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Cbct

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Cbct

  1. 1. 1 CONE BEAM COMPUTED TOMOGRAPHY
  2. 2. CONTENTS 2  Introduction  Principles  Image Acquisition X-ray generation Image detection system Image reconstruction Image display  Clinical considerations  Imaging protocol  Comparison with CT  Artifacts  Applications in Dentistry
  3. 3. INTRODUCTION • It is also known as Dental volumetric tomography, Cone beam volumetric tomography, dental computed tomography and cone beam imaging. • A recent technology initially developed for angiography in 1982. • It is a digital analog of film tomography in a more exact way than is traditional CT • It uses a divergent or “cone“ shaped source of ionizing radiation (conical or pyramidal) and a 2D area detector fixed on a rotating gantry to acquire multiple sequential projection images in one complex scan around the area of interest. 3
  4. 4. • Since the late 1990s it is become possible to produce clinical system (inexpensive & small enough) 4 4
  5. 5. Principles of CBCT 5 5 • Round Cone shaped X-ray beam • 2- D area detector Combine with 3D x ray beam with circular collimation – cone shaped resultant beam • 360 0 rotation around the object – both source and detector mounted on a gantry Uses a cone shaped divergent beam of ionozing radiation like X-rays and a 2D area detector mounted on a rotated gantry to acquire multipalanar sequential projection images in one single scan around the area of interest Projections made in all planes at a time volumetric images obtained
  6. 6. 6 6 • X-ray beams attenuated by patient- detected by the receptor • Raw data assembled by computer algorithm • Generate cross sectional components of image called pixels • CBCT acquires volumetric data. Each unit is called a voxel. • Size of each voxel corresponds to size of pixel of the detector
  7. 7. IMAGE ACQUISITION • Rotation scan exceeding 1800 of an x ray source and area detector. • BASIS IMAGES – During the rotation, many exposures made at fixed interval, providing a single projection images. • The complete series of basis image is k/a PROJECTION DATA 100 – 600 images in single scan 7 7
  8. 8. • Software programs – backprojection filters are applied – to generate 3D volumetric data- reconstruction of images in 3 planes. 8
  9. 9. 4 components for CBCT acquisition: • X-ray generation • Image detection system • Image reconstruction • Image display 9 9
  10. 10. X –ray Generation • Single scan of the patient is made to acquire a data set. • Patient positioning • X-ray generator • Scan Volume • Scan factors 10 10
  11. 11. Patient Positioning 1. Supine 2. Standing Units 3. Seated units Immobilization of patients head is necessary 11 11 Equipment required Large surface area/ physical footprint Not for physically disabled patients Not able to adjust the height in wheelchair bounded patients Most comfortable Not for physically disabled
  12. 12. 12
  13. 13. 13
  14. 14. Upright patient loading and supine 14
  15. 15. X Ray Generator • Scan times are longer than panoramic  due to pulsed exposure. • So, Actual exposure time is markedly less than scanning time • ALARA – CBCT exposure factors should be adjusted on the basis of patient size.( Tube current , tube voltage or both ) • Automatic exposure control – Kvp and mA automatically modulated in near real time by feedback mechanism. 15 15
  16. 16. • Patient exposure depends upon : Presence of pulsed X ray beam Size of the image field 16 16
  17. 17. 17 Scan Volume
  18. 18. Scan Volume • Also called as field of view • It is the amount of area to be exposed in a single scan. Depends on: • Detector size • Geometry of beam projection • Collimation of the beam Shape – cylinder or Spherical Can be selected based on individual requirements. 18
  19. 19. 19
  20. 20. Scan Factors FRAME RATE: Speed with which the images are acquired. Projected images / second frame rate images acquired for reconstruction higher frame rate reduces metallic artifact. frame rate scanner time Patient dose 20 SCAN ARC: It is the trajectory of the scan or the path traveled in a single scan. It is usually 360 degrees. SCAN TIME : < 30 secs. Lesser the scan time , lesser will be the motion artifacts. (limiting factor in voxel resolution)
  21. 21. IMAGE DETECTION • Detection of X rays with an indirect detector • Large area solid state sensor coupled with scintilla layer (cesium iodide) 21 CBCT Image intensifier + charge coupled device Fiberoptic coupling Flat panel area detectors
  22. 22. DETECTORS The detector must be able to: – Record X ray photons – Read off and send signal to the computer – Be ready for the next acquisition many hundreds of times within the single rotation • Rotation is usually performed within times (10-30 seconds) which necessitates frame rate image acquisition times of milliseconds 22
  23. 23. – Flat detectors are composed of a large-area pixel array of hydrogenated amorphous silicon thin-film transistors. X rays are detected indirectly by means of a scintillator, such as terbium activated gadolinium oxysulphide or thallium- doped cesium iodide, which converts X rays into visible light that is subsequently registered in the photo diode array. 23
  24. 24. Grid distortion pattern produced by the image-intensifier detector that affects the image construction and is noted in the image display. When moving away from the center. 24
  25. 25. 25 Image receptor area receiving the signal from the flat-panel detector’s scintillator is flat. Therefore, even at more distant areas from the center of the grid, there is minimal to no distortion of the grid pattern.
