Cone beam computed tomography

5 minute guide to know CBCT

1. CONE BEAM COMPUTED TOMOGRAPHY VASHI NARULA FINAL YEAR

2. INTRODUCTION CBCT IMAGING Most significant technology advancement in maxillofacial imaging Shift from 2 D to a volumetric approach.

3. PRINCIPLES CBCT imaging is performed using a rotating platform carrying an x-ray source and detector

4. 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

5. 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 form raw primary data, which is individually known as - basis, frame or raw image Ususally several hundred 2-D basic images from which the image volume is calculated. Complete series of images is called PROJECTION DATA

6. Frame Axis

7. COMPONENTS OF IMAGE PRODUCTION

8. X-RAY GENERATION Patient Stabilization Sitting , Standing, Supine With all system, immobilization of the patient’s head is more important than position because any movement degrades the final image Immobilization of head by - 1) Chin cup 2) bite fork 3) other head –restraint mechanism

9. CHIN CUP BITE FORK

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

11. Scan Volume/field of view(FOV) * Detector size *Shape * Beam projection geometry * Ability to collimate the beam

12. 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 image qualit y

13. CLASSIFICATION OF CBCT UNIT ACCORDING TO FOV

14. SCANNING OF ROI GREATER THAN FOV OF DETECTOR

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

16. IMAGE DETECTOR larger and bulkier lighter in weight circular basis image area rectangular spherical volume cylindrical vol. cesium iodide scintillator

17. 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

18. 3. 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

19. 2 STAGES OF RECONSTRUCTION PROCESS Preprocessing stage- performed at acquisition computer Inherent pixel imperfections should be corrected Exposure normalization Reconstruction stage- Corrected images are converted into a special representation called a sinogram Sinogram is a composite image developed from multiple projection images

20. CLINICAL CONSIDERATION 1.Patient selection criteria It provides a radiation dose to the patient higher than radiation dose of other dental radiograph When periapical or paranomic cannot provide the necessary information Used as adjunctive diagnostic tool

21. 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

22. 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.

23. 3. Imaging protocol Develop to produce image of optimal quality with the least amount of radiation exposure to the patient

24. 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

25. Spatial resolution Ability of an image to reveal fine detail Determined by P ixel size Beam projection geometry Patient scatter Focal spot size Number of basis image Reconstruction algorithm

26. 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

27. 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

28. IMAGE ARTIFACTS An artifact is any distortion or error in the image

29. 1. INHERENT ARTIFACTS Can arise from limitations in the physical processes Beam projection geometry, reduced trajectory rotational arcs, and image reconstruction can produce 3 type of artifacts

30. Scatter - Result from x-ray photons that are diffracted from their original path after interaction with matter Partial volume averaging- It occur when the selected voxel size of the scan is larger than the size of the object being imaged

31. Cone beam 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.

32. 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 which appear as fine striations in the image

33. 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 b/w 2 dense objects ,create extinction or missing value artifacts

34. 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

35. 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

36. 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 as large as 0.4 to 2 in CBCT Poor soft tissue contrast- Scattered radiation contributes to increased noise of the image which reduces the contrast of the cone beam system

37. APPLICATIONS

38. IMPLANT SITE ASSESMENT Provides cross section view of alveolar bone height, width, and angulations accurate distance from vital structure such as inferior alveolar canal in mandible and maxillary sinus

39. ORTHODONTICS Used in identification of root resorption Display of position of impacted or supernumerary teeth Relation to adjacent structure Cephalometric analysis

40. TMJ provide multiplaner or 3 D image of condyle and surrounding structures

41. MAXILLOFACIAL PATHOSIS Useful in assessment of trauma Visualizing the extent and degree of involvement of benign odontogenic or non odontogenic as well as osteomyelitis

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