of the film rare earth screen combination. The dose reduction percentage that is reported in the global bibliography varies from 20% to up to 40% related to conventional panoramic 52

radiography. In this case, the final image will have an intense noise. However, using the editing techniques that digital imaging systems offer even such an image may be corrected and used for diagnostic purposes. Units which use the technology of Storage Phosphor Plates (SPP) In the middle of the 80 s, Kashima et al. (1985) presented the first digital panoramic radiograph using a Storage Phosphor Plate (SPP). This technique has been improved over the years, until it reached a widespread level of acceptance all over the world. Today, a variety of companies use this technology. In this technique a special plate, with the same size (30x15 cm) as in conventional panoramic radiography, is used. Such plates contain a phosphor layer that deposits the energy of the photon with which it interacted. Hence, the name storage phosphor is used. After its irradiation, the plate is placed in a special scanner or digitizer where it is scanned by radiation of specific wavelength. When a portion of the plate is illuminated, it emits light that is photomultiplied and collected by a digital imaging chip. The image appears through sophisticated electronic processes (photomultiplier, analog to digital converters) in a monitor of a computer. Photostimulable phosphor systems dedicated to dentistry are available from a number of manufacturers. Each system is comprised of the phosphor plates and a laser scanner that interfaces with a computer. The plates can be quite expensive, costing from 500 euro up to 2,000 euro each. While extraoral plates are not as sensitive to scratching as are the intraoral ones, care must still be taken not to scratch or contaminate them. The plates are very sensitive to ambient light which can erase much of the latent image. Furthermore, they need extensive exposure to light in order to completely erase the image before reuse. On the other hand, storage phosphor systems are versatile in that they can be used with a wide range of different x ray systems. It is of particular importance that this technique can be applied upon every conventional panoramic system. The only modification needed, is the placement in the holder of the panoramic machine, of the Storage Phosphor Plate, instead of the combination of filmrare earth screen. The only extra equipment needed, to gain the final image, is the specialized scanner or digitizer. Many manufactures offer themselves the scanner along with the conventional panoramic machine. 53

A basic characteristic of the plates used in this technique, is that they do not have picture elements (pixels). The latter are formed during the procedure of the laser scanning of the plate and their exact size depends on the degree of focalization of the laser radiation. For example, the DenOptix system (Gendex) uses a pixel size of 170 μm, when the resolution in the scanner is determined to be 150 dots per inch (150 dpi). This pixel gives out a maximum theoretical resolution, the Nyquist frequency, equal to 2.95 cycles / mm. The phosphor plates have a very high dynamic range of irradiation. Thus, with the use of this technique the quantity of the radiation targeting the patient (ma) may be reduced, without any loss of the diagnostic quality of the final image. The dose reduction percentage, related to conventional techniques, is approximately at 20%. The time period of the radiation cannot be reduced, as it is always stable for a specific unit depending on the whole rotation movement that it makes. Figure18: Imaging using storage phosphor plate. 54

Figure19: Loading a photostimulable phosphor plate into a soft cassette. Interoperability It is not unusual to review film radiographs that are decades old especially when demonstrating classical radiographic features of disease entities at a continuing education forum. Archived film images that are decades old are usually still of high quality and can be viewed by anyone who happens to have a view box to transmit light through the radiographs. One might question whether the digitized or digital versions will be as readily accessible as the analog film versions decades into the future. The likelihood of being able to retrieve digital images is dependent upon both hardware and software/file format considerations. Regarding hardware issues, one simply needs to back up all files on new media as they become accepted. If one intends to use digital images they periodic storage hardware upgrades must be performed. Regarding the matter of software/file format interoperability, the digital x ray industry and practice management system vendors are presently working together to facilitate digital image interoperability using specifications from the DICOM (Digital Image Communication) standards that were developed initially for medical radiology. This specification includes image format rules and associated information for transmission of radiographs used in dentistry including intraoral surveys and panoramic images. 55

However, no guidelines or specifications will guarantee interoperability. Interoperability needs to be demonstrated practically. Such practical demonstrations were initiated at the ADA Annual Congress in New Orleans in 2002, where ten companies demonstrated that interoperability of their image files could be archived satisfactory. Similar practical demonstrations have been made with DICOM validation at all ADA Annual Sessions, through at least until the time of publication of this book. Each time there are more vendors involved. Interoperability within the DICOM Standard is important so that the dentist can integrate data from different digital sources and read diagnostic images referred from outside sources where different systems may have been used. Otherwise there could be inconvenience both for the patient and for the practitioner. Radiation Dosage Unlike intraoral radiology, the switch to digital panoramic imaging does not generally result in a substantial dose reduction to the patient. In fact it is sometimes necessary to actually increase dosage to optimize image quality when using digital systems. With intraoral x ray film radiography, the emulsion is directly sensitive to x rays, so adding a scintillating screen can improve the efficiency with which x rays are detected. However, for extraoral radiography, an intensifying screen is generally employed and this is not so very different from the scintillating layer used with solid state detectors. Gijbels et al. found no difference in exposure settings or organ doses between analog x ray film and digital panoramic radiography using Photostimulable phosphor plates. COMPARISON BETWEEN FILM AND DIGITAL PANORAMIC IMAGING Digital panoramic radiography includes the following essential properties: Images of diagnostic quality Radiation dose similar or reduced compared to film radiography Lossless archiving (storage of the full original radiographic image) Interoperability of image format so that the patient s information can be conveniently shared when professionally necessary Below there is a summary of the advantages and disadvantages of the two categories. 56

ADVANTAGES DISADVANTAGES FILM TECHNIQUE Low initial cost, especially for manual processing Often already in place No changes or additional training required Known entity proven output Relatively low cost of operation Excellent diagnostic clarity possible if exposed and processed optimally Widely accepted Cost of consumables such as film and processing solutions Cost of processing equipment and darkroom space Time consumption in film processing and processor maintenance Processed film images are rarely optimal Used processing chemicals are toxic to the environment Film radiograph storage and retrieval can be problematic Duplicates made from film radiographs are invariably inferior to the original images DIGITAL X RAY IMAGING Time saving as there is no chemical processing More consistent in quality for the same reason Digital images ease communication with patients Digital images are readily stored and retrieved Digital radiology opens the way to electronic interchange Consultation can be expedited Digital images allow perfect clone duplication and backup Post processing can help optimize the diagnostic yield Digital radiology eliminates environmental silver contamination from spent fixer Added initial cost for equipment if film imaging is already used Need for additional computers, monitors, networking, backup storage Detectors (both solid state and phosphor systems) can add an important cost of the panoramic system Changes in operations, systems and procedures require an investment in time and involve a learning curve Not all digital image formats are identical at this moment, so interoperability can be problematic both in the same office and when making outside referrals Eventually hardware obsolescence Table 2: Advantages and disadvantages of conventional and digital panoramic imaging (Farman, 2007). 57

Equipment There are several different dental panoramic tomographic units. Although they vary in design, all of them consist of three main components: An x ray tubehead and its power supply, producing a narrow fan shaped x ray beam, angled upwards at approximately 8⁰ to the horizontal A cassette and cassette carriage assembly Patient positioning apparatus including light beam markers Figure 20: A modern panoramic x ray unit. The equipment should have a range of tube potential settings, preferably from 60 to 90 kv. The beam height at the receiving slit of cassette holder should not be greater than the film in use (normally 125 mm or 150 mm). The width of the beam should not be greater than 5mm. Equipment should be provided with adequate patient positioning aids incorporating light beam markers. New equipment should provide facilities for field limitation techniques. 58

Almost all modern panoramic machines have a continuous mode of operation and produce a so called continuous image showing an uninterrupted image. X ray production is continuous throughout an uninterrupted tomographic cycle, during which the centers of rotation are adjusted automatically. However, one machine was developed which produced a so called split mode image because the radiographic image is split by a broad, vertical, white, unexposed zone, with duplication of the midline. The split mode equipment is now only of historical interest. There are a number of different panoramic machines available in the market. The machines operate on the same image formation principles and only differ in the added features. Panoramic machines employ a single phase x ray robe that requires an average cool down period of 5 minutes between exposures for maximum tube life. Machines have an upright construction with an adjustable vertical tube height range typically of 3 feet (0.91 m) to 6 feet and 4 inches (1.92 m) from the floor. The entire machine may be freestanding with a heavily weighted base or wall mounted. The cassette holder and x ray tube are attached to a carrier that mechanically rotates them at a preset path and speed around the patient s head during each exposure. These variables stay consistent from patient to patient unless the technologist changes the location or width of the focal trough which is adjustable to accommodate different facial profiles. Two types of cassettes are made for panoramic machines: hard and floppy. The type of cassette to be used is determined by the manufacturer. The standard film sizes available for panoramic imaging are 6 x 12 and 5 x 12. Sizes are also determined by the machine manufacturer. Cassettes incorporate rare earth screens and use 400 speed film. Specific film types may be chosen to optimize bony or soft tissue detail. Most modern panoramic machines allow the patient to stand up for the exposure. A chair or wheelchair can be accommodated. All panoramic machines have some type of guides to help position and hold the patient within the focal zone. This commonly includes a platform for the chin to rest on and a bite stick with grooves for the upper and lower central incisors, placing the oral cavity within the focal plane. For edentulous patients there may be a separate, specialized positioning device that is used in place of the bite stick. Many machines use laser light guides at the infraorbitalmetal (IOM) line and along the midsagital plane to help the technologist adjust the patient into position, as well as guide bars that can be secured alongside the head to help the patient maintain the correct position throughout the exposure. Almost every panoramic machine produces standard panoramic films as well as specialized projections of the (TMJ) such as transmaxillary projections and either single images or multiple cuts. Even the least sophisticated machines typically offer multiple 59

views of the TMJ area including lateral images with the mouth open and closed, producing all 4 images on one film. A wide array of programs specifically designed to evaluate the TMJ are available with more advanced machines. Additional programs allow for multilayered exposures of the area in the frontal, transmaxillary and lateral projections to evaluate fractures, arthritic changes and abnormalities in the size, shape and position of the condylar head. Programs can also assess the articular processes. More sophisticated panoramic machines offer a variety of programs that produce tomographic images of predefined areas of almost any area of the head. Using a series of computer controlled programs that use various tube head and film motions, including multidirectional, these machines can perform coronal, sagittal and cross sectional images through a predetermined area. These programs can be helpful in the assessment and treatment of sinusitis, neoplasm, fractures, foreign bodies, air fluid level detection in the sinuses, mucosal changes of the sinuses and soft tissue calcifications. Many panoramic machines can be fitted with a film holder that supports an 8 x 10 film used for static skull views using the panoramic x ray tube head. Figure 21: The Instrumentarium OP 200D (PaloDEx, Tuusula, Finland) 60