  26. 26. 26
  27. 27. Advantage of flat panel detectors; • The configuration of such detectors is less complicated • Offers greater dynamic range and • Reduced peripheral distortion Disadvantage of flat panel detectors; • These detectors require a slightly greater radiation exposure. 27
  28. 28. VOXEL SIZE • Determinants of voxel size  Focal spot size determine degree of  X ray geometric configuration geo unsharpness  Matrix  Pixel size of solid state detector Object to detector distance Source to object – minimizes geometric unsharpness Source to object – magnified projected image. 28
  29. 29. GRAYSCALE • Ability of the panel to detect subtle contrast differences called as bit depth of the system. • CBCT units use detectors capable of recording grayscale differences of 12 bits or higher. 29
  30. 30. RECONSTRUCTION • Basis projection frames are process to create volumetric data set k/a primary reconstruction. • Single cone beam rotations < 30 sec • 100 – 600 individual projection frames • Data acquired by one computer then transfer to processing computer (workstation) • Reconstruction depends on : Acquisition parameters (voxel size, size of image field, no of projection Hardware Software 30
  31. 31. 31 RECONSTRUCTION PROCESS Once all slices have been reconstructed they combine into single volume of visualization
  32. 32. DISPLAY • The volumetric data set is a compilation of all available voxels. • Reconstruction of images – 3 orthogonal planes 32
  33. 33. 33 MULTIPLANAR REFORMATION Isotropic nature of volumetric data , nonaxial 2 dimension images refers as Multiplanar reformation. This includes : Oblique , curved planar reformation, serial transplanar reformation. Axial image – occlusal image MPR oblique curve line – panoramic Serial cross section 1 mm thick images
  34. 34. RAY SUM IMAGE 34 An axial projection use as reference image Correspond to mid sagittal plane Thickness of this increase due to right and left side of volumetric data set Thickness of the “slab” increases Anatomic noise
  35. 35. THREE DIMENSIONAL VOLUME RENDERING • A TECHNIQUE which allows the visualization of 3D data by integration of large volumes of adjacent voxels and selective display. INDIRECT VOLUME RENDERING Selection of intensity or density of grayscale levels of voxels to be displayed within an entire data set called as segmentation. Requires software Volumetric surface reconstruction with depth. 35
  36. 36. DIRECT VOLUME RENDERING • Simpler process • Maximum Intensity Projection (MIP) • MIP visualization – Evaluating each voxel value along an imaginary projection ray from observer’s eye within a particular volume of interest and represent the high value as a display value 36
  37. 37. CLINICAL CONSIDERATION • PATIENT SELECTION CRITERIA • PATIENT PREPARATION • IMAGING PROTOCOL • IMAGE OPTIMIZATION • REPORTS • ARCHIVING, EXPORT & DISTRIBUTION 37
  38. 38. PATIENT SELECTION CRITERIA • CBCT is more commonly used for diagnostic purpose. • Cone beam exposure is higher than other radiographs, there should be justification of the exposure to the patient so that the total potential diagnostic benefits are greater than individual detriment radiation exposure. 38
  39. 39. PATIENT PREPARATION • Personal radiation barrier protection- Acc to federal legislation- Lead torso apron Pregnant patients & children Highly recommended Lead thyroid collar (when not interfere with scan) • Head Stabilization Chin cups to posterior Lateral head supports Image quality degraded by head movement . 39
  40. 40. • Alignment of area of interest with x-ray beam is critical in imaging • Facial topographic reference planes (middle saggital , frankfort horizontal) or internal references (occlusal plane , palatal plane) aligned with external laser light position. 40
