CBCT basic principle,artifacts,landmarks.pptx

cbct basics

1. CONE-BEAM COMPUTED TOMOGRAPHY Presented by, Dr. Nitha Willy Second year post graduate Department of Oral Medicine and Radiology

2. Also known as : Dental Volumetric Tomography Cone-beam Volumetric Tomography Dental Computed Tomography Cone-beam Imaging Cone beam computed tomography (CBCT) was discovered in Italy in 1997. The first unit created was the NewTom. INTRODUCTION

3. PRINCIPLES OF CONE-BEAM COMPUTED TOMOGRAPHY Uses a cone shaped divergent beam of ionizing radiation like X-rays and a 2D area detector mounted on a rotated gantry to acquire multiplanar sequential projection images in one single scan around the area of interest Multiple planar projections are acquired by rotational scan to produce a volumetric dataset from which interrelational images can be generated .

4. The technical elements for optimal imaging in CBCT comprise a sequence of discrete but interrelated processes, referred to as the imaging chain . These include: • A source of X-radiation. • The object to be imaged • A device capable of detecting the remnant radiation after attenuation by the object. • A mechanism to archive the resultant image. • A method to retrieve and display the image.

5. STAGES OF CBCT IMAGING Image acquisition : X-ray generation & Image detection system Image reconstruction: primary and secondary rconstruction Image display

6. IMAGE ACQUISITION

7. X-ray Generation X-ray Source

8. PROJECTION GEOMETRY

9. The beam is collimated as a cone or pyramid, and a 2D detector array is used to capture the raw data; thus, a single rotation suffices to reconstruct the FOV.

10. During the rotation, many exposures are made at fixed intervals, providing single projection images known as basis images . The complete series of basis images is referred to as the projection data .

11. 11 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.

12. IMAGE DETECTOR CBCT units use either an image intensifier (II) or one of several types of flat panel detectors (FPD) as the image detector II Based systems Indirect FPD systems Direct FPD systems 2. IMAGE DETECTION SYSTEM

13. II BASED SYSTEMS The attenuated X-ray beam is converted into electrons These electrons are then amplified before being reconverted into photons, which are recorded using a charge-coupled device (CCD). These systems tend to be relatively large and most frequently result in circular basis image areas (spherical volumes) rather than rectangular ones (cylindrical volumes). They are prone to geometric distortion and have a relatively narrow dynamic range

14. INDIRECT FPD SYSTEMS An indirect system consists of two components: a scintillator medium which converts X-ray radiation into visible light a photon detector which converts light into an electrical signal, which can then be digitized

15. DIRECT FPD SYSTEMS FPD systems currently comprise an amorphous selenium (a-Se) Cadmium telluride ( CdTe ) cadmium zinc telluride ( CdZnTe ) photoconductor which converts X-ray photons into an electrical charge, directly connected to a TFT or CMOS panel.

16. For each projection image during acquisition, the detector receives incident X-ray photons, collects a charge proportionally to the X-ray intensity at a given point, and sends a signal to the computer. The speed with which a detector performs this acquisition is called the frame rate. FRAME RATE

17. Frame rate is measured in frames, projected images, per second. The maximum frame rate of the detector and rotational speed determines the number of projections that may be acquired. The number of projection images comprising a single scan may be fixed or variable.

18. With a higher frame rate More information to reconstruct the image; therefore, primary reconstruction time is increased. Increase the signal-to-noise ratio, producing images with less noise. In the maxillofacial region, it reduces metallic artifact. Usually accomplished with a longer scan time and hence higher patient dose.

19. VOXELS Voxels are three-dimensional data blocks that representing a specific x-ray absorption. CBCT units capture isotropic voxels. An isotropic voxel is equal in all three dimensions (x, y, and z planes) producing higher resolution images. The voxel sizes currently available in CBCT units range from 0.076 mm to 0.4 mm.

20. Resolution of the final image is determined by the unit’s voxel size. The smaller the voxel size the higher the resolution. The higher the resolution, the higher the radiation dose to the patient as well.

21. The principal determinants of nominal voxel size in CBCT are the x-ray tube focal spot size x-ray geometric configuration the matrix and pixel size of the solid state detector

22. CBCT images are reconstructed as a 3D stack of voxels, in which each voxel is assigned a grey value (i.e., a whole number) according to its estimated X-ray attenuation. A lower grey value corresponds to a lower attenuation, with the lowest grey values corresponding to air. GREY VALUES

23. The ability of CBCT to display differences in attenuation is related to the ability of the detector to detect subtle contrast differences. This parameter is called the bit depth of the system and determines the number of shades of gray available to display the attenuation. All available CBCT units used detectors capable of recording grayscale differences of 12 bits or higher. If a 12-bit detector is used to define the scale, 4096 shades are available to display contrast.

24. DATA ACQUISITION The patient is positioned with the unit. The equipment orbits around the patient in a 180°, 270° or 360° rotation, taking approximately 5–40 seconds, and in one cycle or scan, images a cylindrical or spherical volume referred to as the field of view (FOV). As all the information/data is obtained in the single scan, the patient must remain stationary throughout the exposure.