Patient Positioning The exact positioning techniques vary from one machine to another. However, there are some general requirements that are common to all machines and these can be summarized as follows: Patients should be asked to remove any earrings, jewellery, hair pins, spectacles, dentures or orthodontic appliances. The procedure and equipment movements should be explained, to reassure patients. A protective lead apron should not be used as it can interfere with the final image. Patients should be placed accurately within the machines using the various head positioning devices and light beam marker positioning guides. (In some units the patients face away from the equipment and towards the operator and in others the patient faces the other way round.) Patients should be instructed to place their tongue into the roof of the mouth so that it is in contact with the hard palate and not to move during the exposure cycle (approximately from 9 to 18 seconds). Appropriate exposure setting should be selected, typically in the range of 70 100 kv and 4 12 ma. The positioning of the patient s head within this type of equipment is critical it must be positioned accurately so that the teeth lie within the focal trough. The effects of placing the head too far forward, too far back or asymmetrically on relation to the focal trough are quite easy to take place. The parts of the jaws outside the focal trough will be out of focus. The fan shaped x ray beam causes patient malposition to be represented mainly as distortion in the horizontal plane, i.e. teeth appear too wide or too narrow rather than foreshortened or elongated. These and other positioning errors are shown later. However accurately the patient s head is positioned, the inclination of the incisor teeth, or the underlying skeletal base pattern, may make it impossible to position both the mandibular and maxillary teeth ideally within the focal corridor. 61

Field Limitation Techniques In panoramic radiography, there is the ability to program the equipment to only radiate certain parts of the jaws when specific information is required, rather than the entire dentition. This results in a significant radiation dose reduction. A variety of these socalled field limitation techniques are possible. Figure 22: Diagrams showing the position of the mandible in relation to the focal trough when the patient is not positioned correctly. A) The patient is too close to the film and in front of the focal trough. B) The patient is too far away from the film and behind the focal trough. C) and D) The patient is placed asymmetrically within the machine. 62

Figure 23: Diagrams showing the vertical walls of the focal trough in the incisor region and the relative positions of the teeth with different underlying dental or skeletal abnormalities. A) Class I, B) Gross class II division 1 malocclusion with large overjet. C) Angle s class II skeletal base. D) Angle s class III skeletal base. The shaded areas outside the focal trough will be blurred and out of focus. The two figures below represent the relative movements of the x ray tubehead, cassette carrier and film during an exposure cycle of a continuous mode panoramic unit. Figure 24: Initially the left side of the jaw is imaged (position 1). As the x ray tubehead moves behind the patient s head to image the anterior teeth, the cassette carrier moves in front of the patient s face and the centre of rotation moves forward along the dark arc (arrowed) towards the midline. 63

Figure 25: The x ray tubehead and cassette carrier continue to move around the patient s head to image the opposite side and the centre of rotation moves backwards along the dark arc (arrowed) away from the midline. Throughout the cycle, the film is also continuously moving as illustrated, so that a different part of the film is being exposed at any one moment. 64

CHAPTER 4 RADIATION EFFECTS, DOSES AND PROTECTION CONCERNING QUALITY CONTROLLING ON PANORAMIC RADIOGRAPHY INTRODUCTION X rays have taken their name due to their unknown nature, at the time of their discovery (1895) by Roentgen. Being one of the different types of ionizing radiation and being able to penetrate the human tissues, they are a form of high energy electromagnetic radiation and part of the electromagnetic spectrum, which also includes lower energy radio waves, as well as television and visible light. Radiation Wavelength Photon Energy Radio, television and radar waves 3x104 m to 100 m 4.1x10 11 to 1.2x10 2 ev Infra red 100 m to 700 m 1.2x10 2 ev to 1.8 ev Visible light 700 nm to 400 nm 1.8 ev to 3.1 ev Ultra violet 400 nm to 10 nm 3.1 ev to 124 ev X and gamma rays 10 nm to 0.01 pm 124 ev to 124 MeV Table 1: The electromagnetic spectrum ranging from the low energy (long wavelength) radiowaves to the high energy (short wavelength) x and gamma rays (White, 2000). X rays consist of wave packets of energy, called photons. A photon is equivalent to one quantum of energy. The x ray beam, as used in diagnostic radiology, is made up of millions of individual photons. Although the production and interactions of x rays with matter is an essential knowledge, in this project such a presentation is out of its interest as such background information is presupposed. 65

SOURCES OF RADIATION Everyone is exposed to some form of ionizing radiation from the environment. The sources of radiation may be either natural or artificial. These include: Natural Radiation Artificial Radiation Cosmic radiation from the earth s Fallout from nuclear explosions atmosphere Gamma radiation from the rocks and soil Radioactive waste discharged from nuclear in the earth s crust establishments Radiation from ingested radioisotopes, Medical and dental diagnostic radiation e.g. 40 K, in certain foods Radon and its decay products, 222 Rn is a Radiation from occupational exposure gaseous decay product of uranium that is present naturally in granite. As a gas, radon diffuses readily from rocks through soil and can be trapped in poorly ventilated houses and then breathed into the lungs. Table 2: Natural and artificial sources of radiation (Whaites, 1996). Figure 1: The distribution of natural and artificial radiation. Natural radiation contributes more exposure than artificial radiation. Note that medical x ray diagnosis is the largest component of artificial radiation. 66

In the UK, an individual s average dose from background radiation is approximately 2 msv per year, while in the US it is estimated approximately at 3.6 msv. Having these numbers in mind, one can refer more safely to the numbers of doses of the medical diagnostic radiographies. As indicated above, medical x ray diagnosis produces the largest exposure of all artificial sources of radiation (11%). This should not surprise anyone, as the millions of x ray examinations that are made every year worldwide contribute to the total radiation doses which add up through arithmetic progression. As such techniques are widespread all over the world, it is crucial to find out whether some proportion of this exposure could and should be avoided. The relatively very low doses that are produced from a single x ray examination of any kind should not loosen neither the criteria by which an x ray examination is decided nor the standards of the proper functioning of the x ray units and their proper usage. Classification of the biological effects Three main categories classify the biological damaging effects of ionizing radiation: Somatic Deterministic Effects Somatic Stochastic Effects Genetic Stochastic Effects Somatic Deterministic Effects These are the damaging effects to the body of the person that will definitely result from a specific high dose of radiation. Examples include skin reddening and cataract formation. The severity of the effect is proportional to the dose received and in most cases a threshold dose exists below which there will be no effect. Somatic Stochastic Effects Stochastic effects are those that may develop. Their development is random and depends on the laws of chance or probability. Examples of somatic stochastic effects include leukemia and certain tumors. These damaging effects may be induced when the body is exposed to ant dose of radiation. Experimentally it has not been possible to establish a safe dose (a dose below which stochastic effects do not develop). It is therefore assumed that there is no 67

threshold dose and that every exposure to ionizing radiation carries with it the possibility of inducing a stochastic effect. The lower the radiation dose, the lower the probability of cell damage. However, the severity of the damage is not related to the size of the inducing dose. This is the underlying philosophy behind present radiation projection recommendations. Somatic effects are further subdivided into: Acute or immediate effects, appearing shortly after exposure, e.g. as a result of large whole body doses Chronic or long term effects, becoming evident after a long period of time, the so called latent period (20 years or more), e.g. leukemia. DOSE WHOLE BODY EFFECT 0.25 Sv Nill 0.25 1.0 Sv Slight blood changes, e.g. decrease in white blood cell count 1 2 Sv Vomiting in 3 hours, fatigue, loss of appetite, blood changes. Recovery in a few weeks 2 6 Sv Vomiting in 2 hours, severe blood changes, loss of hair within 2 weeks. Recovery in 1 month to year for 70% 6 10 Sv Vomiting in 1 hour, intestinal damage, and severe blood changes. Death in 2 weeks for 80 100% > 10 Sv Brain damage, coma, death. Table 3: Summary of the main acute effects following large whole body doses of radiation Genetic Stochastic Effects Mutations result from any sudden change to a gene or a chromosome. They can be caused by external factors, such as radiation or may occur spontaneously. Radiation to the reproductive organs may damage the DNA of the sperm or egg cells. This may result in a congenital abnormality in the offspring of the person irradiated. However, there is no certainty in this, so all genetic effects are determined as stochastic. A cause and effect relationship is difficult, if not impossible, to prove. Although ionizing radiation has the potential to cause genetic damage, there are no human data that show convincing evidence of a direct link with radiation. Risk estimates have been based mainly on experiments with mice. It is estimates that a dose to the gonads of 0.5 1.0 Sv would double the spontaneous mutation rate. Once again it is assumed that there is no threshold dose. 68

Harmful Effects Important in Dental Radiology In dentistry, the sizes of the doses used routinely are relatively small and well below the threshold doses required to produce the somatic deterministic effects. However, the somatic and genetic deterministic effects can develop with any dose of ionizing radiation. Dental radiology does not usually involve irradiating the reproductive organs, thus in dentistry somatic stochastic effects are the damaging effects of most concern. The precise mechanism of how x rays can cause damaging effects is not yet fully known but two main mechanisms are thought to be responsible: Direct damage to specific targets within the cell Indirect damage to the cell as a result of the ionization of water or other molecules within the cell A photon strikes upon a molecule of water: 1. H 2 O H 2 O+ + e 2. The positive ion immediately breaks up: H 2 O H + + OH 3. The electron (e ) attaches to a neutral water molecule: H 2 O + e H 2 O 4. The resulting negatively charged molecule dissociates: H 2 O H + OH 5. The electrically neutral H and OH are unstable and highly reactive and called free radicals. They can combine with other free radicals, e.g: H + H H 2 (hydrogen gas) OH + OH H 2 O 2 (hydrogen peroxide) The hydrogen peroxide can then damage the cell by breaking down large molecules like proteins or DNA. Table 4: A diagrammatic summary of the sequence of events following ionization of water molecules leading to indirect damage to the cell (Whaites, 1996) Estimating the Dose and Risk of Panoramic Radiography Quantifying the risk of somatic stochastic effects, such as radiation induced cancer, is complex and controversial. Data from groups exposed to high doses of radiation (i.e. radiotherapy, or survivors of nuclear accidents like Hiroshima or Chernobyl) are analyzes and the results are used to provide an estimate of the risk from low doses of radiation encountered in diagnostic radiology. 69

