Accuracy of linear measurement in Galileos cone beam CT under simulated clinical condition

Transcription

1 University of Iowa Iowa Research Online Theses and Dissertations Spring 2009 Accuracy of linear measurement in Galileos cone beam CT under simulated clinical condition Rumpa Ganguly University of Iowa Copyright 2009 Rumpa Ganguly This thesis is available at Iowa Research Online: Recommended Citation Ganguly, Rumpa. "Accuracy of linear measurement in Galileos cone beam CT under simulated clinical condition." MS (Master of Science) thesis, University of Iowa, Follow this and additional works at: Part of the Other Dentistry Commons

2 ACCURACY OF LINEAR MEASUREMENT IN GALILEOS CONE BEAM CT UNDER SIMULATED CLINICAL CONDITION by Rumpa Ganguly A thesis submitted in partial fulfillment of the requirements for the Master of Science degree in Stomatology in the Graduate College of The University of Iowa May 2009 Thesis Supervisor: Professor Axel Ruprecht

3 Copyright by RUMPA GANGULY 2009 All Rights Reserved

4 Graduate College The University of Iowa Iowa City, Iowa CERTIFICATE OF APPROVAL MASTER'S THESIS This is to certify that the Master's thesis of Rumpa Ganguly has been approved by the Examining Committee for the thesis requirement for the Master of Science degree in Stomatology at the May 2009 graduation Thesis Committee: Axel Ruprecht, Thesis Supervisor Steven Vincent John Hellstein Sherry Timmons Fang Qian

5 To my adorable son, Adi To my dear husband, Dev Shankar for his constant support and encouragement To my loving parents, for their guidance and motivation throughout my life

6 ACKNOWLEDGMENTS My heartfelt gratitude goes to Dr. Axel Ruprecht who not only served as my thesis supervisor but also encouraged and guided me throughout my academic program and residency. He and the other members of my thesis committee, Dr. Steven Vincent, Dr. John Hellstein, Dr. Sherry Timmons and Dr. Fang Qian guided me through the dissertation process through their valuable advice. I thank them all. I extend my appreciation to Patrick Elbert, hospital mortician in the department of Anatomy at The University of Iowa, who arranged for specimens for my research and helped with preparing them for my research; to John Laffoon at Dows-Institute of dental research for helping with the band saw for sectioning my specimens. I would like to thank my colleague Dr. Nidhi Handoo, who assisted me a great deal in setting up and dissection of the specimens for the project. I would also like to thank my other colleagues of past and present, Shawneen, Tunde, Ek, Gayle, Rujuta and Ali for their support. My sincere thanks to my friends in Iowa City, Lopa, Soumya, Soma, Arindam and Kajari, for all the good times we shared together and for being there with me when I needed them. I am deeply indebted to my parents and parents-in-law for their invaluable support throughout my years of academic pursuits. I cannot thank my husband, Dev, enough for his cooperation, understanding, love and support during my years of training. iii

7 TABLE OF CONTENTS LIST OF TABLES...v LIST OF FIGURES... vi CHAPTER I INTRODUCTION...1 Aim...11 Hypotheses...11 CHAPTER II MATERIALS AND METHODS...12 Imaging device...12 Linear distance measurement...12 CHAPTER III RESULTS...37 An overview of statistical methods...37 Data preparation...38 Statistical analysis...38 CHAPTER IV DISCUSSION...46 CHAPTER V CONCLUSION...58 BIBLIOGRAPHY...59 iv

8 LIST OF TABLES Table 1. Measurements obtained from six specimens...34 Table 2. Mean linear measurement values of bone height under each condition...41 Table 3. Mean difference between two measurements made by the same observer...45 v

9 LIST OF FIGURES Figure 1. Imaging device Galileos CBCT unit (Sirona Dental Systems Inc., Bensheim, Germany)...16 Figure 2. Imaging specimen in Galileos CBCT unit...17 Figure 3. CBCT image showing the plane of measurement on the left side of specimen...18 Figure 4. CBCT image showing the plane of measurement on the left side of specimen...19 Figure 5. CBCT image showing the plane of measurement on the right side of specimen...20 Figure 6. CBCT image showing the plane of measurement on the right side of specimen...21 Figure 7. CBCT image showing the plane of measurement on the left side of specimen...22 Figure 8. CBCT image showing the plane of measurement on the right side of specimen...22 Figure 9. CBCT image showing the plane of measurement on the right side of specimen...23 Figure 10. CBCT image showing the plane of measurement on the right side of specimen...24 Figure 11. CBCT image showing the plane of measurement on the right side of specimen...25 Figure 12. CBCT image showing the plane of measurement on the right side of specimen...25 Figure 13. Specimen mounted on a base with the plane of dissection marked consistent with the CBCT image...26 Figure 14. Band saw (Craftsman, Sears Roebuck & co)...27 Figure 15. Tangent locator (Medical Instrument Shop, The University of Iowa)...28 vi

10 Figure 16. Digital vernier calipers (Mitutoyo, Japan)...29 Figure17. Combination square...30 Figure 18. Specimen set up for physical measurement...31 Figure 19. Physical measurement between superior most and inferior most points of the bone with calipers...32 Figure 20. Physical measurement between superior most and inferior most points of the bone with calipers...33 Figure21. Graphical representation of mean values of bone height (CBCT and Physical) 44 vii

