Cone beam ct

CBCT technique in dentistry

1. CONE BEAM COMPUTED TOMOGRAPHY 2020-12-19 1

2. CONTENTS Introduction Principles Image Acquisition X-ray generation Image detection system Image reconstruction Image display Clinical consideration Imaging protocol Comparison with CT Artifacts Applications in Dentistry 2020-12-19 2

3. INTRODUCTION It is also known as Dental computed tomography, Cone beam volumetric tomography, Volumetric computed tomography or Cone beam imaging. A recent technology initially developed for angiography in 1982. It uses cone shaped beam of X-ray photons which can scan the region of interest in a single 360⁰ rotation. Most significant technology advancement in maxillofacial imaging. 2020-12-19 3

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5. Principles CBCT imaging is performed using a rotating platform carrying an x-ray source and detector . 2020-12-19 5

6. A divergent cone shaped or pyramidal source of radiation is directed through region of interest(ROI). X-ray source and detector rotate around a rotation center, fixed within center of the ROI. 2020-12-19 6

7. During rotation, multiple sequential planer projection images are obtained while the x-ray source and detector move through an arc of 180 to 360 degree. Single projection image from raw primary data, which is individually known as basis frame or raw image. Usually several hundred 2- D basic images from which the image volume is calculated. Complete series of images is called Projection data. 2020-12-19 7

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9. Components of image production X-ray generation X-ray detection Image reconstruction 2020-12-19 9

10. X-ray generation Patient stabilization Depending on the unit, CBCT examinations are made with patient sitting, standing and supine . 2020-12-19 10

11. With all system, immobilization of the patients head is more important than position because any movement degrades the final image. Immobilization of head by- Chin cup Bite fork Other head-restraint mechanism 2020-12-19 11

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13. X-ray generator X-ray generation continuous or pulsed When pulsed- exposure time is upto 50% less than scanning time(this technique reduces patients radiation dose) ALARA (As Low As Reasonable Achievable) principle dose optimization states that CBCT exposure factor should be adjusted on the basis of patients size. 2020-12-19 13

14. Scan volume /field of view(FOV) Detector size Shape Beam projection geometry Ability to collimate the beam 2020-12-19 14

15. It is desirable to limit the field size to the smallest volume that images the ROI. This procedure reduces unnecessary exposure to the patient and produces the best image by minimum scattered radiation, which degrade the image quality. 2020-12-19 15

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19. Scan factor Number of images forming the “projection data” throughout the scan is determined by- Detector frame rate(no. of image acquired per second). Completeness of the trajectory arc (180 to 360) Rotation speed of source and detector 2020-12-19 19

20. Image detector Divided into two groups based on detector type: 1. Image intensifier tube/charge- coupled device Larger and bulkier and result in circular basis image area spherical volume 2. Flat panel detector (FPDs) Lighter in weight ,rectangular cylindrical volume, consists of cesium iodide scintillator applied to a thin film transistor made of amorphous silicon. 2020-12-19 20

21. Voxel - Volume element Individual volume element is voxel . Voxels form the volumetric data set. CBCT units provide voxel resolution that are isotropic -equal in all 3 dimension. Determinant of voxel is- Pixel size of detector detector with small pixel Capture few x-ray photon per voxel 2020-12-19 21

22. Reconstruction Basis projection frames Volumetric data a single CBCT rotation take less than 20 sec Produce 100 to 600 individual projection frames Each with more than 1 million pixel with 12 to 16 bits of data assigned to each pixel These data processed to create volumetric data set( voxel ) by a sequence of software algorithms a process known as Reconstruction 2020-12-19 22

23. Stages of reconstruction process 2020-12-19 23

24. 1.Preprocessing stage Performed at acquisition computer. Inherent pixel imperfections should be corrected. Exposure normalization. 2. Reconstruction stage Corrected images are converted into a special representation called a sinogram . Sinogram is a composite image developed from multiple projection images . 2020-12-19 24

25. Clinical Considerations 1.Patient selection criteria It provides a radiation dose higher than other dental radiographic procedures. Panoramic and periapical view cannot provide necessary information for patient diagnosis and treatment. Used as adjunctive diagnostic tool. 2020-12-19 25

26. 2. Patient preparation Appropriate personal radiation barrier protection Leaded apron- for pregnant patients and children Lead thyroid collar- to reduce thyroid exposure Before scan, remove all the Metallic object Eyeglass Jewelry Metallic partial denture 2020-12-19 26

27. Patient motion can be minimized by Head stabilization Chin cups to posterior or lateral head support Patient should be directed to remain still as possible before exposure, to breathe slowly through nose, and to close the eyes. 2020-12-19 27

28. 3. Imaging protocol Develop to produce image of optimal quality with the least amount of radiation exposure to the patient. 2020-12-19 28

29. Exposure setting Quality and quantity of X-ray beam depend on Tube voltage( kVp ) Tube current( mA ) CBCT unit manufacturers approach setting exposure in 2 ways: Selection of fixed exposure setting Allow operator manual adjustment of kVp or mA 2020-12-19 29