The problem of quantifying the risk is compounded because cancer is a common disease, so in any group of individuals studied there is likely to be some incidence of cancer. From the data collected, it has been possible to construct dose response curves showing the relationship between excess cancers and radiation dose. The graphs can be extrapolated to zero (the controversy on risk assessment revolves around exactly how this extrapolation should be done), and a risk factor for induction of cancer by low doses of radiation can be calculated. Figure 2: A typical dose response curve, showing excess cancer incidencee plotted against radiation dose and a linear extrapolation of the data to zero. Epidemiological information is being updated continually and recent reports suggest that the risk from low dose radiation may be considerably greater than thought the present figures at least provide an idea of the comparative previously. However, order of magnitude of the risk involved from different investigations. This in turn helps keep the risks associated with dental radiology in perspective. X ray examination Dental intraoral (x2) Dental Panoramic Tomograph Estimated Risk of Fatal Cancer 1 in 2,000,000 1 in 2,000,000 Skull (PA) 1 in 670,000 Skull (Lat) Chest (PA) 1 in 2,000,000 1 in 1,000,000 Lumbar Spine (AP) Barium swallow Barium enema CT chest CT head 1 in 1 in 1 in 1 in 1 in 29,000 13,000 3,000 2,500 10,000 Table 5: Broad estimate of the risk of a standard adult patient developing a fatal radiation induced malignancy from various x ray examinations (NRPB 1999). 70

In the case of Wall BF and Kendall GM (1983) the weig hted equivalent dose from a panoramic examination was calculated to be 80μSv, corresponding to a lifetime risk of 6 fatal cancer of 1.3x10. Other risk estimates have arrived at varying figures: Danford and Gibbs (1980) estimated the risk to be between 2 and 7x10 6 and Bengtsson (1978) 4.2x10 6. Since much of this work was undertaken, several international organizations have suggested that the risk may be greater than previously estimated. Over the same period of time the design of panoramic machines has changed and rare earth screen/film combinations have become more widely used, resulting in a reduction in radiation doses. Using the most recent tissue weighting factors and risk probability coefficients and assuming the use of rare earth screen/film combinations, White (1992) computed the average effective dose for a panoramic examination to be 6.7μSv; this figure is associated with an estimated risk of fatal malignancy of 0.21x10 6. Despite these encouraging findings, it should be emphasized that the lower levels of risk are associated with new equipment. Horner and Hirschmann (1990) described the various methods of limiting patient dose in panoramic radiography. The facility for field size reduction is associated with a reduction in absorbed dose of 85% and effective dose of 50%, when the TMJs are excluded from the field. However, it is likely that the higher levels of dose and risk reported by previous researchers will remain valid as long as older equipment remains in clinical use. For example, certain types of equipment using a circular scanning motion incorporating three centers of rotation produce doses between 3 and 16 times higher than those with an elliptical system, due to the proximity of rotational centers to the mandible and parotid glands. A study carried out in France showed this type of equipment to be the most widely used. Furthermore, a survey of panoramic equipment in the UK found that a higher dose than appropriate was being delivered during use of 70% of this equipment. Although abdominal lead protection is clearly inappropriate in panoramic radiography, some researchers have recommended the use of a lead thyroid collar in younger patients because of the relatively high anatomical position of the gland. However, because the primary beam does not strike the patient from the front during a panoramic examination, a thyroid shield must logically be placed on the back of the neck. This runs the risk of attenuating useful parts of the primary beam and obscuring areas of the mandible on the radiograph. 71

Therefore, it would seem reasonable to suggest that no lead protection should be used during panoramic radiography. Figure 3: Examples of thyroid lead protection. In the first picture, a lead collar (0.5 mm Pb equivalent). In the second picture, a hand held neck shield (0.5 mm Pb equivalent). 72

Main Methods of Monitoring and Measuring Radiation Dose There are 3 main devices for monitoring and measuring radiation dose: Film Badges Thermoluminescent Dosimeters (TLDs) Badge Extremity Monitor Ionization Chambers Figure 4: Monitor Devices: A. Personal monitoring film badge. B. Personal monitoring TLD badge. C. Ionization bleeper. D. TLD extremity monitor. Film Badges They consist of a blue plastic frame containing a variety of different metal filters and a small radiographic film which reacts to radiation They are worn on the outside of the clothes, usually at the level of the reproductive organs, for 1 3 months before being processed They are the most common form of personal monitoring device currently in use 73

Advantages Provides a permanent record of dose received May be checked and reassessed at a later date Can measure the type and energy of radiation encountered Simple, robust and relatively inexpensive Disadvantages No immediate indication of exposure all information is retrospective Processing is required which may lead to errors The badges are prone to filter loss TLDs They are used for personal monitoring of the whole body and/or the extremities, as well as measuring the skin dose from particular investigations They contain materials such as lithium fluoride (LiF) which absorb radiation and then release the energy in the form of light when heated The intensity of the emitted light is proportional to the radiation energy absorbed originally Personal monitors consist of a yellow or orange plastic holder, worn like the film badge for 1 3 months Advantages The lithium fluoride is re usable Read out measurements are easily automated and rapidly produced Suitable for a wide variety of dose measurements Disadvantages Read out is destructive, giving no permanent record, results cannot be checked or reassessed. Only limited information provided on the type and energy of the radiation Dose gradients are not detectable Relatively expensive 74

Ionization Chambers They are used for personal monitoring (thimble chamber) and by physicists (free air chamber) to measure radiation exposure Radiation produces ionization of the air molecules inside the closed chamber, which results in a measurable discharge and hence a direct read out They are available in many different sizes and forms Advantages The most accurate method of measuring radiation dose Direct read out gives immediate information Disadvantages They give no permanent record of exposure No indication of the type or energy of the radiation Personal ionization monitors are not very sensitive to low energy radiation They are fragile and easily damaged Measurements using phantoms Phantom measurements in dental radiography are usually performed on anthropomorphic phantoms in order to derive organ doses and thereby determine the effective dose and/or the energy imparted to the patient. Measurements in anthropomorphic phantoms are performed using TLDs positioned in drilled holes in the phantom. General principles for the use of TLDs should be followed. Patient Dosimetry For patient dose measurements in panoramic projections, the measured quantity is the air kerma length product, P KL. The P KL is the integral of the free in air profile of the air kerma across the front side of the slit of the secondary collimator. Methods using a CT chamber or a stack of TLDs for the measurement of P KL will be described. The air kermaarea product, P KA, is obtained as P KA =P KL H, where H is the height of the x ray beam at the secondary collimator. 75

List of Equipment The equipment used for panoramic projection comprises: Calibrated cylindrical ionization chamber and electrometer Chamber support Thermometer and barometer TLDs and a jig for mounting the dosimeters in front of the secondary collimator may be used as an alternative to the pencil ionization chamber. Dosimeter thickness and their diameter should be about 1 mm or less and 3 mm, respectively. Dosimeters should have had their individual sensitivity correction factors established or dosimeters within a selected sensitivity range should be chosen Film and a ruler (for screen film systems) Methods The air kerma length product is measured using either a calibrated cylindrical ionization chamber or an array of TLDs. The air kerma length product is immediately obtained using an ionization chamber whereas use of TLDs requires several procedures before the result is registered. When neither a cylindrical ionization chamber not TLD is available, direct film could be used as described by Napier. The latter method requires calibration of the film in terms of air kerma and careful handling of film development (IAEA, 1996). Measurement of the air kerma length product using a cylindrical ionization chamber and electrometer 1. Position the cylindrical ionization chamber in front of the secondary collimator (slit), at the centre of the slit and perpendicular to its length direction 2. Make sure that the space between the chamber and the headrest is sufficient when the secondary collimator is rotated. 76

3. Expose the chamber three times using standard settings of tube voltage, tube load an d exposure cycle and record the dosimeter readings, M 1, and M. 3 M 2 4. Repeat step 3 for other standard settings used in the clinic. 5. Record the temperature and pressure. Figure 5: Experimental arrangement for measurements of the air kerma area product for a dental panoramic unit using a CT chamber Measurement of the air kerma length product using an array of TLDs 1. Select a set of TLDs with the sum of the thickness sufficient to cover a length of three times the slit width. Keep another three dosimeters for measurement of the background signal obtained without irradiation. Label each individual dosimeter so that its readings can be identified in calibrations and measurements. 2. Pack the chips in a tube of PMMA. 3. Position the tube with the TLDs in front of the secondary collimator with the tube axis perpendicularr to the length of the slit and at its center. A jig may be used to hold the tube, including insertions for intraoral film for measurement of the height, H, of the x ray beam. 77

4. Expose TLDs using standard settings of tube voltage, tube loading and exposure cycle. Record the settings. 5. Arrange for the dosimeters to be read. Record the readings, M 1, M 2,, M n, from the exposed dosimeters and the readings, M 01, M 02 and M 03, for the unexposed dosimeters. 6. Repeat measurements for other standard settings used in the clinic. Figure 6: Schematic of the jig used to measure the dose profile across the receiving slit of a panoramic x ray unit using TLDs. Positioning of the jig in front of the receiving slit is facilitated by the triangular windows in the jig, which are centered over the slit. Beam length is measured from the developed films using the image of the solder markers. The inner markers are separated by a distance of 120 mm and the outer markers by 150 mm. The jig is mounted with the diagonal solder markers pointing upwards, as shown, to indicate the orientation of the developed films (after Williams and Montgomery) Measurement of the height of the x ray beam at the secondary collimator slit 1. Position the film in front of the collimator slit, at about the same location as where the ionization chamber was located and expose the film to an optical density of less than 0. 5. 2. Develop the film and measure the height of the x ray beam on the film using a ruler or scanner. 78

