Chapter 10: Special Topics

Transcription

1 Chapter 10: Special Topics Slide set of 71 slides based on the chapter authored by D. McLean and J. Shepherd of the publication (ISBN ): Diagnostic Radiology Physics: A Handbook for Teachers and Students Objective: To familiarize the student with Dental radiography, Mobile Radiography and fluoroscopy, Dual-Energy X-Ray absorptiometry, Conventional tomography and tomosynthesis. Slide set prepared by S. Edyvean International Atomic Energy Agency

2 CHAPTER 10. SPECIAL TOPICS IN RADIOGRAPHY Introduction Dental radiography Mobile Radiography and fluoroscopy Dual-Energy X-Ray absorptiometry Conventional tomography and tomosynthesis Bibliography Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 2

3 10.1. INTRODUCTION Previous chapters covered 2- dimensional imaging Later chapters cover cross-sectional imaging (CT,MR, ultrasound) This chapter presents a number of special X ray imaging modalities and their associated techniques - forming a transition between projection and cross sectional imaging Special X-ray imaging techniques Dental radiography Mobile Radiography and fluoroscopy Dual-Energy X-Ray absorptiometry Conventional tomography and tomosynthesis Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 3

4 10.2. DENTAL RADIOGRAPHY Introduction The tooth can be imaged in three ways Intra oral examination with the x-ray tube external and a bitewing film placed inside the mouth Extra oral examination where both the X-ray tube and detector is external to the patient to form an OPG A conebeam CT image Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 4

5 10.2. DENTAL RADIOGRAPHY Introduction An intra oral examination with bite-wing Is the most common examination, and is a low cost technique Places very small demands on X ray generation since the tooth is a low attenuation static object The image receptor is placed inside the mouth, and irradiated externally. bite-wing film teeth X-ray tube Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 5

6 10.2. DENTAL RADIOGRAPHY Introduction Orthopantomograph (OPG), Two dimensional images when radiographs of the entire set of teeth are required The image receptor and the X ray source are external to the patient The X ray beam is transmitted through the head - demanding significant X ray generation power and complex motion control for the X ray tube and image receptor Image receptors are film or digital detectors Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 6

7 10.2. DENTAL RADIOGRAPHY Introduction Cone-beam dental CT For three dimensional information Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 7

8 10.2. DENTAL RADIOGRAPHY Technology Intra oral radiography The intra oral X ray tube is a small robust device with a stationary target operating with a tube current of only a few ma Dental X ray tube with a stationary anode. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 8

9 10.2. DENTAL RADIOGRAPHY Technology Intra oral radiography (continued) The generator is typically very simple often with fixed tube voltage and tube current allowing output changes only by variations in exposure time. Major concerns with this device are for the stability of the tube head and the collimation of the beam. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 9

10 10.2. DENTAL RADIOGRAPHY Technology Intra oral radiography (continued) International standards require that the focus to the patient surface distance (FSD) be 200 mm. This is assured with the use of a collimating attachment that also restricts the beam to the region of the mouth being radiographed. bite-wing film teeth 200 mm X-ray tube Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 10

11 10.2. DENTAL RADIOGRAPHY Technology Intra oral radiography (continued) The X ray equipment requires periodic QC checking The process of film processing requires more diligent attention. The unscreened film is removed from the light tight moisture protective wrapping and is processed either manually or with varying degrees of automation. Hand processing is probably most common and ideally requires control of temperature and processing time. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 11

12 10.2. DENTAL RADIOGRAPHY Technology Intra oral radiography (continued) For higher volume clinics this can be automated with film mounted on hangers that progress through the development, stop bath, fixation and rinse processes. Typically these devices have timing and temperature control but do not control chemical activity through replenishment. This is achieved in fully automatic processors, however these are typically restricted to major dental hospitals. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 12

13 10.2. DENTAL RADIOGRAPHY Technology Intra oral radiography (continued) The uncertainties in film processing are best controlled through sensitometry. Light sensitometers are rare in dentistry due to the small film format, However adequate results can be achieved by using a simple radiograph of a 3 step wedge This can be easily manufactured by folding the lead foil found in the film wrap or purchased commercially Increasingly digital detectors are replacing film. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 13

14 10.2. DENTAL RADIOGRAPHY Technology Intra oral radiography (continued) Digital image capture can be achieved from an intensifying screen that is linked to a CCD camera through a tapered fibre optic coupling. The electronic signal can be transferred to an acquisition computer either through a direct cable or through blue tooth radio frequency transmission. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 14

