17 Cone Beam Computerised Tomography

Question 1

Computerised tomography began its development in the years following WW2, what was it often called?

Answer 1

CAT scanning - computer axial tomography

Question 2

what are the 3 field of view (FOV)s CBCT’s are described as?

Answer 2

Small FOV - limited/dento-alveolar
Medium FOV - maxillofacial
Large FOV - craniofacial (inc cranial base and the skull vault)

Question 3

What are the advantages of using CBCT?

Answer 3

• much lower radiation dose than medical grade CT
• overcomes the issues of 2D imaging
• Multi-planar reformatting (MPR), allows images to be viewed from all directions
• fast scanning time
• compatible with other software
• good spatial resolution
• conventional images may not give the whole information required for diagnosis and treatment planning

Question 4

In the developing dentition, what may a CBCT be indicated for?

Answer 4

• localisation of an unerupted tooth (small FOV)
• Assessment of external resorption in relation to unerupted teeth (small FOV)
• localisation assessment of an impacted tooth (small FOV)
• Assessment of a cleft palate (small or medium FOV)
• orthognathic surgery assessment - jaw surgery (medium/large FOV)

Question 5

In restorative dentistry, if conventional images prove inadequate, what may a CBCT be indicated for?

Answer 5

Only if conventional radiographs are unhelpful:
- assess periapical lesions, root canal anatomy, furcations, perforations
- dental trauma assessment
- dental anomalies

Question 6

In surgery application, what may a CBCT be indicated for?

Answer 6

• assessment of the lower 8s, where an intimate relationship with the inferior dental canal is suspected
• implant assessment
• assessment of pathological lesions affecting the jaws
• assessment of facial fractures where soft tissue detail is not required
• assessment of the bony elements of the TMJ

Question 7

what FOV do dental practices use?

Answer 7

small FOV

Question 8

What is the effective does (in mSv) of a dento-alveolar CBCT (small FOV)?

Answer 8

1.01 - 0.67

Question 9

What are the disadvantages of CBCT?

Answer 9

• patients must be still to avoid movement artefacts
• soft tissues are not imaged well
• radio-dense materials such as restorations and RF material produce beam hardening artefacts - streak artefacts
• images can be difficult to interpret particularly if a large FOV is used

Question 10

Prior to a CBCT being performed, what is often carried out?

Answer 10

An initial positioning view is usually taken (Scout view)
This ensures that the area of concern is in the field of view

Question 11

How is CBCT image aquired?

Answer 11

The patient is positioned (according to manufacture instructions)
The x-ray tube and the flat panel receptor are opposite each other and rotate around the patient in the same direction at the same time
The receptor sends the information to the computer where an image will appear on the screen
- The x-ray beam is coned shaped and the area of the face and jaws to be scanned is in the path of the beam for the whole rotation, passing through structures in all of the 360 degree direction