Cone beam ct

 Introduction
 Principles
 Image Acquisition
X-ray generation
Image detection system
Image reconstruction
Image display
 Clinical consideration
 Imaging protocol
 Comparison with CT
 Artifacts
 Applications in Dentistry
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 It is also known as Dental computed tomography,
Cone beam volumetric tomography, Volumetric
computed tomography or Cone beam imaging.
 A recent technology initially developed for
angiography in 1982.
 It uses cone shaped beam of X-ray photons which
can scan the region of interest in a single 360⁰
rotation.
 Most significant technology advancement in
maxillofacial imaging.
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 CBCT imaging is performed using a rotating
platform carrying an x-ray source and detector.
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 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.
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 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 from raw primary data,
which is individually known as basis frame or raw
image.
 Usually several hundred 2- D basic images from
which the image volume is calculated.
 Complete series of images is called Projection data.
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 X-ray generation
 X-ray detection
 Image reconstruction
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Patient stabilization
 Depending on the unit, CBCT examinations are
made with patient sitting, standing and supine .
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 With all system, immobilization of the
patients 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
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X-ray generator
X-ray generation
continuous or pulsed
 When pulsed- exposure time is upto 50% less than
scanning time(this technique reduces patients
radiation dose)
 ALARA (As Low As Reasonable Achievable)
principle dose optimization states that CBCT
exposure factor should be adjusted on the basis of
patients size.
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Scan volume /field of view(FOV)
 Detector size
 Shape
 Beam projection geometry
 Ability to collimate the beam
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 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 the image quality.
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Scan factor
 Number of images forming the “projection data”
throughout the scan is determined by-
1. Detector frame rate(no. of image acquired per
second).
2. Completeness of the trajectory arc (180 to 360)
3. Rotation speed of source and detector
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 Divided into two groups based on detector type:
1. Image intensifier tube/charge- coupled device
 Larger and bulkier and result in circular basis image area
spherical volume
2. Flat panel detector (FPDs)
 Lighter in weight ,rectangular cylindrical volume, consists of
cesium iodide scintillator applied to a thin film transistor
made of amorphous silicon.
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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
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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
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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.
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1.Patient selection criteria
• It provides a radiation dose higher than other
dental radiographic procedures.
• Panoramic and periapical view cannot
provide necessary information for patient
diagnosis and treatment.
• Used as adjunctive diagnostic tool.
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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
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• 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.
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3. Imaging protocol
• Develop to produce image of optimal quality
with the least amount of radiation exposure to
the patient.
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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:
i. Selection of fixed exposure setting
ii. Allow operator manual adjustment of kVp or mA
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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
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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
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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.
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 An artifact is any distortion or error in the image.
 Artifacts can be classified according to their etiology.
 Inherent
 Procedure related
 Introduced
 Patient motion artifact
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1.Inherent artifacts
• Can arise from limitations in the physical processes
involved in the acquisition of CBCT data.
• Beam projection geometry of CBCT, reduced
trajectory rotational arcs, and image reconstruction
methods produce 3 type of artifacts.
i. Scatter
ii. Partial volume averaging
iii. Cone beam effect
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Scatter-
• Result from x-ray photons that are diffracted from
their original path after interaction with matter.
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Partial volume averaging-
• It occur when the selected voxel size of the
scan is larger than the size of the object being
imaged.
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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.
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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
3. Sharp edges
• which appear as fine striations in the image.
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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
between 2 dense objects, create extinction or
missing value artifacts.
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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.
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 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
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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 large as 0.4 to 2 in CBCT.
Poor soft tissue contrast-
• Scattered radiation contributes to increases noise of
the image which reduces the contrast of the cone
beam system.
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 Pre-operative evaluation for implant placement.
 Assessment of alveolar bone grafting before and after
orthodontic treatment adjacent to cleft.
 Eruption of teeth at cleft site treated by bone grafts.
 Assessment of relation of roots of teeth with inferior
alveolar canal and floor of maxillary antrum.
 To stimulate condylar growth,bone formation and
orthognathic surgery.
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 3-D cephalometrics in orthodontics.
 Analysis of TMJ space and diagnosis of pathologies
of TMJ.
 Localization of fractured teeth, benign calcifications
and determining the size, shape and extent of various
pathologies of head and neck region.
 Rapid Prototyping: Used to fabricate physical scale
models directly from 3D computer assisted design
data.
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 Nerve mapping: provides the visualization of
the inferior alveolar nerve inorder to reduce
the rates of nerve damage after third molar
(M3) removal and bilateral sagittal split
osteotomy (BSSO).
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CBCT
COMPUTED
TOMOGRAPHY(CT)
 Very thin slice of x-ray beam
is used.
 Images of higher resolution.
 Involves high amount of
radiation.
 Larger space is required.
 Better contrast,soft tissues are
imaged better.
 Higher cost.
 Anisotropic.
 Not suited for implant
imaging.
 Cone beam is used in CBCT.
 Images of lower resolution as
compared to CT
 Involves lower amount of
radiation as compared to CT.
 Smaller space is required
 Soft tissue imaging is not as
good.
 Lesser cost.
 Voxel resolution-isotropic
 Ideal for implant imaging
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 CBCT is an advanced imaging modality that has high
clinical applications in the field of dentistry. CBCT
proved to be a successful investigative modality that
has been used for dental and maxillofacial imaging
with reduction in radiation dose. It is highly accurate
and can provide a three dimensional volumetric data
in axial, sagittal and coronal planes.
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Cone beam ct

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