HVL measurement 1. Set up the x ray equipment for the chosen examination. Suppress the tube movement if possible. 2. Select the tube voltage that would be used for a routine clinical examination. 3. Centre the dosimeter in the x ray beam. The detector should be mounted free in air in such a way as to avoid the effect of scattered radiation. 4. Collimate the beam to achieve conditions for narrow beam geometry. The beam should just cover the detector. This may be difficult for the panoramic equipment unless the width of the beam can be increased. In this case, use a detector with a smaller volume and try to cover the maximum area of the detector. 5. Select a tube loading so that the dosimeter readings with and without the attenuator are within the rated range of the instrument. 6. Expose the detector three times and record measured values M 1, M 2 and M 3. 7. Repeat step 6 for a set of three Al attenuators and the same tube loading as that used for measurements without any attenuator. The thickness of attenuators is selected so that their value encloses the expected HVL of the beam. 8. Expose the detector again without any attenuator (steps 5 and 6) and record the measured value. Calculations Measurements of the air kerma length product using a cylindrical ionization chamber and electrometer 1. Calculate the HVL of the beam by interpolating in measured signal for various attenuator thicknesses. The HVL value measured during a quality assurance program can be also used. 2. Calculate the mean value,, of the dosimeter readings. 79

3. Calculate the air kemra length product, P KL, from the mean dosimeter reading,, using the following equation:, k TP is the correction factor for temperature and pressure,, is the calibration coefficient for the radiation quality Q 0 obtained at the temperature T 0 and pressure P 0 and k Q is the correction factor for the radiation quality Q used during measurement. It is assumed that the leakage signal of the dosimeter can be neglected and that no correction has been applied for this effect. The correction factor, k TP, is given by: 273.2 273.2 Measurement of the air kerma length product using an array of TLDs 1. Calculate the mean value of the background reading,, from background dosimeter readings, M 01, M 02 and M 03 ( 3). 2. For i th dosimeter (i = 1,, n), calculate the background corrected dosimeter reading, M i, from the exposed dosimeter,, and the mean background dosimeter reading,, using the following equation:, where factor f s,i is used to correct for the individual sensitivity of the i th dosimeter. This factor is a constant for dosimeters grouped so that the sensitivity of dosimeters in the group lies within a selected range. 3. For i th dosimeter (i = 1,, n), calculate the air kerma, K i, from the background corrected dosimeter reading,, using the TLD calibration coefficient,,, for the reference radiation quality, Q 0, the correction factor, k Q, for the radiation quality used and the correction factor, k f, that corrects for the effect of fading of the thermoluminescence signal between irradiation of the dosimeter and its readout. The correction factor, k Q, depends on HVL of the beam that should be established using the diagnostic dosimeter:, 80

4. Calculate the air kerma length product, P KL, using the following equation, in which Δd is the thickness of a single TLD: Establishment of the air kerma area product Calculate the air kerma area product using through the following equation. In this equation, P K L is the air kerma length product obtained through the above equation and H is the height of the x ray beam at the secondary collimator slit: Estimation of uncertainties The uncertainty in the measurement of the air kerma can be estimated with a cylindrical ionization chamber during panoramic projections. The value of relative expanded uncertainty for measurements of the air kerma length product is again 6 13%, depending on the measurement scenario selected. Assuming that the maximum difference of a measured slit height from the actual height is 2%, the relative standard uncertainty for this effect is 1.2%. The expanded uncertainty for the measurement of the air kerma area product using a cylindrical chamber is 6.4 13.2%. The uncertainty in measurements made with TLDs is discussed above. The relative expanded uncertainty (k=2) of 10% was adopted. Additional contributions from positioning of the dosimeters and measured slit height have to be included in the overall uncertainty. The value of the relative expanded uncertainty (k=2) in measurements of the air kerma area product using TLDs is about 10.5%. The user should establish the actual measurement uncertainty using the principles described in the relative bibliography. 81

Example A panoramic dental unit is typically operated at 70 kv and 15 ma for a period of 15 s. The air kerma length product was measured at a secondary collimator using a calibrated CT chamber. The reading of the dosimeter wa s 0.923. The calibration coefficients,, and kq, for the dosimeter were 10.02 mgy cm/reading and 0.98, respectively. The correction factor, ktp, for temperature and pressure was 1.002. The measured air kerma length product is: 0.923 10.02 0.98 1.002 9.082 The height of the x ray beam at the secondary collimator slit was measured as 12.5 cm. The air kerma area product is thus calculated as: 9.082 12.5 113.53 For scenario 3 (reference type detector and all corrections applied), the relative expanded uncertainty (k=2) in the measurement is 6.4%.The air kerma area product is written as: 114 7 82

Worksheets Determination of the air kerma length product and the air kerma area product for panoramic projection using a cylindrical diagnostic dosimeter User: Date: Hospital or clinic name: 1. X ray equipment X ray unit and model: Room No.: Slit width: mm X ray beam height (H): mm 2. Dosimeter Dosimeter model: S /N: Dat e of calib: Calibration coefficient (, )*: / / Reference conditions: HVL (mm Al): Pressure P 0 (kpa): Field size: Temperature T 0 ( o C): 3. Exposure conditions Starting tube voltage (kv): Tube current: ma Time: s Ambient conditions: Pressure P (kpa): Temperature T ( o C): k TP = 4. Dosimeter reading and calculation of air kerma length product and air kerma area product** Dosimeter reading (M 1, M 2, M 3 ): Mean dosimeter reading : Pressure P (kpa): Temperature T ( C): HVL (from 5 below) = mm Al o.. = *** k Q = Calculated value of air kerma length product, Calculated value of air kerma area product /10 * This is the calibration coefficient for the whole dosimeter, including the detector and the measurement assembly. For systems with separate calibration coefficients for the detector and the measurement assembly, the overall calibration coefficient is calculated as a product of the two separate calibration coefficients. ** This is an example for one setting. The measurements should be repeated for all settings used in clinical practice *** For dosimeters with a semiconductor detector, k TP = 1 83

5. Determination of HVL Dosimeter readings should be obtained for filter thicknesses that bracket the HVL. The first and last readings, M 01 and M 02 are made at zero filter thickness. Filter thickness (mm Al) 0.00 Dosimeter reading (M) (mgy) Average dosimeter reading,, at zero thickness ) 0.00 Interpolated HVL: mm Al 84

Determination of the air kerma area product using an array of TLDs User: Date: Hospital or clinic name: 1. X ray equipment X ray unit and model: Room No.: Slit width: mm Slit height (H): mm 2. TLDs Identification markings on TLD sachet (if any): Calibration coefficient (, ) for TLDs: mgy / reading Reference beam quality (HVL): mm Al k Q for measurement set up: Dosimeter thickness (Δd): cm 3. Exposure conditions Tube voltage (kv): 4. Dosimeter readings and calculation of the air kerma area product Background reading s (M 1, M 2, M 3 ): Mea n background 3 Fading corre ction (k f ) = No. Dosimete r reading Corrected Individual Air No. Dosimete reading sensitivity kerma r reading Corrected Individual Air reading sensitivity kerma, Air Kerma,, Air kerma length product, Air kerma area product, /10 85

Dose Area Product The dose areaa product DAP is a measurement of the amount of radiation that the patient absorbs. It is usually measured behind the multi leaf collimator, that is, on the side of the patient where the radiation enters the body, by attaching a measuring device in from of the x ray tube and passing a beam through it. The dose area product (DAP) is independent of the distance between the x ray tube and the measuring device because the further away from the x ray tube this measurement is taken, the more the size of the device increases, and the dose itself decreases. The dose to the patient can be calculated by DAP, the size of the measuring device and the distance to the x ray tube and the patient. DAP = (Gy* cm 2 ) Figure 7: X ray tube is as great as dose area for 100cm or 200cm, because the size of the measuring device increases with greater distance to the x ray tube. But the dose itself decreasess with greater distance to the tube. Thus, DAP is the same at each position if the size of the measuring device enables it to detect all of the radiation. There are several DAP meters available globally. The use of a DAP meter can eliminate problems arising from this special characteristic of panoramic units, since it be mounted on the x ray tube and detect all radiation incident on the patient s head. This characteristic makes DAP meter very effective in dose measurements in panoramic radiology. Another advantage of this method is that DAP measurements can be performed in real time patient examinations, since the DAP meter is rel atively transparent and therefore does not interfere with the x ray examination. Figure 8: Real time DAP measurement. 86

Diagnostic Reference Levels (DRLs) deriving from DAP measurements DRLs are defined as dose levels in groups of standard sized patients or standard phantoms, for typical examinations and for broadly defined typed of equipment. These levels should not be exceeded for standard procedures when good and normal practice is applied with regard to diagnostic and technical performance. In this way, DRLs can play an important role in clinical practice to guarantee the performance of diagnostic equipment and as a support to improve techniques and procedures. Many countries have reported diagnostic reference levels for panoramic radiography. In Greece, for this technique, the report of such levels has already started to take place by several publications. The European Commission Radiation Protection Report No 109 (2004), along with the majority of the studies published, considers the frequency curve of a number of examinations and their doses, and proposes the adoption of the 75 percentile as an appropriate DRL value. th Below, there are some indicative values from the European Commission. COUNTRY/DATE OF PUBLICATION Spain 2001 F inland 2000 RESULTS OF SURVEY Occipital ESD: Mean 0.53 mgy Range 0.25 0.87 mgy Third Quartile 0.66 mgy DAP: Mean 94 mgy cm 2 Range 34 254 mgy cm 2 PROPOSED/SET DRLs Occipital ESD: 0.7 mgy 87

COUNTRY/DATE OF PUBLICATION UK 1999 UK 2000 RESULTS OF SURVEY Dose Width Product: Mean 57.4 mgy mm Range 1.7 328 mgy mm Third Quartile 66.7 MGy mm DAP: Mean 11.3 cgy cm 2 Dose Width Product: Mean 65.2 mgy mm Third Quartile 75.8 mgy mm PROPOSED/SET DRLs Dose Width Product: 65mGy mm Table 6: European Commission s Radiation Protection Guidelines in Dental Radiology No 136 (2004), indicative dose values and DRLs, for four European countries. Dose Width Product The Dose Area Product (DAP) is directly correlated to the Dose Width Product (DWP), since DAP is the product of DWP and slit length. As it has been already mentioned, the assessment of patient dose in panoramic radiography is difficult because of the dynamic nature of the imaging process and the narrow width of the x ray beam. The dose quantity used is the product of the absorbed dose in air and the horizontal width of the beam, both measured at the front side of the secondary collimator slit and integrated over a standard exposure cycle. This is referred to as Dose Width Product (DWP) with units of mgy mm. The DWP provides a measurement related to the total amount of radiation to which the patient is exposed. It can be derived either from the product of the peak dose at the centre of the x ray beam and the width of the beam, or from an incremental summation of the dose across the beam. According to P. Doyle s et al report in 2006, the main dos measurement techniques, in terms of DWP, are: 88