  41. 41. • Removal of metallic objects – eyeglasses, jewellery, metallic partial dentures • Not necessary to remove plastic completely removable prosthesis ( unless closed TMJ view or orthodontic view ) • Separate the dentition – tongue depressor , cotton roll This is useful in single arch scan where scatter from metallic restorations in the opposing arch can be reduced. • Direct the patient to remain still n breathe slowly through nose 41
  42. 42. IMAGING PROTOCOL • It is a set of technical exposure parameters • It is developed to produce images of optimal quality with the least amount of radiation exposure to the patient. 42
  43. 43. VOXEL SIZE • Voxel size with which projection images are acquired varies from manufacturer to manufacturer principally on the basis of matrix size of the detector and projection geometry. • Image detector collects information over a series of pixels in horizontal and vertical direction. • voxel size spatial resolution • But higher radiation dose required to the pixel fill factor. 43
  44. 44. SCAN TIME & NO OF PROJECTIONS 44 Limiting the irradiation field to fit the field of view with a reduced exposure dose to the patient and improved image quality because of reduced scattered radiation
  45. 45. IMAGE OPTIMIZATION • To optimize image presentation & facilitate diagnosis it is necessary to adjust contrast/window and brightness/level parameters to favor bony structures. • CBCT software have window/level presets • This is adjusted for each scan • Enhancement can perform by application of sharpening , filtering. 45
  46. 46. REPORTS • Interpreting the resultant volumetric data set: Series of images formatted to display/ image report Cognitive interpretation of the significance of image finding/ interpretive report 46
  47. 47. ARCHIVING, EXPORT,& DISTRIBUTION CBCT imaging produces 2 data products: • Volumetric image data from scan • Image report generated by operator Export of image data – DICOM( Digital Imaging and Communications in Medicine) file format is standard for use in specialized software. 47
  48. 48. ADVANTAGES OF CONE-BEAM CT IN DENTISTRY • Being considerably smaller, CBCT equipment has a greatly reduced physical footprint. • Is approximately one quarter to one fifth the cost of conventional CT. • CBCT provides images of highly contrasting structures and is therefore particularly well suited for the imaging of osseous structures of the craniofacial area. • Rapid Scan time 48
  49. 49. • Beam Limitation • Image accuracy • Reduced patient radiation dose • Interactive display modes applicable to maxillofacial imaging • Multiplanar reformation • Three dimensional Volume Rendering 49
  50. 50. LIMITATIONS OF CONE-BEAM CT IN DENTISTRY • X-ray beam artifacts • Patient related artifacts • Scanner-related artifacts • Cone beam related artifacts The beam projection geometry of the CBCT and the image reconstruction method produce three types of cone-beam related artifacts: (1) partial volume averaging. (2) undersampling (3) cone-beam effect. • Image noise • Poor tissue contrast 50
  51. 51. DIFFERENCE BETWEEN CONE BEAM CT AND MULTISLICE CT 51
  52. 52. 52 CONE BEAM CT MULTISLICE CT Image the whole area in one rotation, then reconstruct slices Image the patient in multiple slices Cone beam Geometry Fan beam Geometry Radiation Dose; 45-477µSv Radiation Dose; =2000µSv Operating voltage 80 – 120Kvp 80 – 140 Kvp Focal Spot size 0.5- 0.8mm 0.5 – 1.2mm 1-13% Annual Background radiation Dose =65% Annual Background radiation Dose Lesser cost Higher Cost Spatial resolution = 0.07-0.4 mm 5 lp/mm Spatial resolution = 0.3-0.4 mm 2-3 lp/mm Sections are not skipped, No loss of diagnostic information Sections may be skipped, diagnostic information may be lost if thicker sections are taken