11 1 CHAPTER I INTRODUCTION Radiography has been one of the frequently applied aids in human biometric research when using images for measurement. It is essential to check for the accuracy of reproduction with respect to enlargement and projection. Without this accuracy errors can be incorporated into the measurement. Accuracy can be affected by the measurement procedure itself due to errors in marking reference points or lines or whether the measurement was obtained directly from the image or indirectly by addition or subtraction of measurements obtained directly. One example of indirect measurement is measuring the spacing of the teeth by using the difference between the arch perimeter and sum of the tooth widths. 1 Measurement is a vital aspect of interpretation, either of anatomical structures or pathological entities. It plays an important role in orthodontic treatment and maxillofacial surgery especially when closely related to vital structures. More recently, an increasing demand for dental implants for rehabilitation of edentulous jaws has raised an interest in the available imaging techniques to perform an accurate preoperative planning. It is essential to measure accurately the height of bone available for implant placement to avoid compromising vital structures such as the inferior alveolar nerve or maxillary sinus during placement of implants. Acquiring this information requires some form of imaging, either two dimensional such as panoramic radiography or three dimensional such as computed tomography depending on the case and experience of the practitioner. 2 There are different types of imaging used in dentistry that have been used for measurement of distances between anatomic structures, dimensions of anatomic or

12 2 pathologic entities and implant site assessment. These are intraoral radiographs, lateral cephalometric radiographs, pantomographs, cross sectional imaging such as conventional tomography, computed tomography, magnetic resonance imaging and more recently cone beam computed tomography. With intraoral radiographs, the implant site can be assessed in terms of the trabecular bone pattern and relationship to adjoining anatomic structures. The advantage of these radiographs are that they are inexpensive, readily available and well tolerated by patients, and provide high resolution images of the implant site and low radiation dose to the patient. 2 The disadvantages of intraoral radiographs are that they have nonreproducible imaging geometry and produce distortions that are inherent to intraoral radiography. 5 The region visualized on an intraoral image is limited in size, sometimes not extending to the inferior alveolar canal or maxillary sinus. Facial-lingual cross-sectional information that is vital for implant site assessment is missing with intraoral radiographs. 6 Lateral cephalometric radiographs can be useful diagnostic aids for implant site assessment. 7, 8, 9 These radiographs provide information regarding the midline region of both the jaws with respect to the bone height, width and angulation. 2 However, because of superimposition of structures of the right and left sides, measurements of bone height and width in the symphyseal region may mask local bone defects. 6 The advantages are low cost, easy acquisition and availability. In conventional tomography, the x-ray beam and receptor move in opposite directions with respect to each other on opposite sides of the patient. A single plane in the patient is well defined. This is called the focal plane. Structures outside this plane are

13 3 blurred. The use of a cephalostat, lasers or plastic positioning devices in conventional tomography is recommended for data registration. Some means of relating the crosssectional image to the actual implant site in the oral cavity is necessary. 10, 11 This is usually done by use of radiographic stents and radiopaque teeth contours. Broadly, there are two types of tomographic movements, linear and multidirectional. The latter consists of circular, spiral, elliptical and hypocycloidal paths of travel. Images from linear tomography often appear streaked, whereas those from multidirectional movement tomography are free of such streaks called parasite lines. 12 A constant magnification is required to accurately perform any kind of measurements on radiographs. Linear tomography provides non-uniform magnification as opposed to multidirectional tomography which provides a constant magnification factor. 13 Deficient blurring of structures outside the focal plane may lead to a reduction of image resolution making visualization of structures difficult. 14 Such cases may lead to overestimation of distances. The disadvantages also include limited availability of such imaging modalities and increased time to acquire the images. Pantomography is a special tomographic technique that produces a panoramic radiograph of a curved surface. This is a curvilinear complex variant of conventional tomography and is also based on the principle of the reciprocal movement of an x ray source and an image receptor around a central point which moves during the image acquisition; the movement and path of motion varies as specified by the manufacturer. 15 In most modern pantomographic units the center of rotation moves along a path defined by the manufacturer (different for each unit) to try to compensate for the noncircular shape of arch. The receptor moves within its carrier at a different rate corresponding to a

14 4 slice in the dental arch. Objects outside and inside the image layer are blurred. Pantomographs provide a comprehensive view of the maxillofacial region, producing an image of both dental arches in a single radiograph, and significantly reducing radiation exposure to the patient in comparison to intraoral radiography. 2 The effective radiation exposure is 35 µsv for a full mouth series with rectangular collimation and E speed films or photostimulable phosphor plates. The effective radiation exposure with pantomographs is 6-26 µsv. 15 In accordance with the ALARA principle exposure of patients to radiation should be avoided unless the benefit from such exposure outweighs the risk from the procedure. Panoramic radiographs are useful for evaluating skeletal and dental pathosis, making dimensional assessments, and determining relative angulations of teeth with respect to other structures. 2 In addition, pantomographs play an important role in implantology by providing information about the vertical dimension of the bone available for implant placement and the locations of certain anatomic structures in the orofacial region. However, dimensional measurements made on pantomographs can involve considerable methodologic error. One major limiting factor of the pantomograph is its inaccuracy in determining the dimensions of structures resulting in magnification and distortion. A pantomographic image alone provides no information regarding the bone thickness and may lead to errors in determining the bone width. 33 Superimposition of structures can lead to poor image quality. The presence of metallic restorations, bone screws etc. can cause metallic artifacts to appear on the image.

