RAD02 - Lecture 7 - CBCT

Question 1

What does CBCT stand for?

Answer 1

Cone beam computed tomography (CBCT)

Question 2

What is CBCT? (4)

Answer 2

A technique that allows the 3D depiction of anatomy and pathology

Producing images similar in appearance -> medical CT

Doses are variable depending on type of unit and scan selected

But generally higher doses compared to -> conventional radiographs

Question 3

Give 6 examples of CBCT digital imaging systems

Answer 3

Question 4

How does the beam shape differ between CBCT and medical CT? (2)

Answer 4

Medical CT -> flat-fan

CBCT -> cone shaped

Question 5

Describe the properties of Accuitomo CBCT imaging system (4)

Answer 5

Images have very small volume (4cm)

This enables high resolution on specific teeth

Voxel size = 0.125mm

Radiation dose is very low - equivalent to 3-4 PAs

Question 6

What are the clinical indications for CBCTs?

(i.e. what are they used to assess)

Answer 6

Developing dentition (4)

Locate unerupted/impacted teeth

External resoprtion in relation -> unerupted teeth

Cleft palate

Plan complex orthodontic/surgical anagement of maxillofacial skeletal abnormalities

Restoring dentition (3)

Peri-apical

Root-canal antomy (multi-rooted teeth)

Complex endodontic treatment

Surgical (5)

Check proximity of lower third molars -> ID canal

Prior to placing implants

Pathological lesions affecting jaw

Bony elements of the TMJ

Maxillary antrum

Question 7

How does the CBCT imaging system work? (4)

Answer 7

1) Cone-shaped x-ray beam orbits once around -> patient

2) Obtains information in a cylindrical or spherical volume (in one cycle)

3) Thus - patient needs to stay still throughout exposure

4) Computer will collate the data in tiny cubes called voxels

Question 8

How do CBCTs differ to panoramics?

Answer 8

There is no focal trough -> doesnt image tissues

Question 9

What are the 3 stages of scanning/image creaction process of CBCTs? (6:4:3)

Answer 9

Data aquisation (6)

You have a cone-shaped beam + special detector

Equipment obrbits around patient in 180^o, 270^o or 360^o

It takes ~5-40secs for one cycle/scan

Images are of cylindrical or spherical volume (this is known as Fov - field of view)

Fov size depends on equipment***

Primary reconstruction (4)

Computer collates the data -> tiny cubes known as voxels

Voxels = 0.076mm³-0.4mm³

Typical scans contain > 100million voxels

Image resolution good but not better than 2D imaging

Secondary reconstruction (3)

Computer manipulation allows viewing -> saggital, axial, coronal planes.

Selecting and moving cursor lines in one image -> automatic movement in other 2 reconstructed slices

This allows images to be scrolled through in real time

Question 10

What are the different Fov (field of view) volume sizes? (3)

Answer 10

4x4cm -> 3 molar teeth or several anteriors (commonly used for single RCT or impacted 8’s)

6x6cm -> upper 3-3, unilateral antrum (commonly used for multiple teeth in orthodontics)

8x8cm -> upper and lower 7-7 at same time (not usually wisdom teeth)

Question 11

What are the 3 planes in which images are separated into after secondary reconstruction?

Answer 11

Saggital

Axial

Coronal

Question 12

Compare scans that rotate 180^o and 360^o

Answer 12

360^o -> takes 17.5 seconds, more slices of information -> ↑ resolution

180^o -> takes 9 seconds, fewer slices of infromation -> ↓ resolution (“nosier”)

Question 13

When is needed to justify any requests for CBCTs? (1)

Answer 13

Previous radiographic images - to determine if CBCTs are appropriate

Question 14

Describe the procedure of taking a CBCT (5)

Answer 14

1) Input patient data

2) Select scan parameters (4)

Volume size

Voxel size/resolution

Scan time

Exposure factors (i.e. field, kV, mA)

3) Position patient (6)

1) Sit back with feet supported

2) Align button -> raise and lower seat until light is at required position.

3) Horizontal light should be on the occlusal plane - which is flat (FFP of ~5^o)

4) Side verticle light - adjusted by moving cursor on comp -> couple of cm infront of ear lobe

5) Midline (saggital) light -> ensures head is not tilted/rotated

6) Secure head with velcro

4) Preview - to check for position and coverage of required area

5) Expose

Question 15

When should you avoid taking CBCTs? (2)

Answer 15

If the patient has alot of metal in region of interest (i.e. amalgam fillings, cast posts)

It will produce star-shaped artefacts on image and make it darker

Question 16

Identify the error the arrows are pointing to

Answer 16

Movement Artefacts

Question 17

What are movement artefacts? (1)

Answer 17

They occur when the patient moves - ususally in young or nervous patients

Question 18

How can you minimise the effect of movement artefacts from 360^o scans? (3)

Answer 18

Reconstruct a 180^o scan from the 360^o captured volume imaging data

This will avoid the need to re-scan the patient

However - it does decrease the resolution

Question 19

What are the advantages of CBCT?

Answer 19

Anatomy/pathology can be viewed in different planes

Allows better visualisation of bony structures

Lower radiation dose < medical CT (~200-300 panoramic radiographs)

Geometrically accurate images

Very good spatial resolution (line pair/mm)

Fast scanning times

Question 20

What are the disadvantages of CBCTs? (11)

Answer 20

Patient needs to keep still -> movement artefacts

Radio dense objects (i.e. metals) -> star, streak, hardening artefacts

Soft tissues are not imaged (in detail)

Cant compare CBCT panoramics -> panoramic radiographs directly

Expensive

Field of view size are not always ideal

Time consuming -> to interprete and review data

Poor resolution (compared to 2D or digital images)

More medico-legal issues -> all data needs to be reported on (problem for dentists who do not know medicine)

Higher doses compared to > conventional imaging

Training required to refer or own equipment

Question 21

What 4 factors determine the radiation dose of a CBCTs?

Answer 21

Type of CBCT machine

Exposure facts (kV, mA, time of exposure)

Volume size (Fov)

Part of jaw/maxillofacial skeleton being imaged

Question 22

Some units offer a Dose Reduction button - when would you select it? (1)

Answer 22

When a high resolution is not required

Question 23

Compare the effective (radiation) dose of different dental examinations

(include: effective dose, equivalent background radiation, risk of cancer)

Answer 23

Dental intra-oral film -> 2 μSv (≈ 8hours BR) -> 1/10mill get cancer

Panoramic -> 20 μSv (≈ 3days of BR) -> 1/1mill get cancer

CBCT -> 5-1073 μSv (≈ 1-179days of BR) -> 1/4mill or 1/19thousand get cancer

BR = natural background radiation