cbct

Question 1

what are the componnts in the cbct machine

Answer 1

head stabiliser
detector
x ray source
control panel

Question 2

cbct principles

Answer 2

x ray source and detector rotate 180/360º around the patient’s head. rotate around rotation centre which is fixed point in the centre of the region of interest

beam is cone shaped, directed through region of interest

hundreds of 2d basis projection images taken of slightly different projections

Back projection image reconstruction using algorithm eg filtered back projection to construct 3d cbct volume

Question 3

how does back projection image reconstruction in cbct work

Answer 3

create attenuation profiles of each basis projection image. attenuation profiles are added together and filters are added

Question 4

strengths of cbct

Answer 4

3 dimensional, able to provide buccal lingual relationship

able to provide more information about spatial relationship

no superimposition so can localise entities more accurately

no distortion/magnification hence measurements more accurate

Question 5

limitations of cbct

Answer 5

increased radiation exposure, increased risk of radiation induced cancer.
15x more radiation exposure than panoramic for similar area of interest (12 vs 177 usv)

poor soft tissue contrast, cannot get good soft tissue information. hounsfields unit not reliable on cbct

metal streak/beam hardening effect which can make diagnosis hard

patient motion artefacts. long duration of cbct scan, patient has to stay very still

Question 6

what gives rise to metal streaking and beam hardening effect on cbct

Answer 6

metal restorations/crowns, zirconium dioxide and titanium implants are very dense, absorb x ray beams as they pass through

metal streaking due to complete absorption of xray, leading to white streaks/splatter

beam hardening as low energy photons are absorbed, leaving behind high energy photons. this causes formation of dark bands as remnent beam energy veru high

Question 7

Indications for CBCT imaging

Answer 7

1. Implant planning (buccal lingual width)
2. Assess relationship of 8 to iac
3. Evaluation of diseases of maxilla and mandible (eg see expansion in buccal lingual direction)
4. Craniofacial evaluation eg cleft palate
5. TMJ evaluation eg osteoarthritis
6. Endodontic evaluation eg atypical pathology, complex root morphology
7. Evaluate paranasal sinuses

Question 8

Inappropriate use of CBCT

Answer 8

• Field of view too large/small
• As replacement of 2D imaging (radiation dose! Avg 15x)
• Caries diagnosis
• Soft tissue pathology (CBCT does not give good soft tissue information)

Question 9

What x ray modality should you use to evaluate soft tissue

Answer 9

MRI

Question 10

What is field of view and how is it controlled

Answer 10

Scan volume. The FOV size and radiation exposure is controlled by collimation of x ray beam

Question 11

What is a voxel

Answer 11

Smallest unit that forms CBCT image. Determines spatial resolution ie sharpness. Smaller voxels = higher spatial resolution

Question 12

What does voxel size determine

Answer 12

Spatial resolution

Question 13

Characteristic of CBCT voxels

Answer 13

Isotropic ie equal size in all 3 dimensions. This allows 3d data to be sectioned in any plane to produce 2d images

Question 14

Advantage and disadvantage of using smaller voxels

Answer 14

Better spatial resolution but higher radiation dose. Use largest voxel size suitable for diagnostic task

Question 15

What is axial, coronal, oblique, sagittal?

Answer 15

Go find egs to check since I cant put pictures in

Axial: horizontal, transverse cut
Coronal: split vertically from left to right
Sagittal: split vertically from front to back
Oblique: to look at structures when they do not lie perfectly 90º

Question 16

What is a curved planar reconstruction

Answer 16

Panoramic reconstruction for cbct

Question 17

What is serial cross sectional cbct used for

Answer 17

measure buccal lingual bone for implant

Question 18

What is volume render on cbct and what is it used for

Answer 18

Surface view, not looking at internal information

For visualising and analysing craniofacial conditions eg osteomas in Gardner’s syndrome

Question 19

What is maximum intensity projection in cbct

Answer 19

Type of view that shows up the structure with highest density

Used for eg localising impacted teeth, demonstrating soft tissue calcification, identifying fractures

Question 20

Basic principles and guidelines for use of cbct

Answer 20

Justification and selection criteria
Medicolegal responsibility
Appropriateness of imaging
Optimisation of dose
Quality assurance 
Adequate training and clinicians using CBCT

Question 21

Medicolegal responsibilities: justification and selection criteria for cbct

Answer 21

History and clinical examination already performed

Benefits outweighs the risk

Add new information to aid management eg change treatment plan/outcome

Should not be performed routinely

Used only when imaging is required and question cannot be answered adequately by lower dose conventional traditional radiography

Question 22

What are the medicolegal responsibilities of clinician who orders CBCT

Answer 22

Must do thorough clinical evaluation of ENTIRE image database

Question 23

How to perform quality assurance for cbct

Answer 23

Light beam/markers for accurate positioning

New Cbct equipment undergo critical examination and detailed acceptance tests to ensure optimal radiation protection

Regular routine tests of cbct equipment to ensure radiation protection has not significantly deteriorated

Question 24

What to include when requesting for cbct

Answer 24

Indication 
FOV/region of interest
Voxel size
Mode
Special instructions eg mouth slightly open