15 5 The inherent problems with 2D imaging led to the need for 3D imaging which can overcome the issues of superimposition, blurring and magnification as these factors compromise measurement accuracy to a large extent. Simultaneously, cross-sectional information which is vital for preoperative planning for implant placement can be available from 3D imaging. Computed tomography which is a 3D imaging modality had long been in use in medical radiology before its use in dental implant imaging. The first modern computed tomography (CT) scanner developed by Godfrey Hounsfield in 1967 was first introduced in clinics in The basic concept of CT includes measurement of attenuation of the x- ray beam through a subject at many positions around the subject and at a sufficient number of angles. It is possible to determine attenuation differences of 0.5% which is sufficient to distinguish between soft tissues. 34 The details in a CT image are a result of computer calculations that give the weighted average of all tissues in a particular voxel (volume elements). 14 Patient positioning and lack of movement are two critical factors necessary to obtain clear CT images. Correct positioning is also important in the performance of applicable linear measurements. 35 CT imaging has undergone technologic improvements over the years by stages called generations. In late 1980s, the acquisition of CT images required 45 to 60 minutes, 36 whereas currently, the time has decreased significantly to approximately 5 seconds. With modern multidetector CT imaging, multiple thin axial slices of data are obtained through the area of interest and added together to form a data volume. Crosssectional and panoramic images are reconstructed from this data through the use of

16 6 software programs. The advantages of CT images are uniform magnification, high contrast images with minimum blurring, simultaneous assessment of multiple implant sites in a single study and multiplanar images. The disadvantages include a high cost, a high radiation dose and metallic streak artifacts if metallic objects such as dental restorations are present. Several studies have been reported on the dimensional accuracy of various systems for mandibular height and width, as well as for size and location of the mandibular canal. The mandibular canal is not always well visualized radiographically, in part because of a lack of the cortical outline in some jaws 10 In some studies, the canal could be easily seen by conventional tomography 37 whereas in others CT gave better results. 13, 37, 38 CT images provide high accuracy of measurement with no significant difference between the measurement of actual landmarks or CT images. 39, 40, 41 However, the high radiation dose, availability and cost limit the use of this modality in the maxillofacial region for preoperative implant planning purposes. Reports of dimensional accuracy also vary, particularly with respect to measurement of the distance from the alveolar crest to the superior border of the inferior alveolar canal. In one report, three forms of tomography (computed and both hypocycloidal and spiral conventional tomography) all underestimated this distance by less than 1 mm compared to a larger inaccuracy with standard panoramic imaging, 37 whereas another study concluded that CT was better than conventional tomography. 42 Linear tomography has been reported to significantly overestimate the distance between the alveolar crest and the top of the canal. 13

17 7 A world-wide survey of CT use for implant imaging reported a tenfold variation between lowest and highest absorbed doses from nine different makes of CT scanners and the protocols being used. 43 Methods of dose reduction for implant imaging include lowering the mas 44, changing the spiral CT pitch from 1:1 to 2:1 45 and reducing the number of slices to the very minimum needed. 46 It is the responsibility of the both the implant surgeon and the radiologist to work together to minimize the CT doses by scanning only the concerned area and by choosing the lowest mas and appropriate pitch that will not significantly degrade the image quality. 2 Magnetic resonance imaging (MRI) is another advanced 3D imaging modality which does not utilize ionizing radiation for the acquiring images. This is an advantage over the other imaging modalities utilized for the purposes of preoperative planning. However, MRI is good for imaging soft tissues but not bone, hence it is not recommended for preoperative planning of implants. Although CT provided cross-sectional information with high accuracy of measurement yet it could not be used routinely for dental implant imaging due to high radiation exposure and high cost leading to limited availability. There was need for a 3D imaging modality which would overcome the issues of medical CT. A dedicated CT for maxillofacial imaging called cone-beam computed tomography (CBCT) was introduced in 1997 by NewTom 9000 in Italy. (Information received by personal communication through with chairman of NewTom, David Vozick on February 19, 2009). It was introduced in literature in , 48 In the past decade the technology of cone beam computed tomography (CBCT) has evolved which allows 3D visualization of the oral and maxillofacial complex at a much smaller radiation dose than that produced by

