Cone Beam CT

Question 1

What is CBCT (Cone beam Computed tomography?

Answer 1

• Form of cross-sectional imaging suitable for assessing radiodense structures
• Used in secondary and primary healthcare (private)

Question 2

What are some non-DMFR uses of CBCT?

Answer 2

• Temporal bone imaging
• paranasal sinus imaging
• Radiotherapy planning

Question 3

What are the 3 basic principles of CBCT?

Answer 3

• Ionising radiation
• Conical/pyramidal x-ray beam and sqaure digital detector that rotates around head
• Captures many 2-dimensional images e..g 200 which are made into cylindrical 3D image

Question 4

What are the unit types and what is the pt positioning of CBCT?

Answer 4

• Can have single or multi-purpose units
• Pt position specific to unit but usally standing or sitting
• Head positioning same as panoramic, horizontal frankfort plane, vertical midsagittal plane

Question 5

What are some benefits of CBCT over plain radiography?

Answer 5

• No superimposition
• Ability to view subject from any angle
• No magnification/distortion
• Allows for volumetric (3D) reconstruction

Question 6

What are some negatives of CBCT over plain radiography?

Answer 6

• Increased radiation dose to pt
• Lower spatial resolution (not as sharp)
• Susceptible to artefacts
• Equipment more expensive
• Images more complicated to manipulate and interpret
• Requires additional training to justify, operate and interpret

Question 7

What are the main CBCT benefits over CT?

Answer 7

• Lower radiation dose
• Potential for sharper images due to higher resolution
• Cheaper initial cost, running and maintenance
• Smaller footprint

Question 8

What are the main benefits of CT over CBCT?

Answer 8

• Differentiate soft tissues better
• Cleaner images due to better signal to noise ratio
• Larger field of view is poss

Question 9

What are some common uses of CBCT in dentistry?

Answer 9

• Clarifying relationship between impacted mandibular third molar and IANC prior to intervention
• Implant placement to measure alveolar bone dimension
• Visualising complex root canal morphology to aid endo txt
• Investigate external root resorption next to impacted teeth
• Assess large cystic jaw lesions and involvement of important anatomical structure

Question 10

What orthogonal plane is this?

Answer 10

• Axial

Question 11

What orthogonal plane is this?

Answer 11

• Sagittal

Question 12

What orthogonal plane is this?

Answer 12

• Coronal

Question 13

What are the used of 3D volume reconstruction?

Answer 13

• Help clinician tp picture the extent/shape of disease
• Can be informative teaching aid for pt

Question 14

What are some drawbacks of 3D volume reconstruction?

Answer 14

• It is a modified reconstruction of data and can create misleading images (poor at showing chin bone)

Question 15

There are some imaging factors/variables that are set before the CBCT scan begins. Give some examples and what principle should be used when deciding

Answer 15

Examples
- Field of view
- Voxel size
- Acquisition time

ALARP principle as each will alter info obtained and pt dose

Question 16

What is the field of View? What does increased size mean for the pt

Answer 16

• Size of the captured vol of data
• Increase size = increased radiation dose + increased number of tissue irradiated + increased scatter + more to report

Question 17

What is the voxel size? What does decreased voxel size give for pt?

Answer 17

• The image resolution (aka 3D pixels)
• Decreased voxel size indirectly leads to increased radiation dose
• Decreased voxel size leads to increased scan time

Question 18

What are the range of options for voxel size of CBCT?

Answer 18

• 0.4mm^3 - 0.085mm^3

Question 19

In an endodontic case what imaging factor choice do you want?

Answer 19

• FOV small as poss e.g. 5x5cm unless large apical pathology
• Smaller voxel size to see fine detail of root canal and apex

Question 20

In an implant planning case what factor imaging choice is correct?

Answer 20

• FOV depends on number/pos of implants
• Large voxel size as only needing to see the bone surrounding

Question 21

What is the radiating dose of panoramic / intraoral and what is the rough guide for CBCT?

Answer 21

Panoramic = 3-24uSv
Intraoral = 4uSv
CBCT = approx 2-3X dose of panoramic

Question 22

What are artefacts ? What are the two main types?

Answer 22

• Visualised structures on scan not present in object investigated
• Movement artefacts
• Streak artefacts

Question 23

When does movement artefact occur? What can it lead to? How is it reduced?

Answer 23

• Occurs if pt not completely still during full exposure (affects whole scan)
• Lead to general blurriness or extra contours
• Reduced by using fixation aids like a chin rest or head strap

Question 24

What are streak artefacts caused by? What are the main issues with them?

Answer 24

• Caused by high-attenuation objects e.g. amalgam
• Can prevent caries assessment adjacent to restorations
• Can prevent assessment of perforations/missed canals in RCTed teeth

Question 25

What are some contra-indications for CBCT?

Answer 25

• If plain radiographs are sufficient
• Pathology requiring soft tissue visualisation i.e. Malignancy or infection spreading in soft tissue
• If high risk of debilitating artefacts
• Pt factors e.g. pt unable to stay still like Parkinsons disease or Pt unable to fit in machine like obsese/bodybuilder

Question 26

What justification do you require for CBCT?

Answer 26

• Preceded by clinical exam
• If plain radiography unable to provide sufficient info
• FGDP guidance to assist