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 CBCT
Image
production
Visualization Interpretation

8. COMPONENTS OF IMAGE
PRODUCTION
X-ray
generation
X-ray
detection
Image
reconstruction

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 -
1) Chin cup
2) bite fork
3) other head –restraint mechanism

10. CHIN CUP
BITE
FORK

11. 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.

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

13. • 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 quality

14. CLASSIFICATION OF CBCT UNIT
ACCORDING TO FOV

15. SCANNING OF ROI GREATER THAN FOV OF
DETECTOR

16. Scan factor
• Number of 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 and bulkier lighter in weight
circular basis image area rectangular
spherical volume cylindrical vol.
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 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

20. 2 STAGES OF RECONSTRUCTION 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

21. 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

22. 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

23. • 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.

24. 3. Imaging protocol
• Develop to produce image of optimal quality with
the least amount of radiation exposure to the
patient
Exposure setting
Spatial resolution

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 image
vi. Reconstruction algorithm

27. 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

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

29. IMAGE ARTIFACTS
• An artifact 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 can
produce 3 type of artifacts
scatter
Partial volume averaging
Cone beam effect

31. 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

32. 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.

33. 2. 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:-
1. misregistration
2. noisier image
• which appear as fine striations in the image

34. 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

36. 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

37. 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

38. 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

39. APPLICATIONS

40. IMPLANT SITE ASSESMENT
• 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

41. ORTHODONTICS
1. Used in identification of root resorption
2. Display of position of impacted or supernumerary
teeth
3. Relation to adjacent structure
4. Cephalometric analysis

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

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

44. THANK YOU