1) In Beam Detector and Film 2) Partial Volume Detector (proposed by the authors) 3) TLD Array When a panoramic x ray unit is installed, radiological parameters (tube potential and tube current) are adjusted so that the density on the resultant film is optimized. This adjustment is dependent on the sensitivity or speed of the film screen combination and the effect of such an adjustment will be reflected in changes in DWP. This parameter is therefore a useful quality control tool but is not directly related to patient risk. A more useful parameter for this is the dose area product. DAP and Effective Dose In medical radiology, including panoramic x ray imaging, DAP can be converted to effective dose E using Monte Carlo generated conversion factors. DAP can also be used in dental radiology for the assessment of E without the need for extensive phantom studies. The calculation of the effective dose can be carried out by multiplying the DAP with a pre determined conversion factor. Poppe et al. (2006) calculated the effective doses E from the DAP and consequent DRLs (by the 75 th percentile) results of their survey, using three conversion factors, found in literature. The same calculations of effective doses E will take place in the 2 nd part of this thesis, according to the results of Tierris et al (2004) from a research based in Athens, Greece in order to create a background of DAP reference levels for future studies, in Greece. 89

J. S. Lee et al (2010) have summarized the DAP and DWP reference levels from a number of well established surveys globally, including their own results. A very indicative table is demonstrated below, taken from this survey. Survey Number of x ray units Gender/ Age/ Size DWP (mgy mm) DAP (mgy cm 2 ) Mean 3 rd Quartile Mean 3 rd Quartile Napier (UK) 387 57 67 Williams an d Montgomery (UK) 16 65 76 113 139 Isoardi and Ropolo (Italy) 5 Adult 74 84 Male 101 117 Tierris et al. 62 Female 85 97 (Greece) Child 68 77 2175 Adult and Hart and Wall (DWP) Child 60 82 (UK) 1910 (DAP) Doyle et a l. (UK) Poppe et al. (Germany) Kim et a l. (Korea) Lee et al. (Korea) 20 65 67 89 90 Large Adu lt 85.7 101.4 Male 76.4 87 50 Female 71.6 84.4 Child 59.3 75.4 36 Adult 72.1 106.7 44 Adult 47.7 60.1 Table 6: DR Ls (from DAP and DWP) in pan oramic radiol ogy (Lee et al 2010). 90

Although the differences between the values of the same parameters may seem quite large, it should be mentioned that despite the fact that the parameters are the same (DAP, DWP), the methods used for their calculation, as well as the number of panoramic x ray units used in each survey, differ from one report to another. This table is indicative of the range of these values, but if a long term study from different authors is to be made in order to achieve some useful DRLs, this should be executed in one country (or more but without significant or determining variations in populations characteristics) with some standard and commonly recognized methods and a minimum number of panoramic x ray units needed for each survey. 91

CHAPTER 5 QUALITY CONTROL PROTOCOLS CODES OF PRACTICE LEGISLATION INTRODUCTION In this chapter a short presentation of Quality Control references will be presented, concerning dental x ray panoramic imaging. 1) Greek Atomic Energy Commission Protocol of Periodical Quality Control Checks on Orthopantograph, October 2006 The protocol of the Greek Atomic Energy Commission focuses on the proper functionality of the panoramic unit exclusively. On these terms it supplies a typical yet substantial quality control upon the modalities. While it is mentioned that all supply, HVL and repeatability checks (including time) are the same as the basic QC checks of conventional radiographic systems, the protocol determines the quality controlling of movement, image quality, field size, radiation dose, FSD and correspondence of radiation field with the alignment slit of the cassette. Taking into consideration the complexity and the difficulty of the technique, the frequency of QC checks could be considered as an expanded one. 92

PROTOCOL OF PERIODICAL QUALITY CONTROL CHECKS FOR ORTHOPANTOMOGRAPHS Parameter Parameter procedure Test Objects Test Voltage Accepte d Limits Frequency of Quality Control Check KVp accuracy and repeatability checks (*), supply, HVL and exposure time repeatability checks are the same with the basic quality control che cks of conventional radiographic systems. Correspondence of radiation field with the alignment slit of the radiographic cassette Check of correspondence of radiation field with the alignment slit of the radiographic cassette Film Annually Movement Rotation of the tube cassette system Check and confirmation of proper movement/rotation of tube cassette system Annually Image Quality Check of the quality of received images Film Annually Field Size Radiation Supply Measurement of the field diameter on the outer extremity of the beam applier Measurement of the radiation supply Film and Led Markers Proper Dosimeter 50 70 KVp with 1 m distance from the radiant 150x10 mm in t he slot 30 80 mgy/ mas Focal to Skin Distance (FSD) Tapeline 30 cm *The supply in panoramic systems is measured with DAP or with pencil beam dosimeters. Annually Annually Requirements for Dental Panoramic and Cephalometric Examinations, by the GAEC. All units must satisfy the requirements of the regulations that are mentioned at the department of medical radio diagnostic machines. Moreover the following must be effective: The units must function with high voltage of at least 60 90 kvp, while the minimum filter that interferes to the used beam must be 2.5mm Al. Specialized systems and equipment of head holding and immobilizing must be present. 93

During panoramic examinations, the dimensions of the radiation field upon the film holder must not exceed the 10 mm x 150 mm. The use of endodentulous tubes for panoramic or simple radiographs is prohibited. 2) Conference of Radiation Control Program Directors, Inc Quality Control Recommendations for Diagnostic Radiography, Volume 1, Dental Facilities, July 2001 This general and integrated Quality Control Program concerns all facilities using dental intraoral, panoramic or cephalometric x ray units. Although it corresponds mainly to the employees of such facilities and secondly to QA experts, this Protocol succeeds in introducing a well based overall quality check of the whole laboratory, including the x ray panoramic unit. The frequency of the tests is well established according to the needs. The following table is indicative of the information contained in this Program. Recommended Quality Control Tests for Dental Facilities TEST FREQUENCY PROCEDURE Dental System Constancy Daily, Prior to Developing Films 1 Test (Intraoral Only) and After Service Processor QC (Extraoral) Daily, Prior to Developing Films 2, Appendix B Darkroom QC Daily and Weekly 3 View boxes Monthly 4 Visual Checklist Quarterly and After Service 5 Repeat Analysis Quarterly 6 Tube Head Boom Quarterly 7 Stability Film and Chemical Storage Quarterly 8 Cassettes and Screens Quarterly or Semiannually(as 9 needed) Darkroom Fog Semiannually* 10A or 10B Lead Apron Check Annually 11 Panoramic Field Alignment Annually 12 Program Review Annually Form 6 Radiation Safety Survey Every Two Year Form 4 * Darkroom fog should beevaluated every time you change the filter, bulb or ilm type and at least every 6 months. 94

3) European Commission Radiation Protection 136 European Guidelines Radiation Protection in Dental Radiology, 2004 The European Commission Guidelines mainly concern the users of the modalities. On these terms the x ray unit Quality Controlling lacks of detailed guideline and information. However, this Quality Assurance Protocol gives a well established general idea concerning the appropriate procedures that all practitioners should consider. The following table demonstrates some quality assurance information considering panoramic radiography. Quality Standards for Panoramic Radiography Patient preparation / instruction adequate Edge to edge incisors No removable metallic foreign bodies (e.g. earrings, spectacles, dentures) No motion artefacts Tongue against roof of mouth Minimisation of spine shadow No patient positioning errors No antero posterior positioning errors (equal vertical and horizontal magnification) No mid sagittal plane positioning errors (symmetrical magnification) No occlusal plane positioning errors Correct positioning of spinal column Correct anatomical coverage Appropriate coverage depending upon the clinical application. Field size limitation should have been used (if available) to exclude structures irrelevant to clinical needs (e.g. limitation of field to teeth and alveolar bone for everyday dental use) Good density and contrast There should be good density and adequate contrast between the enamel and the dentine No cassette / screen problems No lights leaks Good film / screen contact Clean screens Adequate processing and darkroom techniques No pressure marks on film, mo emulsion scratches No roller marks (automatic processing only No evidence of film fog No chemical streaks / splashes / contamination No evidence of inadequate fixation / washing Name / date / left or right marker all legible 95

4) International Atomic Energy Agency Dosimetry in Diagnostic Radiology: An International Code of Practice (Technical Reports Series No. 457), Vienna, 2007 As entitled, this fully detailed protocol concerns dosimetric procedures for all medical x ray techniques. On this basis, the dosimetric one, this scientific code of practice contains integrated and important theoretical and practical information about dosimetric quality control. In the previous chapter the methods and the worksheets of this protocol concerning panoramic imaging were demonstrated. 5) Health Canada Radiation Protection in Dentistry, Recommended Safety Procedures for the Use of Dental x ray Equipment, Safety Code30, 2000 This Code of Practice, published by the Canadian Health Ministry, contains a full guideline of Quality Control Assurance of the whole dental x ray laboratory. Personnel guidelines, facilities requirements, equipment specifications, film and processing handling, Quality Assurance Program, Radiation Reduce and proper shielding are some of the main contents of this well based protocol. The following table demonstrates the Quality Control Program for Dental Radiography, including panoramic imaging. Essential Dental Radiography Quality Control Test Performance Criteria Minimum Frequency Test Film and Film ± 1 step (stepwedge) Daily Processing < ± 0.1 optical density Test Radiogram Visual Daily Retake Record Visual Daily Operation of Darkroom Visual Quarterly Cassettes and Screens Visual Annually Filtration (Reference) Annually and After Service Controlling Timer (Reference) Annually and After Service X ray Tube Shielding (Reference) Annually ad After X ray Tube Housing Service X ray Tube Voltage (Reference) Annually and After Service Irradiation Switch (Reference) Annually and After Service Focal Spot to Skin Distance (Reference) Annually and After Service Beam Alignment and (Reference) Annually and After Service Collimation Patient Radiation Dose (Reference) Annually and After Service 96