15 10.2. DENTAL RADIOGRAPHY Technology OPG (Orthopantomograph) An OPG image is created by complex equipment where the X ray tube and image receptor assembly move in a horizontal plane around the head of the patient. OPG image of the teeth Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 15

16 10.2. DENTAL RADIOGRAPHY Technology OPG (Orthopantomograph) (continued) A narrow beam of radiation is formed by the tube collimation the image receptor moves within the assembly behind a lead aperture The basic movements of the OPG unit around the mandible are illustrated Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 16

17 10.2. DENTAL RADIOGRAPHY Technology OPG (Orthopantomograph) (continued) The device uses the principle of tomography and more importantly the principle of panoramic photography. This process can be illustrated through consideration of the panoramic camera used in photography. Here an acquisition aperture is used to expose an image plate that is moved behind the aperture slit to capture the image of a panorama while the camera simultaneously slowly rotates to scan a scene. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 17

18 10.2. DENTAL RADIOGRAPHY Technology Cone-beam CT CT imaging has been used for some time in dentistry, including the use of custom designed units for dental applications. Their use has become more widespread recently with the advent of cone beam technology Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 18

19 10.2. DENTAL RADIOGRAPHY Technology Cone-beam CT (continued) There are many cone beam CT (CBCT) models available using a variety of acquisition schemes They have in common a flat panel detector for acquisition, typically using either DR technology or an intensifying screen with a CCD camera (see chapter 7). Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 19

20 10.2. DENTAL RADIOGRAPHY Technology Cone-beam CT (continued) Typically a CBCT can acquire a full field of view (FOV) that covers the whole head although acquisitions that are restricted to the mandible with as little as 10% of full FOV are possible. The use of these lower cost CT units opens up new potentials in some areas of dental diagnosis However they have significantly higher dose compared to OPG procedures Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 20

21 10.2. DENTAL RADIOGRAPHY Dental Dosimetry Since dental examinations are amongst the most numerous, the dosimetry of these procedures is of high interest. Relevant principles and measurement techniques of dosimetry can be found in Chapter 21 of this handbook and in the Technical Report Series No.457 Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 21

22 10.2. DENTAL RADIOGRAPHY Dental Dosimetry There are large variations in recorded doses between X- ray facilities. A recent study in Europe showed that for an intra oral bitewing projection the average incident air kerma varied from 1 to 2 mgy with a corresponding KAP measurement of 20 to 40 mgy cm 2. The dose in centres that use slower film would be expected to be significantly higher. Data for OPG examinations from Europe showed KAP values ranging from 40 to 150 mgy cm 2 Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 22

23 10.2. DENTAL RADIOGRAPHY Dental Dosimetry The estimation of a population effective dose is difficult owing to the complex distribution of critical organs There are few radiosensitive organs around the mandible with some exceptions The thyroid, red bone marrow, brain, and salivary glands Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 23

24 10.2. DENTAL RADIOGRAPHY Dental Dosimetry The thyroid is the main radiosensitive organ around the mandible Well collimated X ray units should not directly irradiate this organ but it will probably receive appreciable scattered radiation Other radiosensitive organs include the red bone marrow of the mandible the brain The salivary glands also need to be considered as they are extensively irradiated They are now included as a remainder organ in the calculation of effective dose in accordance with ICRP 103. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 24

25 10.2. DENTAL RADIOGRAPHY Dental Dosimetry OPG examinations estimates of effective dose give average values of ~ 7 msv using weighting factors from ICRP 60 The use of ICRP 103 weighting factors has been variously estimated to increase effective dose in dentistry by 50% to 400%. Since CBCT units operate with a large FOV, their effective doses are considerably higher than for OPG with estimates of dose varying from 60 msv to 550mSv, for full FOV still considerably lower than conventional head CT with effective doses of about 2 msv. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 25

26 10.3. MOBILE RADIOGRAPHY AND FLUOROSCOPY Introduction Mobile X ray equipment ranges from small dental units to CT and MRI units carried in a large vehicle. However this chapter is restricted to simple radiographic and fluoroscopy equipment. Mobile equipment is needed when the patient cannot be brought to a fixed installation for a radiographic examination. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 26

