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Cone beam computed tomography (2)
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- 2. CONTENTS INTRODUCTION PRINCIPLESOF CBCT IMAGING COMPONENTS OF IMAGE PRODUCTION CLINICAL CONSIDERATIONS IMAGE ARTIFACTS ADVANTAGES AND DISADVANTAGES APPLICATIONS CONCLUSION
- 3. INTRODUCTION CBCT IMAGING Mostsignificant technology advancement in maxillofacial imaging In this, the imaging shifts from 2D to a volumetric approach.
- 4. PRINCIPLES CBCT imagingis performed using a rotating platform carrying an x-ray source and detector
- 5. A divergentcone 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
- 6. • 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 • Usually several hundred 2-D basic images are formed from which the image volume is calculated. • Complete series of images is called PROJECTION DATA
- 7. Components of CBCT 1. IMAGE PRODUCTION 2. VISUALISATION 3. INTERPRETATION
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- 9. 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 - Chin cup OR Bite fork
- 10. CHIN CUP BITEFORK
- 11. X-ray generator X-ray generation continuousor 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.
- 12. Scan Volume /field of view(FOV) It is the amount of area to be exposed in a single scan DEPENDS ON - * Detector size and shape * Beam projection geometry * Ability to collimate the beam Shape – cylindrical or spherical
- 13. It isdesirable 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 quality.
- 14. CLASSIFICATION OF CBCTUNIT ACCORDING TO FOV
- 15. SCANNING OF ROIGREATER THAN FOV OF DETECTOR
- 16. Scan factor Numberof images forming the “projection data” throughout the scan is determined by- 1. Detector frame rate (no. of image acquired per sec.) 2. Completeness of the trajectory arc (180 to 360) 3. Rotation speed of source and detector
- 17. IMAGE DETECTOR Larger andbulkier Lighter in weight Circular basis image area Rectangular Spherical volume Clindrical volume Cesium iodide scintillator CBCT units Image intensifier tube/charge-coupled device(II/CCD) Flat panel detector(FPDs)
- 18. 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
- 19. 3. RECONSTRUCTION basis projectionframes 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
- 20. 2 STAGES OFRECONSTRUCTION PROCESS 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 The final image is constructed from the sinogram with a filtered back-projection algorithm.
- 21. CLINICAL CONSIDERATION 1.Patient selectioncriteria It provides a radiation dose to the patient higher than radiation dose of other dental radiograph When periapical or panoramic view cannot provide the necessary information Used as adjunctive diagnostic tool
- 22. 2. Patient preparation Appropriatepersonal 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
- 23. Patient motion canbe 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.
- 24. 3. Imaging protocol Develop to produce image of optimal quality with the least amount of radiation exposure to the patient PARAMETERS Exposure settings Spatial resolution Scan time and number of projections
- 25. 1. Exposure setting Quality and quantity of x-ray beam depend on i. Tube voltage(kVp) ii. Tube current( mA) CBCT unit manufacturers approach setting exposure in 2 ways- 1. Selection of fixed exposure setting 2. Allow operator manual adjustment of kVp or mA
- 26. 2. Spatial resolution Ability of an image to reveal fine detail Determined by i. Pixel size ii. Beam projection geometry iii. Patient scatter iv. Focal spot size v. Number of basis images vi. Reconstruction algorithm
- 27. 3. Scan timeand number of projection adjusting the detector frame rate increase the number of basis image projections reconstructed image with fewer artifacts and better image quality
- 28. 4. Archiving, export,and distribution Process of CBCT imaging produces 2 data products 1. Volumetric image data from the scan 2. Image report generated by the operator Both set of data must be archived and distributed However the export of image data is in Digital Imaging And Communications in Medicine standard version 3 (DICOM v3) file format.
- 29. IMAGE ARTIFACTS Anartifact is any distortion or error in the image Image artifacts Inherent Procedure related Introduced Patient motion artifact
- 30. 1. INHERENT ARTIFACTS Can arise from limitations in the physical processes Beam projection geometry, reduced trajectory rotational arcs, and image reconstruction methods produce 3 type of artifacts Scatter Partial volume averaging Cone beam effect
- 31. Scatter- Result fromx-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
- 33. Cone beam effect- Is a potential source of artifacts, especially in the peripheral portion of scan volume Can result in i. Image distortion ii. Greater peripheral noise Clinically, the effect can be reduced by positioning of ROI in the horizontal plane of x-ray beam.
- 34. 2. Procedure relatedartifacts 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:- 1. misregistration 2. noisier image which appear as fine striations in the image
- 37. 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 1. Distortion of metallic structure as a result of differential absorption, known as cupping artifact 2. Streaks and dark bands, which when present b/w 2 dense objects ,create extinction or missing value artifacts
- 39. 4. Patient motionartifacts 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
- 41. 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
- 42. DISADVANTAGES OF CBCT Imagenoise – 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 Poor soft tissue contrast- Scattered radiation contributes to increased noise of the image which reduces the contrast of the cone beam system
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- 44. IMPLANT SITE ASSESSMENT Provides cross section view of i. alveolar bone height, width, and angulations ii. accurate distance from vital structure such as inferior alveolar canal in mandible and maxillary sinus
- 45. ORTHODONTICS 1. Used inidentification of root resorption 2. Display of position of impacted or supernumerary teeth 3. Relation to adjacent structure 4. Cephalometric analysis
- 46. TMJ provide multiplaneror 3 D image of condyle and surrounding structures
- 47. MAXILLOFACIAL PATHOSIS Usefulin assessment of trauma Visualizing the extent and degree of involvement of benign odontogenic or non odontogenic as well as osteomyelitis
- 48. MANDIBULAR THIRD MOLAR POSITION To check the relationship of the third molar with the inferior alveolar canal during extraction To prevent the nerve damage
- 49. CONCLUSION CBCT imagingis an effective volumetric diagnostic imaging technology that produces accurate, high resolution images of diagnostic quality in formats enabling volumetric visualisationof the osseous structures of the maxillofacial region at lower doses and costs.
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