25. IMAGE RECONSTRUCTION The reconstruction process consists of: Primary reconstruction Secondary or multiplanar reconstruction

26. The most widely used reconstruction algorithm in CBCT is the Feldkamp (FDK) algorithm, which is a modified filtered backprojection (FBP) method. When a larger number of projection angles are combined, the reconstruction represents the original object more accurately .

27. To facilitate reconstruction , data is often acquired by one computer (acquisition computer) and transferred via ethernet connection to a second computer (workstation) for processing. Reconstruction times vary depending on the Acquisition parameters (voxel size, FOV, number of projections) Hardware (processing speed, data put from acquisition to workstation computer) Software (pre- and post-processing, reconstruction algorithm) used. Reconstruction should be accomplished in usually less than 5 min

28. PRIMARY RECONSTRUCTION Having obtained data from the one scan, the computer then divides the volume into tiny cubes or voxels (ranging in size between 0.076 mm3 and 0.4 mm3) and calculates the X-ray absorption in each voxel. Each voxel is allocated a number and then allocated a colour from the grey scale from black through to white. Typically one scan contains over 100 million voxels.

29. SECONDARY OR MULTIPLANAR RECONSTRUCTION

30. The axial plane (X) is a horizontal plane that divides the anatomical features within the FOV into superior and inferior slices The coronal plane (Y) is a vertical plane that divides the anatomical features within the FOV into anterior and posterior slices The sagittal plane (Z) is also a vertical plane that divides the anatomical features within the FOV into right and left slices

31. AXIAL Images in the axial orthogonal plane demonstrate a continuum of anatomy extending from the supraorbital region of the frontal bone to the hyoid bone and vertebral bodies of the third cervical vertebrae.

32. CORONAL Images in the coronal orthogonal plane demonstrate facial and cranial anatomy extending from supraciliary arches of the frontal bone and mental protuberance of the mandibular symphysis to the occipital condyles of the occipital bone and anterior processes of the cervical vertebrae.

33. SAGITTAL Images in the sagittal orthogonal plane demonstrate a continuum of anatomy extending from the midline of the nasal cavity and cranial base laterally to the mastoid sinuses and glenoid fossa of the temporal bone and condylar head of the mandible.

34. Following the primary reconstruction, the computer software then allows the operator to select voxels in the three anatomical orthogonal planes to create sagittal, coronal or axial images A set of sagittal, coronal and axial images appear simultaneously on the computer monitor. The software then enables these image data sets to be scrolled through in real time. For example, by selecting and moving the horizontal cursor up and down on the coronal image, all the axial images can be scrolled through from top to bottom.

35. VOLUME ACQUISITION CBCT units can be assigned into four broad categories based on the vertical and horizontal dimensions of the FOV : • Large (Maxillofacial) • Dentoalveolar (both jaws) • Single jaw/dual TMJ • Small (localized)

36. Image Quality Image quality can be described using four fundamental parameters: Spatial resolution Contrast resolution Noise Artifacts

37. IMAGE DISPLAY There are four independent operations involved in image display for CBCT (Udupa 1999): Reconstruction Visualization Post-processing- Image enhancement and image manipulations Analysis

38. VISUALIZATION The images from CBCT reconstruction are optimized to facilitate visual display by various rendition techniques in both 2- and 3-D

39. POST-PROCESSING In this operation, an observer interacts with the image to alter the representation of features within the image dataset. This involves specific image enhancement techniques.

40. ANALYSIS The assessment of various image characteristics is performed to provide quantitative information from the dataset

41. TECHNIQUE AND POSITIONING Patient preparation ● Patients should be asked to remove any earrings, jewellery , hair pins, spectacles, dentures or orthodontic appliances. ● The procedure and equipment movements should be explained to reassure patients and the importance of remaining stationary throughout the scan should be stressed.

42. EQUIPMENT PREPARATION ● The smallest volume size needed to answer the clinical question should be used to reduce the radiation dose to the patient. Using a smaller volume reduces scatter and potentially improves image quality. ● Optimal exposure factors should be selected to satisfy the diagnostic requirements of the examination. Higher exposure factors may be chosen if a higher spatial resolution is required.

43. ● Optimal reconstructed voxel size should be selected. If choosing a larger voxel size results in a reduced patient dose (due to lower exposure factors being used) then this should be considered as long as the lower resolution is compatible with the aims of the radiographic examination. ● Some machines offer a ‘quick scan’ where the rotation arc is reduced. This feature reduces the number of projections taken and therefore reduces the dose. If the required diagnostic information can be obtained using this scan protocol then it should be selected.

44. Patient positioning The patient should be positioned using the manufacturer’s guidelines to ensure that the correct region of interest is captured. A scout view may be useful to ensure the right part of the jaw is imaged Once positioned correctly, using the light beam markers, immobilization chin cups and head straps must be used to prevent any patient movement There is no need for the routine use of a protective lead apron.

45. There is no need for the routine use of a protective thyroid collar as the thyroid gland does not normally lie in the primary beam, however its use should be considered on a case by case basis particularly in children. If used, it must be positioned so that it does not interfere with the primary beam since this could lead to significant artefacts.