27 10.3. MOBILE RADIOGRAPHY AND FLUOROSCOPY Introduction Limitations of mobile equipment relate to the availability of a suitable electrical power supply, the size and weight of the equipment and the consequent effort required to move it. The equipment design of mobile X ray equipment is varied and innovative in order to maximise the benefit given the above constraints. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 27

28 10.3. MOBILE RADIOGRAPHY AND FLUOROSCOPY Technology Fixed angiographic X ray machines require a capacity to draw up to 100 kw with a high current multiphase supply. Assuming no loss in the high voltage transformer the X ray output power in the secondary circuit will equal that of the primary power drawn from the electrical supply (cf Chapter 6) Therefore a domestic single phase electric supply may typically be limited to 2.4 kw While low power is usually not a limitation for fluoroscopic application this is a challenge for some radiography. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 28

29 10.3. MOBILE RADIOGRAPHY AND FLUOROSCOPY Technology One solution is to charge a capacitor which is discharged across the X ray tube the capacitor discharge mobile. However the tube voltage will fall rapidly during the discharge of the capacitor leading to excessive surface kerma for large patient thicknesses. It is more advantageous to have an integral battery power supply which is converted to a medium to high frequency AC signal (cf chapter 5) This leads to substantial reductions in the thickness of the coils needed in the transformer design. There is also the added advantage that it can be used when there is no electrical power supply available at the examination site. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 29

30 10.3. MOBILE RADIOGRAPHY AND FLUOROSCOPY Technology The variety of possible generator designs leads to the possibility of many types of radiographic waveforms being used in the high voltage circuit for X ray generation. This leads to varying tube outputs and beam qualities for the same radiographic settings of tube voltage and tube current (cf chapter 5) Care is therefore needed when determining dosimetric factors for mobile units. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 30

31 10.3. MOBILE RADIOGRAPHY AND FLUOROSCOPY Image quality Image quality and general quality control for mobile X ray units generally follows that used for fixed units. The use of high fluoroscopic image quality can lead to reduced procedural time, and hence reduced radiation exposure time. An important part of image quality is the setup of viewing monitors and the ambient conditions used for operation. Every effort should be made to view monitors in low ambient lighting conditions. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 31

32 10.3. MOBILE RADIOGRAPHY AND FLUOROSCOPY Radiation protection Mobile X ray equipment raises concerns about occupational and the public radiation exposure, as it is not operated in a purpose-built shielded environment. Assuming all X ray equipment has been checked for tube leakage, the source of radiation of occupational concern during the procedure is scatter from the input surface of the patient. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 32

33 10.3. MOBILE RADIOGRAPHY AND FLUOROSCOPY Radiation protection It is advised that the medical physicist take field measurements of air kerma levels due to the patient scattered radiation using a patient phantom for typical radiographic and fluoroscopic procedures. As mobile radiography may take place in environments where other patients or members of the public may be in close proximity it is essential that good communication exists between the medical physicist and the staff at the location for the radiographic procedure. These staff should attend appropriate radiation safety courses that include information about radiation risk from mobile radiography. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 33

34 10.3. MOBILE RADIOGRAPHY AND FLUOROSCOPY Radiation protection In many cases, such as for mobile chest radiography, the use of good radiographic practice with basic radiation protection allows safe usage in most hospital environments. Simple measurements should be made to demonstrate the safety (or otherwise) of mobile X ray equipment use. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 34

35 10.4. DUAL-ENERGY X-RAY ABSORPTIOMETRY The principle of operation involves two images from the attenuation of a low and a high X-ray energy beam Using special imaging equipment comprising of special beam filtering and near-perfect spatial registration of the two attenuation maps Detector(s) within gantry head x-ray source within couch Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 35

36 10.4. DUAL-ENERGY X-RAY ABSORPTIOMETRY Schematic showing the components of a DXA system. The gantry configuration shows a pencil beam system pinhole source collimator and a single detector These scan the patient to acquire the attneuation data Movement of source and detector to acquire attenuation data (courtesy of J. Shepherd, UCSF). Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 36

37 10.4. DUAL-ENERGY X-RAY ABSORPTIOMETRY Other systems with slit may source collimators and segmented line detectors are called fan-beam systems, and have the advantage of higher spatial resolution and shorter scan times Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 37