18 8 conventional CT. 49 CBCT was initially developed for angiography, but more recent medical applications have included radiotherapy guidance and mammography. 50 CBCT allows 3D visualization of the oral and maxillofacial complex. This imaging modality eliminates the shortcomings of 2D imaging, produces a smaller radiation dose than that of conventional CT and enables clinicians to make more accurate treatment planning decisions, which should lead to more successful surgical procedures. 49 The information obtained from CBCT can be used for evaluation of hard tissues for dental implant placement or grafting, the temporomandibular joint complex, pathosis, anatomic variations and trauma as well as for orthodontic treatment planning. 49 CBCT is particularly helpful in presurgical planning for dental implant placement by localizing the anatomy to be avoided during surgery. 49 It helps to measure the quantity and the quality of the bone available for the placement of implants. 49 CBCT provides submillimeter pixel resolution of projection images leading to high spatial resolution of the image. CBCT is primarily used for investigating bone. Although CBCT is able to depict the associated soft tissue in the region imaged, it is not able to distinguish between different types of soft tissues. CBCT was developed as an alternative to conventional CT to shorten the time of image acquisition of the entire FOV (Field of View) with a comparatively less expensive radiation detector. The lack of patient translational movement results in improved sharpness of the image which is reduced in conventional CT imaging. The reduced time of acquisition also reduces image distortion that may be caused by internal organ movement. The main disadvantage, especially with larger FOVs, is a limitation in image

19 9 quality related to noise and contrast resolution because of detection of large amounts of scattered radiation. 50 CBCT imaging produces images with submillimeter isotropic voxel resolution ranging from as high as 0.4 mm to as low as mm. This results in multiplanar reconstructed images (axial, coronal and sagittal) with a level of spatial resolution accurate enough for measurement in maxillofacial applications where precision in all dimensions is important such as implant site assessment. 50 Depending on the type and model of CBCT device and the field of view (FOV) selected, the effective radiation dose varies from 29 µsv (Galileos default) to 477 µsv (CB MercuRay 12-in FOV) according to the published reports. 54, 55, 56 These doses can be compared to 5 times (Galileos) to 74 times (CB MercuRay) the dose of a single film based panoramic radiograph or 3 to 48 days of background radiation. Patient positioning modifications (tilting the chin) and use of additional personal protection (thyroid collar) can cause significant reduction in dose by up to 40%. 55, 56 Maxillofacial imaging with conventional CT exposed the patient to approximately 2000 µsv of radiation. Thus CBCT significantly reduces the dose by a range of 98.5 to 76.2 %. 46, 57, 58 There are several disadvantages of CBCT. The cone beam projection geometry results in irradiation of a large volume of tissue, resulting in large amount of scattered radiation. This scattered radiation, recorded by the detector does not reflect the actual attenuation of an object along the path of an x-ray beam. This is termed noise and it is proportional to the total mass of tissue irradiated by the primary beam. In addition there may be added noise of the detector system and from variations in the homogeneity of the incident beam. The increased divergence of the cone beam results in a pronounced heel

20 10 effect leading to nonuniformity of the beam causing increased noise on images. 50 The portions of the image at the edge of the imaging volume show peripheral noise due to the cone beam effect. The beam, on encountering metal restorations in the mouth is attenuated, producing information voids that result in streak artifacts in the images that can obstruct the surrounding anatomy. Manufacturers attempt to remove noise and streak artifacts during reconstruction of the raw data by using specific algorithms and filters. There may also be patient motion artifact on the images which causes image degradation. CBCT images also have poor soft tissue contrast. In addition to increasing noise in the image, scattered radiation also reduces contrast of the CBCT system by adding background signals that are not representative of the anatomy leading to inferior image quality. 50 As with other imaging modalities, the question of accuracy of measurements arose with CBCT. Accuracy of measurements with respect to distance is vital for procedures such as implant surgery or other surgical procedures in close proximity to vital structures such as the inferior alveolar canal or maxillary sinus as well as for orthodontic treatment. Several studies have been carried out to determine the accuracy of CBCT. However these have been done with dry skulls without the soft tissue component. These studies have shown linear measurement to be accurate on CBCT images. 63, 64, 65 59, 60, 61, 62, Although the previous studies have shown CBCT to be accurate, some of the accuracy may be due to the increase in contrast when soft tissues are replaced by air, and decreased scatter due to absence of soft tissues. It is important to determine if the

21 11 accuracy of measurement is maintained with soft tissues intact as this would simulate a clinical situation more closely. Aim The aim of this study was to determine if the linear measurements made in Galileos CBCT in the presence of soft tissue using cadaver heads are accurate. Hypotheses 1. There is no significant difference in linear distance measurement between the Galileos CBCT and Physical measurements on the right side in the presence of soft tissue. 2. There is no significant difference in linear distance measurement between the Galileos CBCT and Physical measurements on the left side in the presence of soft tissue. 3. There is no significant difference in the overall linear distance measurement between the Galileos CBCT and Physical measurements in the presence of soft tissue.