  53. 53. 53 CONE BEAM CT MULTISLICE CT Soft tissue imaging is not as good Better contrast; soft tissues are imaged better Voxel dimension depends on pixel size on area detector Depends on slice thickness Voxel resolution – Isotropic Anisotropic Poor contrast resolution Good contrast resolution Not meant for imaging malignancy Ideal for malignancy as contrast radiology is very well imaged ; invasion into soft tissues is well detected Reduced artifacts from dental restorations Increased contrast; streaking artifacts are more marked Ideal for implant imaging Not suited for implant imaging The machine has a smaller size Larger machines
  54. 54. 55 IMAGE ARTIFACTS
  55. 55. 56 Artifacts Patient related Scanner related Cone beam related Acquisition
  56. 56. ACQUISITION ARTIFACTS 57 1. Beam hardening- As an x-ray beam passes through an object lower energy photons are absorbed in preference to higher energy photons. CUPPING ARTIFACT STREAKS & DARK BANDS
  57. 57. In clinical practice it is advisable to reduce field size , modify patient position , separate dental arches to avoid beam hardening 58
  58. 58. PATIENT RELATED ARTIFACTS • Patient motion – unsharpness in image reconstruction Minimize by restraining head • Remove metallic objects – to avoid beam hardening 59
  59. 59. 60Motion blur, double cortices
  60. 60. • Motion artifact from swallowing 61
  61. 61. ALAISING ARTIFACT / MOIRE PATTERN • Alaising artifacts appear as slightly wavy lines that diverge outwards toward the periphery of a cone beam image. • Cause – By undersampling of structures. • Related to the size of the dexels within the detector. • Dexels - measure the energy of the incident x-ray or light photons 62
  62. 62. 63
  63. 63. IMAGE NOISE • Random variation in the number of x-ray photons in the beam as it exits an object and strikes the image detector produces a grainy or mottle appearance within the image. • Inc voxel size reduces grainy app but spatial resolution and detection of small object reduced 64
  64. 64. SCANNER RELATED ARTIFACTS • Circular / ring steaks • Result from imperfections in scanner detection • Cause – repetitive reading at each angular position of detector. 65
  65. 65. CONE BEAM RELATED ARTIFACTS Beam projection geometry and image reconstruction causes these artifacts: 1. PARTIAL VOLUME AVERAGING – when selected voxel size of the scan is larger than the size of object being imaged. Eg. A voxel of 1mm in size on a side may contain both bone and soft tissue. Displayed pixel have different brightness value Boundaries of image – “step” appearance “Selection of smallest acquisition voxel “ 66
  66. 66. • 2. UNDERSAMPLING- Undersampling of the object can occur when too few basic projections are provided for image reconstruction. Reduced data sample leads to sharp edges, noisier images Fine striations in the image . Importance of this artifact is in diagnosis. 67
  67. 67. 3. CONE BEAM EFFECT • Potential source of artifacts • Seen in peripheral portions of scan volume Because of divergence of x ray beam as it rotates around the patient in horizontal plane, structures at top and bottom of the image field only be exposed ( x ray beam is in opposite side of patient) Peripheral area – less denser More image noise. 68
  68. 68. • Results – image distortion, streaking artifacts , greater peripheral noise . To minimize – Positioning the ROI in horizontal plane of the x ray beam. 69
  69. 69. APPLICATIONS IN DENTISTRY 70
  70. 70. • CBCT had a substantial impact on maxillofacial imaging. • Applied to diagnosis in all areas of dentistry & now into treatment application. 71
  71. 71. INDICATIONS • Implant site assessment • Extension of pathologies • Bone quality • Maxillary sinus • TMJ • Incisive foramen • Mandibular canal • Diagnostic requirements in endodontics, orthodontics, periodontics, maxillofacial surgery 72
  72. 72. IMPLANT SITE ASSESSMENT 73 Cross sectional images of alveolar bone height, width and angulation Accurately depicts vital structures Useful series of image – axial , reformatted panoramic & serial transplaner images
  73. 73. PREOPERATIVE IMPLANT PLANNING 74
