Cone Beam CT Flashcards
What does cross-sectional mean?
Able to take “slices” out of the subject.
Why are cross-sectional images useful?
Very useful for looking at things WITHIN the tissues WITHOUT SUPERIMPOSITION.
What is CBCT?
- Cone beam computed tomography.
- A form of CROSS-SECTIONAL imaging suitable for assessing RADIODENSE STRUCTURES.
Where is CBCT available on the NHS?
Only in SECONDARY CARE.
(can receive it in PRIMARY care if going PRIVATE).
What are the 2 main parts of a CBCT machine?
- Square digital detector.
- Conical/ pyramidal X-ray beam.
X ray beam and detector always OPPOSITE each other, area of interest caught in the middle.
What happens during image acquisition of CBCT?
Conical/ pyramidal X-ray beam and square digital detector rotate around head - NO MORE THAN 1 FULL ROTATION.
- Captures many 2D images (ex. 200) which are reconstructed into a cylindrical 3D image.
How many rotations does the CBCT machine make?
NO MORE THAN 1 FULL ROTATION.
What is the head positioning for CBCT?
- Horizontal: Frankfort plane (head level with ground).
- Vertical: midsagittal plane.
4 benefits of CBCR over plain radiography?
- No superimposition.
- Ability to view subject from any angle.
- No magnification/. distortion.
- Allows for 3D reconstruction.
6 downsides of CBCT over plain radiography?
- Increased radiation dose.
- Lower spatial resolution (less sharp).
- Susceptible to artefacts.
- Equipment more expensive.
- Images more complicated to manipulate and interpret.
- Requires additional training (to justify, operate and interpret).
What is CBCT good at showing? What is it bad at showing?
- Good for looking at teeth and bone.
- Bad at looking at fat, muscle, mucosa as cannot show enough contrast between them.
4 benefits of CBCT over CT?
- Lower radiation dose.
- Potential for “sharper” images (higher resolution)
- Cheaper
- Smaller (easier to fit in practice).
4 benefits of CT over CBCT?
- Able to differentiate soft tissues better.
- Cleaner images (better signal to noise ratio).
- Larger field of view possible.
5 common uses of CBCT in dentistry?
- Clarifying relationship between impacted mandibular 3rd molar and ID canal (AFTER PLAIN RADIOGRAPH SUGGESTED A CLOSE RELATIONSHIP).
- Measuring alveolar bone dimensions to plan implant treatment.
- Visualizing complex root canal morphology to aid endodontic treatment.
- Investigating root resorption next to impacted/ ectopic teeth.
- Assessing large cystic jaw lesions and their involvement of important anatomical structures.
What is the most common way to look at CBCT images?
Using the ORTHOGONAL PLANES:
- Axial (above/below.
- Sagittal (side).
- Coronal (front/back).
2 benefits of 3D volume reconstruction?
- May help clinician to picture EXTENT/SHAPE of disease.
- Teaching aid for patients.
1 drawback of 3D volume reconstruction?
- Can create misleading images - particularly poor at showing THIN BONE.
When are the imaging factors/ variables set?
Set BEFORE the scan starts.
What does changing the imaging factors change? How are these variables decided?
- Changes the INFORMATION OBTAINED & PATIENT DOSE.
- Decided on a CASE-BY CASE BASIS using ALARP.
3 examples of imaging factors/ variables?
- Field of view.
- Voxel size.
- Acquisition time (ex. 10 secs).
What is the field of view?
The size of the captured volume of data.
2 disadvantages of scatter?
- Can irradiate structures around the patient.
- Result in worse contrast on the image.
Why do we want to have the smallest FOV possible?
Results in a REDUCED RADIATION DOSE.
What is scatter?
X ray photons pass through the structure but rather than being absorbed/ going through and reaching the receptor, they bounce off to different directions.
What does an increased FOV result in?
- Increased radiation dose.
- Increased number of tissues irradiated.
- Increased scatter.
What determines the image resolution?
Voxels (3D pixels).
What is the size of CBCT voxels compared to intraoral voxels?
NEVER AS SMALL AS INTRAORAL RADIOGRAPH PIXELS.
What determines the voxel size?
Depends on the CLINICAL CASE.
What is the effect of decreasing voxel size (3)?
- “sharper” image.
- Increased radiation dose.
- Increased scan time (increases risk of patient moving).
- Increased processing time and requires more space to store (more data).
What is the typical range of voxel size?
0.4mm3 to 0.085mm3
What is a typical FOV and voxel size for endodontic cases?
- FOV as small as possible (ex. 5x5cm) unless large apical pathology as we are only looking at the one tooth.
- Smaller voxel size (to see root canal detail).
What is a typical FOV and voxel size for implant planning cases?
- FOV depends on number/position of implants.
- Larger voxel size
Roughly, what is the dose of CBCT compared to panoramic radiographs?
Dento-alveolar CBCT is 2-3 times the dose of a panoramic radiograph.
What is the approximate effective dose for CBCT?
13-82 μSv
What is the approximate effective dose for CT?
474-1160μSv
What is the approximate effective dose for panoramic?
3-24μSv
What is the approximate effective dose for intraoral?
4μSv
Define artefact
Visualized structures on the scan that were not present in the object investigated.
What are the 2 main types of artefacts?
- Movement
- Streak
How does a movement artefact occur? Which part of the scan does it affect?
- Occurs if patient not completely still during the full exposure.
- Affects the WHOLE SCAN.
What can a movement artefact cause?
General blurriness or extra contours.
How can a movement artefact be reduced?
Using FIXATION AIDS.
- Chin rest, head strap, telling patient to stay still etc.
What causes streak artefacts?
- Usually caused by HIGH ATTENTUATION OBJECTS (ex. amalgam fillings, crowns, implants etc).
2 main issues of streak artefacts?
- Can prevent CARIES ASSESSMENT adjacent to restorations.
- Can prevent assessment of PERFORATIONS/ MISSED CANALS in RCTd teeth.
4 contraindications to CBCT?
- Plain radiographs are sufficient.
- Pathology requiring soft tissue visualization (malignancy, infection spreading into soft tissue).
- High risk of debilitating artefacts (ex. many amalgam fillings, crowns etc).
- Patient factors.
2 patient factors that contraindicate CBCT?
- patient unable to stay still (ex. parkinson’s, learning difficulties, uncooperative child).
- Patient unable to fit in machine (ex. unfavourable shoulder to neck ratio - obese + bodybuilder, kyphosis).
What is the role of undergraduate dentists in CBCT?
Current UK undergraduate teaching deemed INADEQUATE TO REFER, JUSTIFY, PERFORM OR INTERPRET CBCT.
What is the recommended post-graduate training for CBCT in the UK?
- Level 1: for all IRMER duty holders.
- Level 2: for those justifying, performing or interpreting CBCT.
What are 2 requirements to be able to JUSTIFY a CBCT?
- Must ALWAYS be preceded by a CLINICAL EXAMINATION.
- Only considered if plain radiography unable to provide sufficient information.
What document can be used to help clinicians in justifying CBCT?
- Selection Criteria for Dental Radiography by FACULTY OF GENERAL DENTAL PRACTICE FGDP
You take a CBCT and find that the ID canal sits between the roots of an tooth with pericoronitis. What is a possible management option?
- XLA would cause nerve damage.
- Can do CORONECTOMY.