22 12 CHAPTER II MATERIALS AND METHODS Imaging device Three dimensional imaging data was acquired in a Galileos (Sirona Dental Systems Inc., Bensheim, Germany). (Figure 1) The Galileos consists of an x-ray generator and an image intensifier as detector aligned and mounted across from each other on a U arm. The radiation source/detector unit completes a 200 rotation around the patient s head, acquiring 200 projected images 1 apart. During the examination, the patient sits or stands in the rotation center. The position of the patient s head in the image field is determined either by a chin support or a craniostat. Tube voltage is fixed at 85 kv and tube current/exposure time product is fixed at 42 mas. The scan time is 14 seconds. The x-ray detector component consists of a 9-inch (23 cm) image intensifier and a charge-couple device camera. Each of the 200 captured projections is represented by a 1024 x 1024 pixel matrix, the pixels being defined by a 12-bit grayscale. The fixed field of view size is 15 cm resulting in a scan volume of 15 x 15 x 15 cm. Reconstructed threedimensional data is saved together with the original two-dimensional projection views in a proprietary data format file. Linear distance measurement The current study was based on 6 embalmed cadaver heads with intact soft tissue provided by the Department of Anatomy and Cell biology of the College of Medicine, The University of Iowa. The heads were sectioned such that the maxillary and mandibular alveolar arches were preserved along with the surrounding soft tissues. These

23 13 specimens were resized by slicing off tissues superior and posterior to the temporomandibular joints, such that only tissues surrounding the jaws were maintained. The specimens were mounted on a base of dental stone in order to ensure that the vertical orientation of the specimen was consistent for CBCT imaging and thereafter for sectioning with band saw. Subsequently fiduciary markers made of gutta percha, were placed on either side of the mandible along the buccal and lingual alveolar ridges such that the buccal and lingual markers were in alignment. The selection of marker was based on the radiopacity of the material and size of the marker such that they were more radiopaque than the surrounding tissues and small enough to be not visible in more than 1-2 orthogonal slices. Three markers were placed on the buccal surface of the alveolar bone such that they were aligned and contiguous. A marker was placed lingually such that it was in alignment with the most anterior bead as visible to the naked eye. The purpose of placing three markers buccally was to ensure that at least one of the markers was in alignment with the lingual bead confirmed on imaging. This plane of alignment would determine the plane of measurement for both CBCT and physical. The heads were then imaged using the Galileos cone-beam computed tomography unit. (Figure 1) The specimen was placed on a pedestal/stand in order to be placed within the unit for scanning. The scan was performed at 85 kvp and 42 mas. The total scan time for each specimen was 14 seconds, the field of view (FOV) was 15 cm and 3D volume consisted of 512 x 512 x 512 isotropic voxels (volume elements) each of 0.3 mm in size. Once the images were reconstructed, the images were viewed to check for the alignment of the beads on the orthoradial slices. The buccal bead that appeared to be best aligned with the lingual bead on the orthoradial slice was selected and noted on the specimen. An

24 14 indelible ink marker was used on the specimen to indicate the proposed slicing plane which connected the lingual bead with the buccal bead offering proper alignment.(figure 13) In the CBCT images, the cross-sectional image that showed the lingual and the buccal marker in alignment and completely in focus was selected as the plane of measurement. On this cross-sectional image, a horizontal tangent was drawn electronically using the measurement tool in Sidexis touching the superiormost point on the bone and another touching the inferiormost point of the bone. The vertical height was measured between these two tangents using the measurement tool in Sidexis. (Figure 3-12) The vertical distance was similarly measured between two tangents on the contralateral side of the mandible. Three measurements were obtained for each side of the mandible on three different days. All measurements were made by a single observer. Similar procedure was followed for the remaining five specimens. The specimens were dissected after imaging such that the maxilla with its adjoining soft tissue was removed. The mandible with surrounding soft tissues was retained. The specimens were then sliced along the plane marked previously using indelible ink. A band saw (Craftsman, Sears Roebuck & co.) was used to make the sections. (Figure 14) The specimens were kept in the same vertical orientation as in CBCT using the base made for each specimen. An instrument designed in the medical instrument shop (The University of Iowa) was used to make a tangent to determine the highest and the lowest point. The purpose of using this instrument was to reproduce in the specimen, the superiormost and inferiormost points of the bone as in the image. (Figure

25 15 15) These points were again marked with indelible ink. The distance between the two points was measured using a pair of digital vernier calipers (Mitutoyo, Japan). (Figure 16) Measurement of the distance between the superiormost and inferiormost point of a given section of bone at a particular site was taken as the mean of the measurements on either side of the sectioned bone. This mean measurement was used as the physical measurement of the height of bone for the particular site. Three measurements were made at each site on three different days. In order to ensure that the measurement with vernier calipers was absolutely in the vertical plane, a 6 combination square was used. (Figure 17) The measurement was made such that the reading on the caliper was facing away from the measurer to avoid bias. (Figure 20) The measurements of the height of bone obtained from CBCT images and caliper measurements from six specimens were compared.