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31. 2.Spatial resolution Ability of an image to reveal fine detail. Determined by Pixel size Beam projection geometry Patient scatter Focal spot size Number of basis image Reconstruction algorithm 2020-12-19 31

32. 3. Scan time and number of projection Adjusting the detector frame rate Increase the number of basis image projections Reconstructed image with fewer artifacts and better image quality 2020-12-19 32

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34. 4. Archiving , export and distribution Process of CBCT imaging produces 2 data products Volumetric image data from the scan Image report generated by the operator Both set of data must be archived and distributed . 2020-12-19 34

35. Image Artifacts An artifact is any distortion or error in the image. Artifacts can be classified according to their etiology. Inherent Procedure related Introduced Patient motion artifact 2020-12-19 35

36. 1.Inherent artifacts Can arise from limitations in the physical processes involved in the acquisition of CBCT data. Beam projection geometry of CBCT, reduced trajectory rotational arcs, and image reconstruction methods produce 3 type of artifacts. Scatter Partial volume averaging Cone beam effect 2020-12-19 36

37. Scatter- Result from x-ray photons that are diffracted from their original path after interaction with matter. 2020-12-19 37

38. Partial volume averaging- I t occur when the selected voxel size of the scan is larger than the size of the object being imaged. 2020-12-19 38

39. Cone b e am effect- Is a potential source of artifacts, especially in the peripheral portion of scan volume. Can result in Image distortion Greater peripheral noise Clinically, the effect can be reduced by positioning of ROI in the horizontal plane of x-ray beam. 2020-12-19 39

40. 2.Procedure related artifacts Under sampling of the object can occur when too few basis projections are provided for image reconstruction or when rotational trajectory arc are incomplete. Reduced data sample leads to: Misregistration Noisier image Sharp edges which appear as fine striations in the image. 2020-12-19 40

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42. 3.Introduced artifacts An x-ray beam pass through an object, lower energy photons are absorbed in preference to higher energy photons, this phenomenon is known as beam hardening. Can result in 2 type of artifacts Distortion of metallic structure as a result of differential absorption, known as cupping artifact . Streaks and dark bands, which when present between 2 dense objects, create extinction or missing value artifacts. 2020-12-19 42

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44. 4. Patient motion artifacts Can cause misregistration of data which appear as double contours in the reconstructed image. Problem can be minimized by restraining the head and using a short scan time as possible . 2020-12-19 44

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46. Advantages of CBCT Less cost Less space required Rapid, quick scanning time Radiation dose reduction Image accuracy Reduced image artifacts Unlimited number of views Imaging can be obtained at any angle Superior representation of bony structure Powerful diagnostic 3D planning tool 2020-12-19 46

47. Disadvantages of CBCT Image noise- Because radiation from the source transmitted through tissue in the body, the receptor receives non uniform information from radiation scattered in many directions termed as noise . Noise is 0.05 to 0.15 with conventional CT and can be large as 0.4 to 2 in CBCT. Poor soft tissue contrast- Scattered radiation contributes to increases noise of the image which reduces the contrast of the cone beam system. 2020-12-19 47

48. Uses in dentistry Pre-operative evaluation for implant placement. Assessment of alveolar bone grafting before and after orthodontic treatment adjacent to cleft. Eruption of teeth at cleft site treated by bone grafts. Assessment of relation of roots of teeth with inferior alveolar canal and floor of maxillary antrum . To stimulate condylar growth,bone formation and orthognathic surgery. 2020-12-19 48

49. 3-D cephalometrics in orthodontics. Analysis of TMJ space and diagnosis of pathologies of TMJ. Localization of fractured teeth, benign calcifications and determining the size, shape and extent of various pathologies of head and neck region. Rapid Prototyping: Used to fabricate physical scale models directly from 3D computer assisted design data. 2020-12-19 49

50. Nerve mapping : provides the visualization of the inferior alveolar nerve inorder to reduce the rates of nerve damage after third molar (M3) removal and bilateral sagittal split osteotomy (BSSO). 2020-12-19 50

51. cbct Computed tomography(ct) Very thin slice of x-ray beam is used. Images of higher resolution. Involves high amount of radiation. Larger space is required. Better contrast,soft tissues are imaged better. Higher cost. Anisotropic. Not suited for implant imaging. Cone beam is used in CBCT. Images of lower resolution as compared to CT Involves lower amount of radiation as compared to CT. Smaller space is required Soft tissue imaging is not as good. Lesser cost. Voxel resolution-isotropic Ideal for implant imaging 2020-12-19 51

52. Conclusion CBCT is an advanced imaging modality that has high clinical applications in the field of dentistry . CBCT proved to be a successful investigative modality that has been used for dental and maxillofacial imaging with reduction in radiation dose. It is highly accurate and can provide a three dimensional volumetric data in axial, sagittal and coronal planes. 2020-12-19 52

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