Radiology 17 - CBCT Flashcards
What does CBCT stand for?
Cone beam computed tomography
Where is CBCT used currently?
Hospitals and private (non NHS) specialist practices.
Who developed x-ray computed tomography?
Geoffrey Newbold Hounsfield
What is a unit of measurement associated with CT scans?
Quantitative measure of radiodensity on CT scans is the Hounsfield.
DIFFERENT HOUNSFIELD NUMBER FOR EVERY TISSUE.
air = -1000HU, water = 0, cortical bone = +1000
What is the kV of a CT scanner?
120kV
What is a tomographic image?
Looks at a slice of tissue and these slices can be stacked together to form a 3D image.
What is the CBCT scanning process? (3 stages)
- Data acquisition
- equipment orbits around patient (5-40 seconds) and imaged a CYLINDRICAL or SPHERICAL volumed called FIELD OF VIEW (can be small, medium or large). - Primary reconstruction
- after obtaining the data, computer divides the volume into tiny tubes or VOXELS.
- each voxel is allocated a number and then allocated a color from the grey scale from black through white. - Secondary or multiplanar reconstruction
- operator can select voxels in the three anatomical orthogonal planes (sagittal, coronal or axial).
What are 4 advantages of CT?
- not subject to same magnification and distortion as radiographs.
- multi-planar (avoids superimposition).
- images both bone and soft tissue.
- faster than MRI.
What are 5 disadvantages of CT?
- patient dose
- artefacts
- harder to interpret
- may require intravenous contrast to distinguish tissues (iodine based) –> risk of reaction
- expensive
what is dentascan?
CT software mainly available for implant planning.
2 reasons why use CBCT?
- lower radiation dose than medical grade CT
- overcomes issues of 2D imaging
What is positioning/ scout view in CBCT?
- Scout view: initial positioning view is taken to ensure that the area of concern is in the field of view
- very short exposure, produced lateral image.
How is the CBCT image acquired?
- Patient instructed to remove jewellery etc
- Patient either seated or supine depending on machine
- X-ray tube and flat panel receptor are OPPOSITE each other and ROTATE around patient in SAME DIRECTION at SAME TIME.
- receptor then sends information to computer where an image appears on the screen
- CONE SHAPED BEAM, AREA TO BE SCANNED IS IN THE PATH OF THE BEAM FOR WHOLE ROTATION (360 DEGREES)
When is CBCT indicated?
To investigate developing dentition, restorative dentistry (few), and surgery.
Important ones: cleft palate, TMJ bone surfaces.
see slides 20-22.
What affects the dose?
- Exposure factors set by the manufacturer: kV, time, mA (amount of current passed through the x-ray tube)
- scan times can be varied
- volume size - FOV
- type of equipment used
- part of maxillofacial region being imaged
