RAD02 - Lecture 7 - CBCT Flashcards
What does CBCT stand for?
Cone beam computed tomography (CBCT)
What is CBCT? (4)
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
Give 6 examples of CBCT digital imaging systems
How does the beam shape differ between CBCT and medical CT? (2)
Medical CT -> flat-fan
CBCT -> cone shaped
Describe the properties of Accuitomo CBCT imaging system (4)
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
What are the clinical indications for CBCTs?
(i.e. what are they used to assess)
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
How does the CBCT imaging system work? (4)
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
How do CBCTs differ to panoramics?
There is no focal trough -> doesnt image tissues
What are the 3 stages of scanning/image creaction process of CBCTs? (6:4:3)
Data aquisation (6)
You have a cone-shaped beam + special detector
Equipment obrbits around patient in 180o, 270o or 360o
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
What are the different Fov (field of view) volume sizes? (3)
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)
What are the 3 planes in which images are separated into after secondary reconstruction?
Saggital
Axial
Coronal
Compare scans that rotate 180o and 360o
360o -> takes 17.5 seconds, more slices of information -> ↑ resolution
180o -> takes 9 seconds, fewer slices of infromation -> ↓ resolution (“nosier”)
When is needed to justify any requests for CBCTs? (1)
Previous radiographic images - to determine if CBCTs are appropriate
Describe the procedure of taking a CBCT (5)
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 ~5o)
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
When should you avoid taking CBCTs? (2)
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
Identify the error the arrows are pointing to
Movement Artefacts
What are movement artefacts? (1)
They occur when the patient moves - ususally in young or nervous patients
How can you minimise the effect of movement artefacts from 360o scans? (3)
Reconstruct a 180o scan from the 360o captured volume imaging data
This will avoid the need to re-scan the patient
However - it does decrease the resolution
What are the advantages of CBCT?
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
What are the disadvantages of CBCTs? (11)
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
What 4 factors determine the radiation dose of a CBCTs?
Type of CBCT machine
Exposure facts (kV, mA, time of exposure)
Volume size (Fov)
Part of jaw/maxillofacial skeleton being imaged
Some units offer a Dose Reduction button - when would you select it? (1)
When a high resolution is not required
Compare the effective (radiation) dose of different dental examinations
(include: effective dose, equivalent background radiation, risk of cancer)
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