38 The process for determining material composition can be outlined from consideration of the total attenuation of an X-ray flux passing through a subject as represented by the following formula where I o is the unattenuated X-ray intensity before it passes through a N materials with a thicknesses, t i, µ i is the total linear attenuation, (µ/ρ) i is the mass attenuation coefficient of the ith material, and ξ i is the ith areal density = ρ i t i DUAL-ENERGY X-RAY ABSORPTIOMETRY = = = = = = = = N 1 i i i N 1 i i i i i i N 1 i i i N 1 i i i ξ ρ µ O ρ t ρ µ O ) ρ ρ ( µt O µt O e I e I e I e I I Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 38

39 10.4. DUAL-ENERGY X-RAY ABSORPTIOMETRY DXA can only solve for two materials simultaneously. However, three materials may be quantified: bone, lean, and fat mass, by using three fundamental assumptions, 1.X-ray transmission through the body for the two energy windows can be accurately described by exponential attenuation processes 2.Pixels of the human body image can describe two-components i.e. either soft tissue and bone mineral, or, when bone is not present, fat and lean mass. Thus, although DXA can only solve for two compartments within individual pixels, it can describe a 3-component model for body composition. 3.The soft tissue overlaying the bone in the image has a composition and X-ray properties that can be predicted by the composition and X-ray properties of the tissue near, but not overlaying, the bone. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 39

40 For example - simplified DXA equations will be derived for two monochromatic X-ray exposures with different energies (a high and low energy). The full solution would require integration of the attenuation across the x-ray spectrum for each energy. The attenuation equation for each exposure results in the following two equations: where the H and L superscripts represent the high and low energy X-ray beams respectively, and the s and b subscripts represent soft tissue and bone DUAL-ENERGY X-RAY ABSORPTIOMETRY + = b L b s L s e I I O L ξ ρ µ ξ ρ µ + = b H b s H s e I I O H ξ ρ µ ξ ρ µ Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 40

41 The solution of these equations for the areal density of bone is given by Where R S is commonly referred to as the ratio value for soft tissue measured for tissue surrounding but not containing the bone DUAL-ENERGY X-RAY ABSORPTIOMETRY ) ( ln ln abmd areal BoneMineral Density R I I I I R S H b L b L O L H O H S b = = ρ µ ρ µ ξ H S L S R S = ρ µ ρ µ Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 41

42 10.4. DUAL-ENERGY X-RAY ABSORPTIOMETRY Principle of DXA is shown with 4 intensity profiles The high energy absorption profile is multiplied by the soft tissue R-value, R s, which accounts for differences in high and low energy absorption of soft tissue. R s is calculated for pixels that do not contain bone (drawing courtesy of J. Shepherd, UCSF). Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 42

43 10.4. DUAL-ENERGY X-RAY ABSORPTIOMETRY In order to use a DXA unit to determine bone mineral density the DXA unit must be calibrated with a phantom suitable for a particular examination, for example spine, and for a particular DXA system type. Universal phantoms that can be used between different types of systems have been developed, however the calibration of DXA units is an important practical subject essential for the viability of DXA usage. Examples of standard phantoms available Bio-Imaging Technologies, Inc European spine Phantom - QRM Hologic Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 43

44 10.4. DUAL-ENERGY X-RAY ABSORPTIOMETRY The T-score and the Z-score are parameters used. The T-score is the primary diagnostic value used for osteoporosis. The T-score is inversely related to fracture risk. By international convention, the T-score is the difference between the patient s abmd and a young reference abmd in units of the population standard deviation: T score = abmd patient SD abmd Young AdultMean Young AdultMean where SD is the standard deviation of the population of young adults. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 44

45 10.4. DUAL-ENERGY X-RAY ABSORPTIOMETRY The Z-score is used to diagnose low bone mass in young adults and children. It is the difference between the patient s abmd and an age- and typically ethnicity-matched reference abmd and standard deviations: Z score = abmd patient SD abmd Age,Ethnicity matched AdultMean Age,Ethnicity matched AdultMean The reference values used to calculate T and Z-scores are derived from normative databases of local populations. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 45

46 10.4. DUAL-ENERGY X-RAY ABSORPTIOMETRY More information on the standards used to calculate T and Z-scores can be found in the Postitions of the International Society for Clinical Densitometry ( Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 46

47 10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS The usefulness of sectional images, that remove the image of unwanted overlying tissues, has been well understood since the early days of X ray imaging. The formation of such images is through an analogue process known as conventional tomography. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 47

48 10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS Principles Conventional tomography uses the principle of image blurring to remove overlying structures from a radiological image while allowing one section of body to remain in focus. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 48

