The document discusses Cone-Beam Computed Tomography (CBCT), detailing its principles, image acquisition, detection systems, and reconstruction processes. It highlights the technical elements required for optimal imaging, the various artifacts that can occur, and the advantages and disadvantages of using CBCT in dental radiology. Additionally, it emphasizes the importance of patient positioning and equipment preparation to improve imaging quality and minimize radiation exposure.

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
1. Image acquisition : X-ray generation & Image detection system
2. Image reconstruction: primary and secondary rconstruction
3. Image display

6. IMAGE ACQUISITION

7. 1. 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:
1. a scintillator medium which converts X-ray radiation into visible light
2. 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

32. 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.

33. 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.

34. 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.

35.  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.

36. 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)

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

38. 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

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

40. 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.

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

42. 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.

43. 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.

44. ● 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.

45. 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.

46. • 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.

47. ARTIFACTS

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

49. 2. According to where they occur in the imaging chain
• image acquisition
• patient-related artifacts
• The scanner itself
• The beam projection geometry

50. 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

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

52. 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

54. 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

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

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

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

59. NORMALANATOMICAL LANDMARKS

60. AT THE SUPRA-
ORBITAL LEVEL

61. AT THE SUPERIOR-
ORBITAL LEVEL

62. AT THE MID-ORBITAL
LEVEL

63. AT THE INFERIOR
ORBITAL LEVEL

64. AT THE ZYGOMATIC
ARCH LEVEL

65. AT THE MID MAXILLARY
SINUS LEVEL

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

67. AT THE LEVEL OF THE
MANDIBULAR FORAMEN

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

69. AT THE LEVEL OF THE
MENTAL FORAMEN

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

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

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

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

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

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

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

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

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

79. AT THE LEVEL OF THE
MANDIBULAR CONDYLES

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

81. MIDSAGITTAL
ORTHOGONAL CBCT
IMAGE

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

83. AT THE LEVEL OF THE
NASOLACRIMAL DUCT

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

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

86. AT THE LEVEL OF THE
RIGHT INFRAORBITAL
FORAMEN