6) Care Quality Commission The Ionizing Radiation (Medical Exposure) Regulations, Great Britain, 2006 This Protocol stands mostly for the determination of the legitimacy code concerning medial exposures. On this basis, it information is general and representative rather than mainly scientific and technically. However, this Protocol clarifies a clear spectrum of the medical x ray techniques, so that a responsible application can be made. This protocol creates an appropriate basis for well established Quality Control Programs. 7) Greek Ministry of Health, Greek Regulations for Radiation Protection, 2001. The Greek Radiation Protection Regulation clarifies the legitimate basis, on the grounds of which Quality Assurance Programs can be developed. 97

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SECTION II EXPERIMENTAL PART 99

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CHAPTER 6 CALCULATION OF THE EFFECTIVE DOSE (E), USING THE DRLs of TIERRIS ET AL. (2004) INTRODUCTION In this section, a calculation of the effective dose E will be carried out, from the results of the research of Tierris et al (2004). Effective dose E can be calculated by multiplying the DAP with a pre determined conversion factor., METHOD Three conversion factors will be used from literature according to the survey of Poppe et al (2006). This survey used these 3 factors for the determination of effective dose E according to the DAP results of a research on 50 panoramic units of different vendors. The conversion factors are: 1) Williams and Montgomery s (2000) conversion factor of 0.06 msv Gy 1 cm 2 (calculated using the effective dose obtained from literature by White). 2) Helmrot and Alm Carlsson (2002) published a conversion factor including salivary glands of 0.08 msv Gy 1 cm 2, for panoramic examinations, using a multimaterial compound hard tissue phantom. 3) Visser (2000) made an extensive study with a specially designed for x ray energies anthropomorphic tissue equivalent phantom and found a conversion factor of 0.21 msv Gy 1 cm 2. Compared with the E,DAP conversion factors by Williams and Montgomery, and Helmrot and Carlsson, Visser s factor differs by a factor of 3.5 and 2.6 respectively. The discrepancy of the conversion factors may result from the different measuring techniques used and different calculation schemes adopted when calculating effective doses. Lecomber et al (2000) have shown that the salivary gland is exposed to high doses 101

in dental panoramic radiology and its inclusion in the list of remainder organs when calculating effective dose has been questioned. Poppe et al. (2006) mark that the effective dose calculated using the conversion factor from Visser (2000) shows that the risk associated with a panoramic radiography is equivalent to a chest examination. Therefore, the suggestion is to take the values derived by Visser (2000) as an upper dose limit as his study has been carried out under extensive consideration in order to most closely resemble the realistic conditions of application in dental procedures. Tierris et al (2004), measured in real time patient examinations DAP values in 62 panoramic x ray units of the private and public sector in Athens, Greece, with the use of a DAP meter, in order to determine corresponding DRLs. The results are shown on the following table: Exposure Mean (kv) Mean (ma) Mean Exposure Time (s) Mean DAP (mgy cm 2 ) DAP Reference Levels (3 rd Quartile) (mgy cm 2 ) Male 72.4 10.5 15.2 101 117 Female 68.3 10.1 14.9 85 97 Child 64.3 9.7 14.8 68 77 Table 1: Mean exposures parameters for 3 different exposure types, mean measured DAP and DAP reference levels in 62 panoramic x ray units, by Tierris et al (2004). CALCULATION The calculation of the effective dose E now will take place, multiplying the DAP Reference Levels of Tierris et al. (2004) with each conversion factor shown above. The results are demonstrated in the following table. DAP Reference Levels (mgy cm 2 ) Effe ctive Dose E (μsv) Exposure Williams and Helmrot and Visser Montgomery Carlsson Male 117 7.02 9.36 24.57 Female 97 5.82 7.76 20.37 Child 77 4.62 6.16 16.17 Table 2: Calculation of Effective Dose (E), using DRLs from Tierris et al, multiplied by three different conversion factors found in literature (Poppe et al., 2006) 102

CONCLUSION The values in the table above are remarkably higher than the respective values of Poppe et al. (2006). Indeed, their higher than most results on the surveys mentioned on Chapter 4. This may be a result from the fact that 8 x ray units from the survey of Tierris et al. (2004) gave significantly high DAP values compared to the high DAP values of other researchers. Excluding the 8 highest DAP doses from each research, it is noticeable that the values lie in the same range. According to Poppe et al.(2006) it would be appropriate to consider the values derived from Visser (2000) as an upper dose limit. 103

CHAPTER 7 QUALITY CONTROL OF A PANORAMIC CEPHALOMETRIC UNIT (INSTRUMENTARIUM OC/OD-200D) A.1 LABORATORY DESCRIPTION EQUIPMENT RECORD A.1.1 Equipment Description The type of the machine as well as its apparatus along with the specifications of its partial segments, have been recorded. Particularly: Generator The power supply generator of the tube is made by TOSHIBA Company. Generator Voltage : Current : Time : Filter : Focal Spot Size : CE Mark : Digital Detector Type : Sensor Pixel Size : Image Pixel Size : Image Field Height : Resolution : CE Mark : 57 85 kvp 20 16 ma 2.7 17.6 s (panoramic), 5 20 s (cephalometric) 2.5 mm Al 0.5 x 0.5 mm OK Table 1: Generator Specifications. Charged Coupled Device (CCD) 48 x 48 μm 96 x 96 μm 147 mm 221 mm 5.5 lp/mm OK Table 2: Digital Detector Specifications. 104

Both modes of the unit, panoramic and cephalometric, function in fan beam field. As the system uses digital detectors (CCD) it is connected with a workstation computer and the images are printed in films by a digital thermo printer (Dicom Printer, Agfa Drystar 4500M). Picture 1: Panoramic Cephalometric Unit, Instrumentarium OC/OD 200D, Toshiba Picture 2: Workstation and Digital Printer. 105

A.1.2 Ventilation Air Condition Illumination The details concerning illumination, ventilation and air condition of the laboratory were recorded. Their appropriateness was evaluated so that the optimal functional conditions of the units, as well as the personnel working conditions and the comfort of the examinees, are ensured. SYSTEM FUNCTIONALITY Ventilation Excellent Air condition Excellent Illumination Excellent Notes: The illumination and the ventilation of the laboratory are mainly artificial. Table 3: Ventilation, Air condition and Illumination Conditions. 106

A.2. CONTROLS A.2.1. General Apparatus Controls A.2.1.1. Inspectional Control of the Unit Components The task of this control is the contribution to the assurance of the mechanical and electrical function of the x ray system. For this reason the motion of the tube was checked (visually and acoustically). The movements and the mechanical condition of all the partial systems were checked. The function of the illuminating display signs on the console was checked. Finally, the physical condition of the cables was checked. CONTROL RESULT Tube Motion Excellent Movement/Rotation of tube detector system Excellent Physical Condition of cables Excellent Table 4: Inspectional Control of the Unit Components. A.2.1.2. Optical and Acoustic Communication between Examiner Examinee Concerning the confirmation of the optimal inspection upon the examinee, the appropriate acoustic communication, as well as the appropriate optical contact between the examiner and the examinee, were checked. CONTROL RESULT Audio Communication System No intercom present Optical contact examiner examinee Excellent (by lead glass window) Table 5: Audio Optical Control. A.2.1.3. Presence of Technical Manuals and Maintenace/Functioning Log Book CONTROL User Manuals Reference Manuals RESULT Present Present Table 6: Documentation Control. 107

A.2.2. Radioprotection Control A.2.2.1. Spatial Characterization Chamber Signage Control Result Controlled Areas Record System Chamber Superintended Areas Record Controlling (Consoling) Space Non controlled (Public) Areas Record Patient Lounge Chamber, Dentistry Space, Doctor s Office Presence of Warning Signs Yes Presence of Optical and Audio Sign during Yes Exposures Notes: Table 7: Spatial Chamber Control. A.2.2.2. Verification of Radioprotection Report Shield Control For the transaction of the control measurements an x ray dose rate meter was used (Survey Meter, Inovision 451P) with a minimum measurement capability of 0.001 μsv/h. The shielding for the primary as well as the secondary (scattered) radiation was checked. It was also checked whether the shielding is constant, sufficiently covering the door frames and whether a lead incrustation is present in the monitoring (consoling) window. Moreover, a dose rate measurement was taken upon the interfaces of the space. Control Result Presence of Radioprotection Report Yes (by Radiophysicists H. Delis S. Skiadopoulos, April 2007 Workload (mamin/week) According to the Radioprotection Report, 210 mamin/week Control of Shielding Constancy Acceptable (No inconstancies from the measurements on the shielded interfaces) Control of lead glass in the monitoring Physical Condition: Excellent window Table 8: Radioprotection Report Shield Control. 108

Dose Rate Measurements in Neighboring Spaces Measurement Dose Rate (μsv/mah) Spatial Characterization High Voltage (kvp): 57 Tube Current (ma): 2 Time (s): 17.6 Console Chamber Door Mount Superintended Areas (1 Monitoring Window 0.23 μsv/hr for 1mA tube current) Toilet (WC) 0.05 Public Areas (0.1 μsv/hr for Waiting Room Mount 1mA tube current) Table 9: Dose Rate Measurements in Neighboring Spaces. The system functions in fan beam field (both for panoramic and cephalometric modes), approximately 147 x 3 mm for panoramic mode and 221 x 3 mm for cephalometric mode. A.2.2.3. Record and Control of Physical Condition of Radioprotection Apparatus Concerning the protection of the laboratory personnel, the public and the examinees from causeless exposure on x ray, the laboratory has been supplied with the following radioprotection apparatus for the personnel and examinees: One (1) full body protective apron of equivalent thickness 0.30 mm Pb. One (1) full body protective apron with a protective thyroid collar of equivalent thickness 0.30 mm Pb. A.2.2.4. Tube Head Escape Measurements of exposure rate were made, in 1m distance, with an appropriate x ray dosimeter, calibrated at energies 20 150 kev, with minimum measurement capability 0.01 μsv/h. The maximum exposure rate measured was 0.08 msv/h in 1m distance (acceptance limit: 1 msv/h in 1m distance). 109

A.2.3. Beam Geometry Control A.2.3.1. Conjunction of Radiation Field with the Alignment Slit of the Digital Detector For this control, a radiotherapy film was used, positioned in front of alignment slit. Upon the film the position of the slit was marked. The total conjunction between the radiation field and alignment slit of the digital detector was confirmed. The system functions in fan beam field (both for panoramic and cephalometric modes), approximately 147 x 3 mm for panoramic mode and 221 x 3 mm for cephalometric mode. A.2.3.2. Measurement of Minimum Distance Focus Examinee With a direct measurement the distance Focus Examinee was determined (approximately 18cm for panoramic mode and more than 1 m (approximately 1.4 m) for cephalometric mode. A.2.3.3. FFD Control The FFD (Focus to Detector Distance), as mentioned in the technical manuals of the unit, is 487 mm for panoramic mode and 1600 mm for cephalometric. 110

