A CT (computed tomography) scanner is a machine that is shaped like a doughnut that rotates around the patient’s body. The machine contains one or more X-ray sources and, on the opposite side, some X-ray detectors. Unlike a conventional X-ray image, which shows only a few levels of tissue density, the X-ray detector can register hundreds of levels of density. It sends this information to a computer, which processes the data and shows the results as an image on a monitor. CT images usually show the body as “slices”, in which the different tissues can be seen in detail. In some machines, this information can be used to produce a three-dimensional reconstruction of the area scanned.
CT scanning has revolutionized the diagnosis and treatment of tumours, abscesses, and haemorrhages in the brain, as well as head injuries and strokes. The procedure is also used to locate and show tumours, to investigate a wide range of diseases, and to aid needle biopsy in organs of the trunk.
Newer types of CT scanners use a spiral technique: the scanner rotates around the body as the patient is moved slowly forwards on a bed, causing the X-ray beams to follow a spiral course. Images can be made of hollow organs such as the colon (a procedure known as “virtual colonoscopy”). For some procedures, injected or swallowed contrast media (chemicals that are opaque to X-rays) may be used to make certain tissues more easily visible.
The images produced during CT scanning can be stored digitally or on conventional X-ray film. (For details of the procedure, see Performing a CT scan box, opposite.)
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