26 Figure1. Imaging device Galileos CBCT unit (Sirona Dental Systems Inc., Bensheim, Germany) 16

27 Figure 2.Imaging specimen in Galileos CBCT unit 17

28 Figure 3.CBCT image showing the plane of measurement on the left side of specimen 18

29 Figure 4.CBCT image showing the plane of measurement on the left side of specimen 19

30 Figure 5.CBCT image showing the plane of measurement on the right side of specimen 20

31 Figure 6.CBCT image showing the plane of measurement on the right side of specimen 21

32 22 Figure 7.CBCT image showing the plane of measurement on the left side of specimen Figure 8.CBCT image showing the plane of measurement on the right side of specimen

33 Figure 9.CBCT image showing the plane of measurement on the right side of specimen 23

34 Figure 10.CBCT image showing the plane of measurement on the right side of specimen 24

35 25 Figure 11.CBCT image showing the plane of measurement on the right side of specimen Figure 12.CBCT image showing the plane of measurement on the right side of specimen

36 Figure 13.Specimen mounted on a base with the plane of dissection marked consistent with the CBCT image 26

37 Figure 14.Band saw (Craftsman, Sears Roebuck & co) 27

38 Figure 15.Tangent locator (Medical Instrument Shop, The University of Iowa) 28

39 Figure 16.Digital vernier calipers (Mitutoyo, Japan) 29

40 Figure 17.Combination square 30

41 31 Combination square Tongue Mandible Superior most point of measurement Soft tissue of chin Inferior most point of measurement Tangent locator Base Figure 18.Specimen set up for physical measurement

42 Figure 19.Physical measurement between superior most and inferior most points of the bone with calipers 32

43 Figure 20.Physical measurement between superior most and inferior most points of the bone with calipers 33

44 34 Table 1. Measurements obtained from six specimens Specimen Measurement Serial Measurement Measurement type number on Right side on Left side (mm) (mm) Case 1 CBCT Physical Case 2 CBCT Physical

45 35 Table1.continued Case 3 CBCT Physical Case 4 CBCT Physical Case 5 CBCT

46 36 Table1.continued Case 5 Physical Case 6 CBCT Physical

47 37 CHAPTER III RESULTS An overview of statistical methods Descriptive statistics were calculated. A paired sample t-test was used to determine whether there was a significant difference between the average values of the same measurement made under two different conditions (CBCT vs. Physical). The same test was also used to test for the difference between first and second or first and third or second and third measurements made by the same observer. In addition, the intraclass correlation was computed as a measure of agreement between the first and second or first and third or second and third measurements which were made by a single-observer. The following is an approximate guide for interpreting an agreement between two measurements that corresponds to an intraclass correlation coefficient. i) 0=No agreement ii) iii) iv) =Poor agreement =Fair agreement =Moderate agreement v) = Substantial agreement vi) vii) = Strong (or almost perfect) agreement 1.00= Perfect agreement All tests had a 0.05 level of statistical significance. SAS for Windows (v9.1, SAS Institute Inc, Cary, NC, USA) was used for the data analysis.

48 38 Data preparation Six randomly selected cadaver heads were used in this study. Six paired (left and right) measurements were made from each specimen using two methods, comprising 3 pairs of CBCT and 3 pairs of Physical. In order to assure the independence of samples for performing the appropriate statistical analysis, for each method, the average of three measurements at left or right side or the average of 6 measurements from the same specimen was used for the data analysis. Therefore, there were a total of 6 pairedsamples that were used for each method in this study. Statistical analysis Descriptive statistics are summarized in Table 2. A. Testing a difference between CBCT and Physical at right side In order to compare the two measurements made from the same head at right side with the two methods, a new variable called Diff_R (Diff_R=CBCT_R Physical_R) was created. A paired-sample t-test was used to determine whether the mean difference measurement value of two measurements at right side was significantly equal to zero, which would indicate no statistically significant difference between the two measurements. The data revealed that overall there was no statistically significant difference between CBCT and Physical at right side (p=0.2298). The mean difference value is presented in Table 2. B. Testing a difference between CBCT and Physical at left side

49 39 In order to compare the two measurements made from the same head at left side with the two methods, a new variable called Diff_L (Diff_L=CBCT_L Physical_L) was created. A paired-sample t-test was used to determine whether the mean difference measurement value of two measurements at left side was significantly equal to zero, which would indicate no statistically significant difference between the two measurements. The data revealed that overall there was no statistically significant difference between CBCT and Physical at left side (p=0.3554). The mean difference value is presented in Table 2. C. Testing a overall difference between CBCT and Physical In order to compare the overall two measurements made from the same head with the two methods, a new variable called DiffCP (DiffCP=CBCT Physical) was created. A paired-sample t-test was used to determine whether the mean difference measurement value of two measurements was significantly equal to zero, which would indicate no statistically significant difference between the two measurements. The data revealed that overall there was no statistically significant difference between CBCT and Physical (p=0.2684). The mean difference value is presented in Table 2. D. Reliability of measurement Four measurements were made per specimen, two by calipers and two on CBCT images for both sides of the specimen. A total of 24 measurements were thus obtained for all 6 specimens. These measurements were repeated 3 times for each specimen. The

50 40 measurements were assigned variables M1, M2 and M3 for the first, second and third measurements respectively. In order to evaluate the reliability of duplicate measurements made by a single observer, three new variables Diff_M12 (Diff_M12=first measurement second measurement), Diff_M13 (Diff_M13=first measurement third measurement), and Diff_M23 (Diff_M23=second measurement third measurement) were created. A paired-samples t-test was used to determine if the mean difference between the two measurements was equal to zero. The data revealed that there were no statistically significant differences between first and second measurements (p=0.1237), between first and third measurements (p=0.5608), and between second and third measurements (p=0.5809). The mean differences between each of those two measurements are presented in Table 3. In addition, intraclass correlation was computed as a measure of intra-observer agreement between first and second or first and third or second and third measurements. Data showed that intraclass coefficient was significantly different from zero (p< for each instance), and intraclass coefficients of , and for each instance indicated strong agreement between the duplicated measurements made by a single observer.