A.2.4. Beam Quality Control A.2.4.1. Accuracy of High Voltage Values For this specific control an electronic meter was used (Victoreen, x ray test device, Model 4000M + SI). Five (5) high voltage values were measured for various options of charge (cephalometric mode). The results are listed in the following table. Nominal High Charge Measured High Deflection (%) Voltage Value (kvp) (mas) Voltage Value (kvp) 60 12.0 58.92 1.80 60 16.0 58.76 2.07 60 20.0 58.12 3.13 66 12.0 65.07 1.41 66 16.0 64.80 1.82 66 20.0 64.64 2.06 70 12.0 70.03 0.04 70 16.0 69.81 0.27 70 20.0 69.84 0.23 76 12.0 76.67 0.88 76 16.0 76.64 0.84 76 20.0 76.21 0.28 80 19.2 80.15 0.19 80 19.2 80.22 0.27 80 19.2 79.91 0.11 80 9.6 80.24 0.30 80 4.8 80.23 0.29 80 36.0 80.45 0.56 80 24.0 80.01 0.01 80 8.0 80.11 0.14 80 12.0 80.63 0.79 80 16.0 80.63 0.79 80 20.0 80.32 0.40 Table 10: Accuracy of High Voltage Values Control. The deflections listed above are acceptable, as the maximum acceptance limit ± 10% of the nominal high voltage values. The maximum deflection of the measured high voltage values is 3.13%. 111

A.2.4.2. Repeatability of High Voltage Values For nominal high voltage value 80 kvp and charge 19.2 mas, five (5) high voltage values were measured (cephalometric mode). The results of the measurements are listed in the table below: Nominal High Voltage Value (kvp) Charge (mas) Measured High Voltage Value (kvp) 80.15 80.22 80 19.2 79.91 80.24 80.23 Note: For repeatability the coefficient of variation is used. Table 11: Repeatability of High Voltrage Values Control. Repeatability (%) 0.17 From the analysis, the repeatability of the high voltage values is 0.17%. This value is acceptable, as the maximum limit of acceptance is ± 5%. 112

A.2.4.3. Half Value Layer (HVL) of the beam Tube Total Filtering For this control a pencil shaped dosimeter was used along with a multimeter (Victoreen). The dosimeter was placed in parallel with the slit. For the calculation of the HVL different aluminum pieces of increasing thicknesses were used. The measurements are shown in the table below. Thickness Al (mm) Dose (%) 0.00 0.33 1.00 2.30 3.30 100.0 90.59 77.73 59.47 46.47 Table 12: Half Value Layer Estimation. The HVL that comes from the measurements above is HVL=3.0 mm Al. From the following curve it comes that the total filtering of the tube is approximately 3.1 mm Al, with a minimum acceptance limit of 2.5 mm Al. Thus, the total filtering of the tube is inside the acceptance limits. 113

A.2.5. Beam Quantity Control A.2.5.1. Timer Accuracy For this specific control an electronic meter was used (Victoreen, x ray device, Model 4000M+ SI). Six (6) time values were measured (in cephalometric mode) for various current options. The results of the measurements are listed in the following table. Nominal Time Value (s) Measured Time Value (s) Deflection (%) 1.0 0.9967 0.33 1.6 1.5970 0.19 2.0 1.9980 0.10 1.0 0.9991 0.09 1.6 1.6010 0.06 2.0 2.0020 0.10 1.0 0.9998.02 1.6 1.6020 0.13 2.0 2.0030 0.15 1.0 0.9995 0.05 1.0 1.6030 0.19 2.0 2.0020 0.10 1.6 1.6030 0.19 1.6 1.6030 0.19 1.6 1.6030 0.19 0.8 0.7999 0.01 0.4 0.3987 0.33 3.0 3.0070 0.23 2.0 2.0040 0.20 2.0 2.0030 0.15 2.0 2.0030 0.15 2.0 2.0030 0.15 2.0 2.0030 0.15 Table 13: Timer Accuracy Control. From the analysis it comes that for all the used current options the highest deflection of the measured value of the exposure time compared to its nominal value is ± 0.33%. The above deflections are acceptable as the maximum limit of acceptance is nominal value of the exposure time for time values >0.1s. ± 10% of the 114

A.2.5.2. Timer Repeatability The table below includes the results of the timer repeatability control for two (2) values of the exposure time (cephalometric mode). Nominal Time Value (s) Measured Time Value (s) Repeatability (%) 1.5970 1.6 1.6010 1.6020 1.6030 1.6030 1.6030 0.146 1.9980 2.0 2.0020 2.0030 2.0040 2.0030 2.0030 2.0030 2.0030 0.092 Table 14: Timer Repeatability Control. The minimum repeatability is 0.146%, which is included in the acceptance limits (acceptance limit: ± 5%). A.2.5.3. Tube Supply Linearity and Repeatability For this control a pencil shaped dosimeter was used along with a multimeter (Victoreen). The dosimeter was placed in parallel with the slit and the necessary corrections were made concerning the calibration coefficient of the dosimeter and the active magnitude of the dosimeter in relation to the radiation field, while a reduction of the values was made for 100 cm distance. For most used current values (ma) and for high voltage 80 kvp, the absorbed dose (mgy) in the panoramic system was measured for functional time 17.6 s. For the nominal values of kvp and mas the supply was calculated in μgy/mas in 1 m distance. The linearity was calculated according to the equation: 115

The measurements are listed in the following table: High Voltage (kvp) Current (ma) Supply (μgy/mas) Linearity (%) 4 42.33 5 41.95 80 8 40.03 8.6 10 44.02 13 47.61 Table 15: Tube Supply Linearity Control. High Voltage (kvp) Current (ma) Supply (μgy/mas) Repeatability (%) 40.03 80 8 40.06 39.42 39.98 1.2 Table 16: Tube Supply Repeatability Control. The minimum radiation supply is 39.42 μgy/mas in 100 cm distance and 80 kvp voltage. The current linearity of the tube is 8.6% (for various ma, 15% limit) while the repeatability of the supply is 1.2% (with constant ma) with 5% limit. Thus, the current linearity and the repeatability of the supply are within the acceptance limits. A.2.6. Automatic Exposure Selection System The density scale calibration was checked depending on the tube current and the Dose Area Product (DAP). The results are shown in the following curves for high voltage 66 kvp. The relation between the density scale and the charge, as well as the equivalent between the density scale and the DAP are depicted in the following graphs. 116

Graph 1: Density scale and charge relation. Graph 2: Density Scale and DAP relation. 117

A.2.7. Typical Patient Doses According to the manufacturing company and as the unit functions in fan beam field, the doses of the patients during the exposures of the examinations are relatively low. Indicative dose values for panoramic examinations are demonstrated in the following table. High Voltage (kv) Current (ma) Dose (μsv) 57 2 1.90 63 10 10.9 66 13 16.8 70 13 20.4 73 8 15.5 77 8 18.7 81 13 31.3 85 13 36.0 Table 17: Typical Patient Doses Estimation. For cephalometric examinations with high voltage 85 kv and current 13 ma, as the manufacturing company proposes, the dose values in relation to the time for profile and anteroposterior projection are demonstrated in the following table. Time (s) 8 10 16 20 Dose (μsv) 2.7 3.4 5.4 6.8 Table 18: Dose Values in relation to Exposure Time. A.2.8. Image Quality Control Using the auto control quality system of the unit, which radiates a field with 15 levels of increasing density, with voltage 57 kv and current 2 ma, the attached film was produced and the presence of 15 density levels was confirmed, as indicated by the system manuals. 118

119

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APPENDIX I THE DIAGNOSTIC VALUE OF THE PANORAMIC RADIOGRAPH A CRITICAL VIEW THE POPULARITY OF PANORAMIC IMAGING The use of radiographic systems has become an inextricable procedure of the western medicine. If in the second half of the 20 th century the radiographic units were rising their popularity, becoming a must tool on the various diagnostic fields of the modern medical practice, in the beginning of the 21 st century their presence has been coincided with the term of diagnosis itself. Yet, the question of whether this use is following an appropriate handling is a fundamental issue of the scientific debate. Along with the uprising of the use of x ray imaging comes the increase of the radiation doses, concerning not only the patient, but also the professionals involved in these procedures, as well as the general public. More than ever, the issue of using x ray techniques as an inevitable and necessary diagnostic tool among the others is becoming of the highest importance. While the dental radiographic techniques import relative doses of the lowest levels, their extensive use is positioning them very high in hierarchy of the most frequent radiographic techniques in general. Thus, the high radiation doses which are induced by dental radiographic systems are indicating that a potential misuse is a fact. A misuse that concerns the correct use of such units by the professional stuff, a proper form of selection criteria and the quality of the existing and functioning dental radiographic units. Panoramic radiographic units meet a high popularity in dental medicine. Various scientific estimations report that every year, in the developed countries, millions of panoramic radiographies are taking place and thousands of panoramic units are installed and functioning. (see rushton biblio)older researches have estimated the numbers of panoramic units and radiographies in western countries. In the middle of the 90 s, in the UK, there were 128

approximately 3250 panoramic x ray units and around 1.5 million of panoramic radiographies were taken annually. In France, while the proportion of dental radiography made up by panoramic examinations is less than in the UK, the number of panoramic films taken exceeds that of the UK, reaching a number of 1.7 million. This means a greater use of radiography of all kinds, as in France most of the panoramic exposures are carries out by radiologists. In the US, during the 80 s more than 25000 panoramic x ray units were used with an extensive rate of usage. In Australia, in 1988, 6% of practitioners used panoramic radiography. In Greece, the Greek Atomic Energy Commission (GAEC) instituted as the national competent authority responsible for nuclear safety and radiation protection issuesreports that there are 10.000 dental radiographic units installed, authorized and licensed for use. Among them the number of panoramic units is respectfully high. These figures obviously underestimate the true scale of use of panoramic radiology, as films produced in private practice, hospitals and within the community dental services are not included. THE QUALITY OF THE PANORAMIC IMAGES Consequently, the crucial question of whether this extended use of panoramic imaging is properly justified remains. Although a panoramic radiograph includes an important substance of information, it is important to realize whether this information itself is essential for oral diagnose and treatment. A combination of factors in panoramic radiology which reduce its diagnostic quality should be taken into consideration by all physicians. These factors are: The limitations imposed by the film/screen/cassette combination Tomographic blur Super imposed tissue and ghost shadows The overlap of adjacent teeth Variations in magnification Panoramic radiology, being a modified form of tomography, blurs the images of anatomical structures above and below the in focus layer, which ranges from 4.5 to 12 mm in the anterior regions and is two or three times greater in the molar regions. In this 129