  74. 74. A diagnostic stent is made with radiographic markers and inserted at the time of scan DICOM data imported to third party software application Assess and plan surgical & prosthetic components 75
  75. 75. ORTHODONTICS & 3D CEPHALOMETRY • In diagnosis, assessment & analysis of maxillofacial orthodontic & orthopedic anomalies. • Palatal morphological features & dimensions • Tooth inclination, torque, root resorption, alveolar bone width 76
  76. 76. TMJ and pharangeal airway space visualization Ray sum technique – provide both conventional two & three dimension cephalometric image (simulated panoramic, lateral, submentovertex, posteroanterior cephalometric images) 77
  77. 77. 78
  78. 78. 3D cephalometry : Dentaloskeletal relationships Facial esthetics Potential for growth & development 79
  79. 79. LOCALIZATION OF INFERIOR ALVEOLAR CANAL 80 Accurate assessment of the position of canal reduce injury to the nerve while 3 molar surgeries . Panoramic imaging is adequate but in case of superimposition 3D imaging is advisable
  80. 80. TEMPOROMANDIBULAR JOINT • Diagnosis of bone morphologic features, joint space and dynamic functions. • Degenerative joint disease • Developmental anomaly of condyle • Ankylosis • Rheumatoid arthritis 81
  81. 81. 82
  82. 82. MAXILLOFACIAL COMPLEX • Impacted canine , supernumerary teeth, fractured or split teeth, periapical lesions , periodontal disease, 83
  83. 83. Fracture , widening of PDL space – suggestive of tooth subluxation 84
  84. 84. Benign calcifications (tonsiloliths , lymphnodes, salivary gland stones) Phlebolith Useful for trauma Osteomyelitis – extent & degree of involvement 85
  85. 85. IN ENDODONTICS 86 2D image True extent of lesion
  86. 86. 87
  87. 87. IN PERIODONTICS 88 Extent of the lesion in facio-lingual and axial view
  88. 88. 89
  89. 89. RAPID PROTOTYPING • A group of related processes and technique that are used to fabricate physical scale models directly from 3D computer assisted design data. • It creates life size, dimensionally accurate model of anatomic structures k/a biomodels. 90
  90. 90. • DICOM data imported to proprietary software can be used to compute 3D images generated by voxel values which are segmented from the background. • Models produced used for presurgical planning for the cases caused by trauma, tumor resection, distraction osteogenesis, dental implants • Reduces surgical and anesthetic time. 91
  91. 91. References • Oral Radiology : Principles and Interpretation. 5th ed. Stuart C. White & Michael J. Pharoah. • Dental Applications of Computerized Tomography. Stephen L . G. Rothman • Fundamentals of Special Radiographic Procedures.5th ed. Albert M. Snopek. • Christensen’s physics of Diagnostic Radiology.4 th edition. Thomas S. Curry, III , James E.Dowdey, Robert C.Murry, JR. • Dental Radiography, Principles and Techniques.2 nd edition.Joen Iannucci Haring, Laura Jansen. • The efficiency of a computerized caries detector in Intraoral Digital Radiography JADA 133 (7) 183-90 July 2002. 92
  92. 92. • Dental Radiography- Haring Jansen. • Does digital Radiography increases the number of intraoral radiographs. 2003. Dento Maxillofacial Radiology ;32 (2); 124-7. • Randolph Todd, Cone Beam Computed Tomography Updated Technology for Endodontic Diagnosis. 2014;Dent Clin N Am 58;523–543. • Scott R. Makins,Artifacts Interfering with Interpretation of Cone Beam Computed Tomography Images.2014; Dent Clin N Am 58;485–495 • Kenneth Abramovitch,Dwight D. Rice;Basic Principles of Cone Beam Computed Tomography.2014; Dent Clin N Am 58 ;463–484 • M. Loubelea et al , Comparison between effective radiation dose of CBCT and MSCT scanners for dentomaxillofacial application.2008; European Journal of Radiology. • Scott R. Makins, Artifacts Interfering with Interpretation of Cone Beam Computed Tomography Images. 2014;Dent Clin N Am 58 ;485–495 93
  93. 93. THANK YOU 94
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