Computed tomography

Computed tomography (CT) uses X-rays to produce an image of a cross-section of the body. Instead of recording the body from just one direction, a CT scanner takes measurements from many angles and uses them to reconstruct a slice-like image.

How a CT image is produced

An X-ray tube moves around the patient, directing a narrow, monochromatic beam through the body from different angles. A monochromatic beam contains photons of one energy.

An array of detectors measures the X-rays that pass through the body. As X-rays travel through tissue, some are absorbed or scattered, reducing the intensity that reaches the detectors. This reduction is called attenuation. Different detector signals reflect differences in attenuation along the paths taken by the X-rays.

A computer processes the measurements from many directions to reconstruct an image of the cross-section. Because the image represents a slice rather than a projection through the whole body, internal structures can be distinguished without the same degree of overlap seen in a conventional projection X-ray image.

CT: detail, cost and radiation

CT can provide detailed images, but the equipment and its operation are generally more costly than conventional X-ray imaging. CT also uses ionising radiation, which can damage tissue and increase cancer risk. A CT examination often delivers a higher radiation dose than a conventional X-ray examination.

The detail obtained, the cost and the safety of a comparison depend on the imaging methods and the particular examinations being compared.