way, the transfer of information from the attenuated x ray beam to intensifying screens and then to the film, inevitably degrades this information. This degradation is increasing to a variable degree by shadows of soft tissues and surrounding air. Ghost images of the spine and the mandible reduce the diagnostic quality and the presence of air between the dorsum and the hard palate leads to a band of relative overexposure of the roots of the maxillary teeth and alveolar bone. Variations in the horizontal angle of the slit x ray beam and the line of the dental arches result in some amount of overlap of contact points of teeth, particularly in the premolar regions. To continue with, in panoramic radiography there is a magnification factor from 10% to 30%. However, the degree of horizontal magnification varies considerably, depending upon the relationship of the structure to the image layer. Thus, inaccuracies in patient positioning lead to discrepancies between vertical and horizontal magnification of teeth, with consequent distortion of shape. As a conclusion, the quality of any radiograph is dependent upon accurate technique (including the quality of the x ray unit) and careful processing. Panoramic radiography poses particular challenges in both of these aspects of image production. Accurate positioning and preparation of patients is needed to ensure the image is not distorted or affected by ghost images, while quality control is critical when screen film is processed. TECHNICAL AND PROCESSING FAULTS AFFECTING THE IMAGE QUALITY In 1999, Rushton, Horner and Worthington concluded that the quality of panoramic radiographs was considerably lower than standards recently set back then for primary dental care. However, the quality of panoramic radiography could be improved by careful attention to radiographic technique and processing. In a study, including 41 dentists and a total of 1,813 panoramic radiographs, only 0.8% o radiographs were free of faults, while 66.2% were diagnostically acceptable (containing errors which did not detract from the diagnostic utility and 33.0% were unacceptable. When all 1,813 radiographs were considered, the mean number of technical faults per radiograph was 2.75 (SD=1.48). The mean number of processing faults per radiograph was 2.96 (SD=1.55). 130

Faults n % Tongue not in contact with palate 1,298 71.6 Antero posterior positioning errors 1,066 58.8 Absence of orientation (left/right) markers 642 35.4 Occlusal plane errors 568 3.3 Incorrect sagittal plane 508 28.0 Slumped position 267 14.7 Foreign objects/ghost shadows 164 9.0 Lower border of mandible off film 164 9.0 Poor film/screen contact 60 3.3 Overlap of upper and lower teeth 56 3.1 Movement artifact 35 2.0 Table 1: Ranking of technical faults observed on the 1,813 radiographs examined in the study. n=number of radiographs; %=percentage of radiographs showing the fault. The percentages add up more than 100% because most radiographs exhibited more than one technical fault (Rushton et al, 1999). Faults n % Screen artifacts 1,284 70.8 Automatic processors roller marks 752 41.5 Localized film fog 719 39.7 Faults in contrast (too low) 715 39.4 Faults in density (too pale) 659 36.3 Pressure artifacts 377 20.8 Chemical streaks/contamination 271 14.9 Emulsion scratches 248 13.7 Faults in density (too dark) 101 5.6 Generalized film fog 47 2.6 Inadequate fixation/washing 46 2.6 Developer/fixer splashes 16 0.9 Faults in contrast (too high) 6 0.3 Table 2: Ranking of processing faults observed on 1,813 radiographs examined in the study. n = number of radiographs; %=percentage of radiographs showing the fault. The percentages add up to more than 100% as most radiographs exhibit more than one processing fault (Rushton et al, 1999) 131

When the 599 (33.0%) unacceptable radiographs were considered in isolation, the mean number of technical faults per radiograph was 3.54 (SD=147), while the mean number of processing faults was 3.63 (SD=1.48). The most frequent faults were antero posterior positioning errors and faults in film density and contrast. Faults n % Antero posterior positioning T 324 54.1 errors Faults in density (too pale) P 241 40.2 Faults in contrast (too low) P 227 37.9 Incorrect sagittal plane T 144 24.0 Occlusal plane errors T 131 21.9 Slumped position T 54 9.0 Screen artifacts P 31 5.2 Generalized film fog P 23 3.8 Foreign objects/ghost shadows T 21 3.5 Localized film fog P 20 3.3 Automatic processor roller marks P 12 2.0 Poor film/screen contact T 9 1.5 Tongue not in contact with palate T 5 0.8 Patient movement T 5 0.8 Chemical streaks/contamination P 4 0.7 Developer/fixer splashes P 3 0.5 Inadequate fixation/washing P 3 0.5 Table 3: Ranking of technical (T) and processing (P) faults observed on the 599 inadequate radiographs and which directly contributed to their inadequacy. n=number of radiographs; %=percentage of radiographs showing the fault. The percentage adds up to more than 100% because inadequacy was frequently due to more than one fault (Rushton et al, 1999) Analysis of variance identified highly significant differences in the numbers of technical (F=13.72, two degrees of freedom); P < 0.001) and processing (F=12.40, two degrees of freedom; P < 0.001) faults between the dentists. Similarly, the proportion of unacceptable radiographs varied markedly from dentist to dentist, from 10% to 72%. The highest proportion of excellent radiographs recorded was 11.1% with no other dentist achieving a figure exceeding 4%. 132

On the following table, there are the results of the repeatability assessment by the observers. Values of k exceeding 0.75 indicate excellent agreement beyond chance, values between 0.4 and 0.75 indicate fair to good agreement beyond chance, while values below 0.4 indicate poor agreement. Fault assessment % k Agreement Overall acceptability 91.2 0.79 (0.67, 0.91) Tongue not in contact with palate 88.0 0.69 (0.55, 0.84) Antero posterior positioning errors 83.2 0.73 (0.63, 0.83) Absence of orientation (left/right) markers 95.2 0.87 (0.77, 0.97) Occlusal plane errors 80.8 0.60 (0.46, 0.74) Incorrect sagittal plane 76.8 0.50 (0.35, 0.66) Slumped position 87.2 0.57 (0.39, 0.76) Foreign objects/ghost shadows 99.2 0.93 (0.79, 1.00) Lower border of mandible off film 100.0 1.00 (1.00, 1.00) Screen artifacts 79.2 0.56 (0.42, 0.71) Automatic processor roller marks 70.4 0.41 (0.26, 0.57) Localized film fog 94.4 0.88 (0.80, 0.97) Faults in contrast 84.4 0.66 (0.52, 0.80) Faults in density 81.6 0.64 (0.51, 0.77) Pressure artifacts 90.4 0.72 (0.58, 0.87) Chemical streaks/contamination 88.8 0.40 (0.14, 0.66) Emulsion scratches 99.2 0.97 (0.91, 1.00) Table 4: Repeatability of assessments. Percentage agreement between first and second assessments and the kappa statistic (k) are shown. For k values, 95% confidence intervals are shown in brackets (Rushton et al, 1999). 133

FILM FAULT FREQUENCY WITHIN PANORAMIC RADIOGRAPHS TAKEN IN GENERAL DENTAL PRACTICE. In a sample of 2,641 panoramic films that were assessed within published studies, the proportion of unacceptable films and range of faults was 18.2% and 33.0%. Anterior/Posterior positioning errors 54.1% Faults in density and contrast 13.0% and 40.2% Incorrect sagittal plane 24.0% Occlusal plan errors 21.9% Slumped patient position 9.0% Screen artifacts 5.2% Film fog 3.8% and 7.0% Foreign objects/ghost Shadows 3.5% Poor film/screen contact 1.5% TABLE 5: Film fault frequency within panoramic radiographs taken in general practice (European Commission, 2004) In another study, performed in Turkey (Akarslan et al, 2003), through a sample of 460 panoramic radiographs, 173 of them, composing a percentage of 37.61%, were found to have no errors according to the criteria set. The most common positioning error was found to be a radiolucent area palatoglossal airspace over the roots of the maxillary teeth (46.3%) due to the patient s tongue not being raised against the palate during exposure time (see fig. 1). Superimposition of the hyoid bone with the body of the mandible (23.6%) and superimposition of the vertebral column on to the anterior teeth (22.17%) were the next most common errors (see fig. 2 and 3). The least seen positioning errors were the widening of the anterior teeth due to the patient biting the bite block too far back (1.3%) and the vertical overlap of the anterior teeth due to the patient s not biting the bite block, or not using a bite block during the exposure (2.39%). 134

On the evaluated radiographs, the most frequent technical errors were too high density (16.52%) and too low density (15.65%), respectively (see fig. 4 and 5). The least common error was found to be the presence of dirty or bent films (0.21%) (see fig. 6) Error n (number) % Shadow of airway above tongue 213 46.30 Superimposition of hyoid bone 121 26.30 Vertebral column superimposed on anterior teeth 102 22.17 Density too high 76 16.52 Density too low 72 15.65 Occlusal plane tipped down 62 13.47 Asymmetrical placement of teeth 53 11.52 Other miscellaneous errors 45 9.78 Occlusal plane tipped up 43 9.34 Film fogged 34 7.39 Blurring of anterior teeth 32 6.95 Stains on film 29 6.30 Superimposition of spine on other structures 26 5.65 Narrowed anterior teeth 26 5.65 Radiopaque artifact 24 5.21 Patient movement 15 3.26 Vertical overlap of anterior teeth 11 2.39 Marks on film 7 1.52 Widening of anterior teeth 6 1.30 Films dirty or bent 1 0.21 Table 6: Frequency of errors at the evaluated panoramic radiographs. Some films had more than one error so the percentages add up to more than 100 percent (Akarslan et al, 2003). 135

Figure 1: Palatoglossal airspace over the roots of the maxillary teeth. Figure 2: Superimposition of the hyoid bone on the body of the mandible. Figure 3: Vertebral column superimposed over the anterior teeth. 136

Figure 4: Film density too high. Figure 5: Film density too low. Figure 6: Bent film. 137