Introduction To CBCT

Question 1

Principles of CBCT Imaging

Answer 1

• x ray source and detector rotates 180 degrees to 360 degrees around patient’s head
• x ray is cone-shaped and directed through region of interest
• rotation centre is fixed in the center of ROI
• hundreds of 2D basis projection images taken (all slightly different)
• projection data used to construct 3D CBCT
• algorithms (filtered back projection) are used

Question 2

Strengths of CBCT

Answer 2

• 3D (buccal lingual info)
• spatial relationships
• no superimposition
• no distortion or magnification
• measurements are accurate

Question 3

Limitations of CBCT

Answer 3

• Higher radiation than 2D
• Lack of soft tissue contrast
• unreliability of hounsfield units (measurement of radio density)
• presence of artefacts (metal streak, beam hardening, patient motion)

Question 4

Causes of metal artefacts and beam hardening

Answer 4

• Dense objects (metal crowns, titanium and zirconium dioxide implants) absorb x-ray beams as they pass through
• metal artefacts (white streaks/scatter) result from complete absorption of x-rays
• beam hardening (dark bands due to absorption of low-energy (soft) x ray beams leaving only high energy (hard) x ray beams

Question 5

Describe patient motion artefacts

Answer 5

• Appearance of double lines/structures on CBCT scan
• Related to scanning duration
• Get patient to close eyes/stabilise head using head straps

Question 6

Indications of CBCT (8)

Answer 6

1) Implant planning
2) Assessment of 3rd molars and relationships with IAC, other impacted teeth
3) evaluation of intraosseous pathology (cysts and tumours)
4) craniofacial evaluation
5) evaluation of trauma
6) endodontic evaluation
7) evaluation of paranasal sinuses
8) TMJ evaluation

Question 7

What to check for implant planning

Answer 7

• alveolar bone width and height
• anatomical danger zones (maxillary sinus, IDN canal)
• pathology that may affect implant placement

Question 8

What to check for impacted teeth

Answer 8

• interruption of IDN canal
• mesiodens, maxillary canines, mandibular premolars proximity to vital structures
• surgical approach (buccal vs palatal/lingual)
• presence of pathology
• presence of root resorption

Question 9

What to check for intraosseous pathology

Answer 9

L: location
E: edges
S: size/shape
I: internal contents
O: effects on other structures
N: number

Question 10

What is craniofacial evaluation?

Answer 10

For orthognathic surgeries or cleft patients

Question 11

List the inappropriate uses of CBCT imaging

Answer 11

• FOV too large/small
• replace 2D imaging
• caries diagnosis
• diagnose soft tissue pathology

Question 12

What controls FOV size and radiation exposure?

Answer 12

Collimating of ray beam
FOV should be as small as possible to minimise patient dose

Question 13

Describe Voxel

Answer 13

• smallest 3D unit that forms the CBCT image
• smaller voxel = higher spatial resolution = higher dose
• isotopic (equal size in all 3 dimensions)

Question 14

Describe maximum intensity projection

Answer 14

• selection of only the highest voxel gray scale values for display
• only shows bone
• useful for localising impacted teeth
• demonstrating soft tissue calcifications (less dense than other stuff)
• identifying TMJ
• identifying fractures
• craniofacial analysis