51 Table 2. Mean linear measurement values of bone height under each condition Lower Upper Variable Specimens Mean Standard Minimum Maximum Median 95% CI for 95% CI for (N) (mm) deviation (mm) (mm) (mm) mean mean (mm) (mm) CBCT_R Physical_R Diff_R

52 Table 2 continued Variable Specimens Mean Standard Minimum Maximum Median Lower Upper (N) (mm) deviation (mm) (mm) (mm) 95% CI for 95% CI for mean mean (mm) (mm) CBCT_L Physical_L *Diff_L

53 Table 2 continued Lower Upper Variable Specimens Mean Standard Minimum Maximum Median 95% CI for 95% CI for (N) (mm) deviation (mm) (mm) (mm) mean mean (mm) (mm) CBCT Physical *DiffCP

54 44 1 Mean value right side 2 Mean value left side 3 Overall mean Figure 21.Graphical representation of mean values of bone height Physical) (CBCT and

55 Table 3. Mean difference between two measurements made by the same observer Variable Specimens Mean Standard Minimum Maximum Median P-value* (N) (mm) deviation (mm) (mm) (mm) Diff_M Diff_M Diff_M *Paired sample t-test 45

56 46 CHAPTER IV DISCUSSION Radiological evaluation is necessary for information on quantity and quality of bone available for implant placement and to localize the anatomical structures. There are certain basic principles of radiography that should guide the clinician in selecting an appropriate imaging technique and judging whether the resultant images are of required diagnostic quality. First, there should be an adequate number and type of images to provide the needed anatomic information. In implant imaging, this includes the quantity and quality of bone as well as the location of anatomic structures, which generally require multiple images at right angles to each other. Second, the selected imaging modality should be precise and minimally distorted which is governed by ideal positioning of the patient, imaging receptor and x-ray beam. Third, there must be a way of relating the images with the patient s anatomy such as with the use of a stent with radiopaque markers for edentulous regions of the jaws. The exact location of the longitudinal and cross-sectional views can thus be determined with respect to the edentulous region of the jaw. Additionally, all images should be of adequate density and contrast and free of artifacts that might interfere with interpretation of images. Finally, the risks and benefits of an imaging technique should be weighed in that the radiation dose to the patient and the financial cost of the imaging technique should be taken into consideration. The ALARA (as low as reasonably achievable) principle should govern the selection of imaging technique when more than one technique is suitable in a particular case. 2 Dental implant imaging should provide information about the implant site with regards to the (1) presence of disease (2) location of anatomic structures to be avoided

57 47 during implant placement such as maxillary sinus, nasopalatine canal, inferior alveolar canal, mental foramen, mental canal and submandibular salivary gland fossa (3) osseous morphology such as knife edge ridges, developmental variations, postextraction irregularities, enlarged marrow spaces, cortical irregularity and thickness and trabecular bone density; and (4) amount of bone available for implant placement and orientation of the alveolar bone. Lingually inclined bony contours which usually occur near the posterior region of the mandible may lead to osseous undercuts which may lead to a poor prognosis of an implant treatment plan. If the implant is not placed at a favorable angle due to inadequate bone, then the functional loading of the implant may be adversely affected. 2 Lekholm and Zarb 3 developed a grading scheme for the quality of bone in the proposed implant site in terms of relative proportion and density of cortical and medullary bone. According to this scheme, the bone of the alveolar process is divided into 4 classes: (1) almost the entire jawbone is composed of homogeneous compact bone, (2) a thick layer of compact bone surrounds a core of dense trabecular bone, (3) a thin layer of compact bone surrounds a core of dense trabecular bone of favorable strength, and (4) a thin layer of compact bone surrounds a core of low density trabecular bone. Cross-sectional imaging is required to apply this scheme for implant site assessment. There is another system for bone quality classification proposed by Lindh et al 4 according to which the periapical radiographs grade the bone as dense, sparse or alternating dense and sparse trabeculation. This method is less specific than the one proposed by Lekholm and Zarb. 3

58 48 In pantomographs and other two-dimensional images, information such as the width of the bone is lacking and even the apparent height of available bone measured may not be accurate due to distortion caused by positioning errors and variable magnification. 66, 67 The position of the object between the x ray source and the receptor is responsible for the magnification seen on a pantomograph. In the sharply depicted layer the image is free of distortion which means that the magnification factor is the same for both vertical and horizontal planes. 16 Objects outside this layer will appear distorted in the image because of the difference between the velocity of the receptor and the velocity of the projection of the object on the receptor and because of the position of the object in relation to the tube and receptor. The panoramic image is affected by both magnification errors and displacement. Distortion, displacement, and magnification cause changes in the dimensions in the image of depicted structures on radiographs compared to those of the actual structures. 17 The magnification factor varies from one manufacturer to another because of different projection geometries; this variation results in differences in magnification and in the amounts of distortion and displacement of images of structures relative to each other. 17 Non-uniform magnification with panoramic images results in 15% to 220% enlargement of structures. 18, 19, 20 The major problem resulting in errors is patient positioning 21, 22, 23, 24, 25, 26 which can result in geometric distortion of the image. This distortion has a horizontal and a vertical component. The vertical distortion is determined by the size of the x-ray focal spot and the distance between the patient s arch and the image receptor. The smaller the focal spot and the less the distance between the patient s arch and receptor, the less is the vertical distortion. The horizontal dimensions are affected by the continuously moving rotation center of the beam, and