46. ARTIFACTS

47. Artifacts can be classified based on Their appearance in the image : Streaks Shadings rings/bands miscellaneous

48. According to where they occur in the imaging chain image acquisition patient-related artifacts The scanner itself T he beam projection geometry

49. Appearance of artifact Definition Possible causes Streaks Intense straight lines (dark or bright) across the image Aliasing, partial volume, motion, metal, beam hardening, noise, mechanical failure Shadings Dark or bright areas, particularly near objects of high contrast Partial volume, beam hardening, scatter radiation, incomplete projections Rings/bands Rings (full or arcs) or bands superimposed on the image Calibration error, crude interpolation in the reconstruction, offset projection Miscellaneous Cupping, densitometric inaccuracy Beam hardening, scatter radiation, reconstruction algorithm

50. IMAGE ACQUISITION ARTIFACTS X-ray beam related Scatter Beam hardening Cupping Streaks and dark bands Extinction Exponential edge gradient effect

51. PATIENT-RELATED ARTIFACTS Patient motion during the CBCT gantry rotation can cause misregistration of data, and most visibly appear as a “double contour” Subtler motion artifacts presenting as image unsharpness and loss of resolution may also originate. Motion blur is a well-known artifact that, in the case of MDC, led to the development of ultrafast acquisition times below 1 s

52. SCANNER-RELATED ARTIFACTS Scanner-related artifacts present as circular or concentric dark rings in the axial plane centered about the location of the axis of rotation. Partial Volume Effect Sampling Artifacts Cone Beam Effect Local Tomography Offset Projection

53. Advantages of Three-Dimensional Digital Imaging Lower radiation dose Brief scanning time Anatomically accurate images Ability to save and easily transport images Very good spatial resolution Compatible with implant and cephalometric planning software.

54. Disadvantages The patient has to remain absolutely stationary throughout the scan to avoid movement artefacts Soft tissues not imaged in detail Computer constructed panoramic type images are not directly comparable with conventional panoramic radiographs – particular care is needed in their interpretation Radiodense objects such as restorations and root filling materials can produce artefacts

55. REFERENCES Maxillofacial Cone Beam Computed Tomography- Principles, Techniques and Clinical Applications: Scarfe Eric Whaites Nicholas Drage Essentials of dental radiography and dental radiology Interpretation basics of cone beam computed tomography Dental Radiography- principles and techniques: Lannucci and Howerton-5 th edition Textbook of dental and maxillofacial radiology- Freny radiology- 2 nd edition White & Pharoah: 8 th edition

56. NORMAL ANATOMICAL LANDMARKS

57. AT THE SUPRA-ORBITAL LEVEL

58. AT THE SUPERIOR-ORBITAL LEVEL

59. AT THE MID-ORBITAL LEVEL

60. AT THE INFERIOR ORBITAL LEVEL

61. AT THE ZYGOMATIC ARCH LEVEL

62. AT THE MID MAXILLARY SINUS LEVEL

63. AT THE LEVEL OF THE SUPERIOR PORTION OF THE MAXILLARY ALVEOLUS

64. AT THE LEVEL OF THE MANDIBULAR FORAMEN

65. AT THE LEVEL OF THE ALVEOLAR PROCESS OF THE MANDIBLE

66. AT THE LEVEL OF THE MENTAL FORAMEN

67. AT THE LEVEL OF THE LOWER BORDER OF THE ANTERIOR MANDIBULAR SYMPHYSIS

68. AT THE LEVEL OF THE FRONTAL SINUS AND ANTERIOR TEETH

69. AT THE LEVEL OF THE ANTERIOR MAXILLARY SINUS AND PREMOLAR TEETH

70. AT THE LEVEL OF THE ANTERIOR ORBITAL RIM AND ZYGOMATIC BONE

71. AT THE LEVEL OF THE ZYGOMATIC ARCH AND POSTERIOR TEETH

72. AT THE LEVEL OF THE MIDDLE OF THE MAXILLARY SINUS

73. AT THE LEVEL OF THE MIDDLE OF THE ZYGOMATIC ARCH AND THIRD MOLARS

74. AT THE LEVEL OF THE ANTERIOR SPHENOID SINUS, MAXILLARY TUBEROSITY AND THIRD MOLARS

75. AT THE LEVEL OF THE CORONOID PROCESS AND PTERYGOID PLATES

76. AT THE LEVEL OF THE MANDIBULAR CONDYLES

77. AT THE LEVEL OF THE BODY OF C1 AND ODONTOID PROCESS OF C2 OF THE UPPER CERVICAL SPINE

78. MIDSAGITTAL ORTHOGONAL CBCT IMAGE

79. AT THE LEVEL OF THE MIDDLE OF THE RIGHT NASAL APERTURE

80. AT THE LEVEL OF THE NASOLACRIMAL DUCT

81. AT THE LEVEL OF THE MEDIAL WALL OF THE RIGHT MAXILLARY SINUS

82. AT THE LEVEL OF THE MEDIAL WALL OF THE RIGHT MAXILLARY SINUS

83. AT THE LEVEL OF THE RIGHT INFRAORBITAL FORAMEN