Name the 4 alternative imaging modalities.
- CT computed tomography
- MRI magnetic resonance imaging
- ultrasound
- nuclear medicine
what are the main issues with plain radiographs? 7 things
- limited use for soft tissues
- ionising radiation
- sensitivity and specificity
- only show retrospective view (active or arrested caries?)
- significant demineralisation necessary to detect change.
- 2D image = superimposition & multiple view needed for localisation
give 3 positives of plain radiographs.
- cheap
- readily available
- relatively low dose
what kind of radiation is used for CT scan?
Xray photons
how does the CT scanner move?
it rotates around the patient with an X-ray beam at one side of the ring and an X-ray receptor at the other side
what does the CT scanner do as it rotates around the patient?
splits the patient into little cubes called voxels
define a voxel
pixels with volume
what is the size range for a voxel?
0.008-0.04 mm
what is the benefit and disadvantage of using small voxel?
good = high definition bad = greater exposure time = higher dose
how is the CT number determined?
the CT scanner works out how much attenuation of the beam has occurred in each voxel
In a CT what colour is 1. bone 2. soft tissue 3. air
- white 2. grey 3. black (just like X-ray)
Define windowing?
manipulation of an image to allow bettie visualisation of more subtle changes between tissues.
what 2 types of windowing can you get?
Soft tissue windowing and bone windowing
what shows up in the soft tissue windowing?
bone is totally white
what shows up in the bone windowing?
the bone is different shades depending upon the density
what are the 4 advantages of a CT scan?
- images both bone and soft tissue
- speed - faster than MRI
- multiplanar
- low cost and more available compared to MRI
what are the 4 disadvantages of a CT scan?
- high dose to patient
- artefacts will interfere (i.e. amalgam)
- expensive in comparison to plain radiograph
- allergy to iodine based intravenous contrast commonly used to distinguish tissues
give reasons for the use of CT scans in the head and neck (6)
- intracranial bleed, infarct, tumour
- trauma (orbit, cranial base, cervical spine)
- orthognathic surgery and treatment planning
- evaluation of bony lesions
- salivary glands
- neoplasia (primary and metastases)
what does CBCT stand for?
cone-beam computer tomography
what is CBCT good for looking at?
bone (not soft tissues)
3 advantages of CBCT
- 3D
- multiplanar
- same dose as conventional CT
what is the difference between CBCT and normal CT?
normal CT = computer takes multiple slices (32 or 64) and adds together
CBCT = computer takes image as a whole volume and you split it up
what are the 4 planes that data can be reconstructed into?
- axial
- coronal
- sagittal
- specific ones e.g. panoramic
Indications for use of CBCT
- Trauma
- Pathology
- Implantology
- endodontics
- localisation of impacted teeth that affect adjacent structures
