Computed tomography (CT), also known as computerized axial tomography (CAT) and CAT scanning, was developed between 1967 and 1970 by the English scientist Godfrey Hounsfield. The first clinical results were reported in 1972 (Ambrose & Hounsfield, 1972a,b). Hounsfield successfully harnessed conventional X-rays and a computer to produce cross-sectional brain images of unprecedented clarity (Hounsfield, 1973).

All CT scanners share a number of key features. A finely collimated X-ray beam moves around the patient's head within a metallic shell called a gantry. The extent of X-ray absorption by the tissues is measured by detectors which fluoresce or ionize in direct proportion to the intensity of the X-ray beam striking them after traversing the patient's head. The degree of fluorescence or ionization is converted into digital information by a computer. Using one of several suitable computer algorithms, cross-sectional images of the irradiated slices of brain are constructed in which each of the individual elements of the image, called ‘pixels’ (shorthand for picture elements), is allocated a level of brightness somewhere between black and white. Approximately 256 shades of gray are available for use in modern CT scanners. Low-density structures such as fat and water appear black and high-density structures such as bone and calcium appear white.

CT scanner types

The original CT scanners were of the ‘ctranslate–rotate’ type. While continuously emitting a finely collimated, pencil shaped X-ray beam, the tube and detectors traversed (translated) across the head in a straight line.