CT scanning

The development of computerized tomography (CT scanning) constituted a dramatic advance in radiological techniques, especially in respect of producing an image of the brain. Although conventional X-rays are used, detection of any abnormality is by a crystal or gas system of far greater sensitivity; thus lower-power X-rays can be used, with an increase in safety.

In a CT scanner the X-ray tube and detector rotate around the patient. A ‘slice’ of tissue through the head or body is scanned and a computer reconstitutes the image. During a head (or brain) scan, for example, a single scanning movement takes about four minutes, and determines the relative densities of perhaps 25,000 areas, each about 1.5mm2. Moreover, whereas a conventional X-ray film can display about 20 different tissue densities between the lightest and darkest parts of the image, the CT scan has a range of some 2,000 densities. This amazing accuracy enables the interpreting doctor or radiologist to differentiate fat, blood, muscle, brain tissue and the cerebrospinal fluid that bathes it – all in or near the radiodense skull bones. By making a series of ‘slices’ at one centimetre intervals, something like a three-dimensional image of the body is achieved.