Computed Tomography CT & CBCT Flashcards
What is Hounsfield unit?
= is a quantitative scale for measuring proton attenuation
lowest -1000 - darkest = air
highest +3000 units - brightest = compact bone
What are the indications for multislice CT?
Head and Neck - not used in general dentistry
• Trauma
• Pathology: Jaw pathology (cysts, tumours, malignancies, etc.), Sinus pathology, Salivary gland pathology, Oral cancer
• TM Joints
• Non-specific dental pain - Rule out neuropathic pain
• Dentistry (low dose protocol): Implant dentistry, Other: Special needs dentistry
What are the indications for cone beam CT?
- Dental Trauma
- Third molar assessment
- Supernumerary teeth
- Impacted Canines
- Implant assessment
- Periodontal assessment - 3D bone loss
- Complex endodontic therapy - High res for root canal morphology
- Root resorption
- Root fractures
- Orthodontic assessment
- Orthognathic surgical planning
- Digital dentistry
- 3D printing eg. surgical guides
- TM Joint (limited to osseous)
What are the CBCT artefacts?
- Poor soft tissue contrast
- Noise
- Under-sampling
- Partial volume averaging
- Beam hardening
- Cone beam effect
- Detector related
- Ring artefact
- Dead pixel
- Patient related: Movement, Tremor, Child, Restorations, High density restorations eg. amalgams, crowns, implants, etc.
What is a CT?
Fan-shaped, continuous, rotational x-ray beam.
Creates “Slices” of cross-sectional / transverse / axial images.
Hundreds to thousands of acquired slices are later reconstructed (= digitally stacked together) to from volumetric imaging
Describe the modern CT machine
X-ray tube & detectors continuously revolve around the patient while the patient table moves through the gantry
Continuous helix of data is acquired as the x-ray beam moves down the patient
How long does it take to scan the head/jaw?
1 second
What is the radiation dose of 1 PA?
0.004mSv (1 day of background radiation)
What is the radiation dose of 1 OPG?
(2-3 days of background radiation)
What is the yearly background radiation dose in Australia?
1.5mSv/year in Australia
What is the radiation dose of 1 lat ceph?
~0.005 mSv
What is the radiation dose of 1 CBCT?
iCAT large FOV (maxillofacial region): ~0.07 mSv (2-3 weeks of background radiation)
What is the radiation dose of 1 CBCT?
iCAT large FOV (maxillofacial region): ~0.07 mSv (2-3 weeks of background radiation)
Planmeca 3D small FOV: ~0.03mSv (1 week of background radiation)
CBCT dose varies depending on:
FOV voxel size mA kVp rotation trajectory
What are the most pronounced artefacts?
- Patient movement artefact
- CBCT takes up to 30 seconds to scan
- Restoration artefacts (or just dense object artefacts)
- Cupping artefact (shape distortion)
- Scatter/Streaking (white lines)
- Beam hardening (black lines)