59 49 change significantly with the faciolingual positioning of the object and the length of the effective projection radius. The horizontal magnification also varies from the anterior to posterior regions as the width of the focal trough is narrower in the anterior region than the posterior region. The vertical magnification factor varies less between different faciolingual object positioning and moreover is a linear function. 27 Pantomographic images reportedly produce 50% to 70% horizontal distortion and 10% to 32% vertical distortion 28, 29, 30, 31 This distortion factor and the inconsistency in enlargement lead to inaccurate determinations of implant lengths based on linear measurements from pantomographic images. 18 To overcome this problem, the use of a surgical stent to determine implant placement is suggested when making a panoramic image. Small radiopaque markers placed over potential implant sites can be used to determine the distortion factor by measuring the actual size of the markers and finding their difference from the size as measured on the panoramic image. The true height of the residual alveolar ridge at the prospective implant can be calculated by multiplying this distortion factor with the distance measured radiographically from the crest of alveolar process to the superior border of the inferior alveolar canal or floor of the maxillary sinus. 28 Direct measurements on pantomographs cannot be used without mathematical correction for the magnification factor. The x-ray beam is directed from below upwards approximately at 5 in the mandible and 15 in the maxilla such that the objects located to the lingual aspect of the jaw are projected higher than facially positioned objects. This causes objects to be projected at levels different from their true positions. Therefore, the lowest and highest parts of rounded objects are not the parts that are reproduced on the image. 32 Furthermore, due to superimposition of the cervical spine over the anterior region of the

60 50 jaws and due to inherent blurring resulting from a narrow focal trough in the anterior region, it is not possible to obtain diagnostically accurate information from this region on a pantomograph. Thus cross-sectional imaging is recommended to measure bone quantity in all three dimensions and also accurately localize the anatomical structures such as the inferior alveolar canal, maxillary sinus etc. Conventional tomography with complex motions is a cost effective method with low radiation risk that is recommended for most of the cases but provides slices limited in breadth. Conventional tomography is time consuming and the image quality depends on the skill of the operator as the positioning of the anatomical structure in relation to the image layer has to be accurate. Variation from this position may lead to blurring due to superimposition of osseous and soft tissue structures. 68 These variations lead to discrepancy of measurements both intraobserver and interobserver. Many factors must be evaluated when trying to decide which type of crosssectional imaging to use, including number of implant sites, degree of bone resorption or history of bone grafting that may affect the precision needed, accuracy and reliability of the imaging modality, convenience and cost and radiation dose. Both underestimating and overestimating the amount of bone and the location of anatomic structures can affect the success of implant placement. More complex implant cases, such as those involving patients who have had facial trauma or surgery for malignancy or those who have received ridge augmentation, may necessitate more complex imaging such as CT for evaluation of the reconstructed bone before implant placement whereas conventional tomography may be adequate for the majority of implant cases. CT is more appropriate

61 51 when multiple implants are considered as it provides slices through the entire region of interest. Conventional CT provides cross-sectional views through the area of interest and overcomes the issue of blurring associated with conventional tomography but this is achieved at a higher cost and increased radiation to the patient. The measurement error is generally required to be less than 1 mm on images made for implant treatment. 69 In studies 67, 70 using cadaver mandibles, measurement error was found to be less than 1 mm in 94% cases of CT, 39% cases of conventional tomography, 53% cases of intraoral radiography and 17% cases of panoramic radiography. CBCT is a new technology that provides cross-sectional images without superimposition or blurring 47, 48 and reduces the risk of radiation significantly. CBCT provides 3D imaging dedicated to the maxillofacial region at low cost and low dose of radiation. Imaging is accomplished by using a rotating gantry to which an x- ray source and detector are fixed. A divergent pyramidal or cone-shaped source of ionizing radiation is directed through the middle of the area of interest onto the x-ray detector on the opposite side. 50 The x-ray source and detector rotate around a rotation fulcrum fixed within the center of the region of interest. 50 During the rotation, multiple (from 150 to more than 600) sequential planar projection images of the field of view (FOV) are acquired in a complete, or sometimes partial, arc. 50 This procedure varies from a conventional spiral CT, which uses a fan-shaped x-ray beam in a helical progression to acquire individual image slices of the FOV and then stacks the slices to 54, 71 obtain a 3D representation. 50 Each slice requires a separate scan and separate 2D reconstruction. Because CBCT exposure incorporates the entire FOV, only one rotational sequence of the gantry is necessary to acquire enough data for image reconstruction. 50