Computed Tomography

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Computed Tomography (CT)

Introduced in1972 for brain imaging. Provides a detailed cross sectional view of structures with good contrast.

Uses ionising radiation to produce a series of cross sectional images of bone and soft tissue – computer reconstructs a complete cross section of part with minimal superimposition of structures.

Uses a rotating fan of x-ray finely collimated beams with a fixed ring of detectors – x-ray tube rotates about the patient in an arc to obtain the image. The x-ray beam, as it emerges is measured by photoelectric detectors – computer then displays the measurement as a cross-sectional image.

Tissues are displayed on a grey scale according to attenuation value.
Denser structures (eg cortical bone) appear light
Low density areas (eg air) appear dark

Better than MRI for congenital and bone deformities, bone tumours, fractures (stress fractures are more difficult), tarsal coalitions

• shows image without superimposition of structures seen on plain x-ray
• displays cross sectional anatomical data
• better than plain x-ray for soft tissue imaging (higher soft tissue contrast sensitivity)
• good for assessing cortical bones.
• many disease process can be detected earlier with CT than on plain x-ray
• all body tissues can be displayed (eg use in neurosurgery)
• important role in cancer staging
• can be used for 3-D reconstructions (eg in facial surgery)

Contrast resolution is inferior to MRI; sagittal slices difficult; uses higher doses of ionising radiation (CT accounts for 40% of medical radiation dose, but is used in only 4% of radiology examinations; ; capital costs of equipment

Use of ‘conventional’ CT scanning is decreasing – being superseded by spiral scanning and MRI is being more widely used.

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