49 10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS Principles During image acquisition the X ray tube is in motion The image receptor moves simultaneously in the opposite direction The projected image in the focal plane moves in same direction as the image receptor focal plane movement of X-ray tube objects in focal plane movement of images movement of image receptor Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 49

50 10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS Principles The section in focus is the focal plane Regions of the body above and below the focal plane are increasingly blurred as their distance from this plane increases. focal plane movement of X-ray tube fulcrum or pivot point Only the red triangle remains in the same position on the image receptor Illustration of final image movement of image receptor Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 50

51 10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS Principles Conventional tomography with curved focal plane Conventional tomography is easily be extended for use in dental radiography by acquiring a curved focal lane The principle is to use a variable speed for the image receptor The focal plane is known as the focal trough The variable speed gives rise to a curved curved focal trough Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 51

52 10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS Principles Conventional tomography with curved focal plane The principle is that if the image receptor speed is increased the focal plane moves upwards in this example (and vice versa) Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 52

53 10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS Principles For the curved focal trough the image receptor speed changes during motion In this example The X ray tube moves at constant speed to the right The image receptor accelerates to the left during motion Consequently the focal plane moves away from the image receptor. Note the thickness of the focal trough changes in accordance to distance from the image receptor focal plane X-ray tube moving at constant speed focal trough Accelerating movement of image receptor Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 53

54 10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS Principles Tomosynthesis - a development of conventional tomography with the use of digital technology to digitally change the speed of the image receptor In this case one acquisition run might consist of 10 individual X ray images each read and erased in sequence throughout the one tube movement. The images are digitally added to reconstruct different focal planes in the body. It can be seen that the focal plane can be altered by advancing or retarding each image in the series by an increasing amount. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 54

55 10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS Principles Illustration of Tomosynthesis The X ray tube moves at a constant speed to the right The image receptor moves at a constant speed to the left. In this figure 4 samplings of the image are shown at tube positions A, B, C and D. Tomographic images focused on planes I, II and III are created by combining the 4 sampled images with appropriate offsets Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 55

56 10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS Principles Tomosynthesis is a method for performing highresolution limited-angle tomography It can be treated as a special case of computed tomography in which data is acquired over a limited angular range. The computed image can then be obtained using the various CT reconstruction methods. Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 56

57 10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS Tomographic applications Conventional tomography has been almost completely replaced by computed tomography in the modern radiology department. Areas where it is still used are intravenous pyelograms (IVPs) where contrast in the kidney can be conveniently placed within the focal plane to allow clear visualisation of the contrast agent. This examination is largely replaced by CT. pantomographic dental radiography (Orthopantomogram - OPG) OPG IVP Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 57

58 10.5.CONVENTIONAL TOMOGRAPHY AND TOMOSYNTHESIS Tomographic applications Conventional tomography requires one tube acquisition for each focal plane image or slice. Therefore examinations requiring many slices are inherently high dose procedures. The use of tomosynthesis, on the other hand, requires only one tube motion to capture enough data to reconstruct multiple slices within the body. Today it is an emerging technology, with its most notable application so far being in mammography Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 58

59 BIBLIOGRAPHY CENTRE FOR EVIDENCE-BASED PURCHASING, Digital cone beam tomography (DCBT) systems. CEP 10048; NHS PASA March 2010 [online] (2010). CEPproducts.htm. HELMROT, E., ALM CARLSSON, G., Measurement of radiation dose in dental radiology, Radiation Protection Dosimetry 114 (2005) INTERNATIONAL ATOMIC ENERGY AGENCY, Guidelines for the use of DXA in measuring bone density and soft tissue body composition, Rep. TBA,, Vienna (2010). Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 59

60 BIBLIOGRAPHY LANGLAND, O.E., LANGLAIS, R.P., Principles of Dental Imaging, Williams & Wilkins, Baltimore, MA (1997). LUDLOW, J.B., DAVIES-LUDLOW, L.E., BROOKS, S.L., HOWERTON, W.B., Dosimetry of 3 CBCT devices for oral and maxillofacial radiology: CB Mercuray, NewTom 3G and i-cat, Dentomaxillofacial Radiology 35 (2006) WILLIAMS, J.R., MONTGOMERY, A., Measurement of dose in panoramic dental radiology, British Journal of Radiology 73 (2000) Diagnostic Radiology Physics: a Handbook for Teachers and Students chapter 10, 60