Radiology 17 Flashcards
CBCT
Cone Beam Computerised Tomography
Hounsfield scale
quantitative measure of radio density on CT scans
Hounsfield unit: air, water and cortical bone
Air = -1000 HU
Water = 0 HU
Cortical bone = +1000HU
Basic set up of a CT scanner
- High kV tube - 120kV – much higher than dental machines
tomographic image – looks at a slice of tissue and these slices can be stacked together in order that 3D images can be produced
x-rays pan through body
attenuation of beam measured by detectors
computer assesses radio-opacity of each voxel
different Hounsfield number for every tissue
Voxel
Volume of pixel
When might a CT scan be used?
Intracranial
bleed, infarct, tumour
Trauma
Evaluation of osseous lesions
Salivary glands
Neoplasia (benign/malignant)
primary tumour
metastases
Orthognathic assessment and treatment planning -
Implant planning
Advantages of CT scans
Not subject to same magnification and distortion as plain radiographs
Multi-planar (manipulate, reconstruct images)
avoids superimposition
Images bone and soft tissue
Differentiate different tissues
(Head injury, see bone and brain)
Speed (cf MRI)
Both CT and MRI are still expensive pieces of equipment.
Disadvantages of CT scans
Patient Dose
Artefacts (metallic objects, amalgam in head and neck)
May require intravenous contrast to distinguish tissues (iodine based, risk of reaction)
Expensive in comparison with plain radiography
Interpretation more difficult
Dentascan
software that was available mainly for implant planning
Why is CBCT used over a CT?
- Much lower radiation dose than medical grade CT
- Overcomes the issues of 2D imaging
- Conventional images may not give the whole information required for diagnosis & treatment planning
- Some machines have a large field of view (FOV) and are usually within hospital departments
- Dental practices are more likely to have small FOV machines
Positioning or scout view
quick initial scan to check area of concern is in the FOV
CBCT - How is the image acquired?
- Patient positioned either seated or lying supine, depending on machine.
- The x-ray tube and the flat panel receptor are opposite each other and rotate around the patient in the same direction at the same time.
- The receptor send the information to the computer where an image will appear on the screen
- The x-ray beam is cone shaped and the area of the face and jaws to be scanned is in the path of the beam for the whole rotation, passing through structures in all of the 360 degree directions.
How is the image displayed?
- A DPT type image is usually the first image displayed on the PC screen.
- The imaging software used will determine how the view can be modified
- If multi-planar imaging is available then three slices will be displayed – axial, coronal and sagittal.
- Some software will allow these planes to be viewed obliquely – which is very useful if you want to slice through the dental arch.
- 3D reconstruction can also be available.
When would you use CBCT on a developing dentition?
- Localisation of unerupted tooth e.g. maxillary canine
- Assessment of any resorption from unerupted tooth
- Cleft palate patients
- Orthognathic surgery assessment
CBCT: What affects the dose?
- Some exposure factures are set by the manufacturer – kV, time & mA
- Scan times can be varied
- Volume size – FOV
- Type of equipment used
- Part of maxillofacial region being imaged
I-CAT panoramic
Used by orthodontists to create a 3D view of your teeth and the craniofacial structure.
