Tomography achieves this result by simply moving an x-ray source in one direction as the x-ray film is moved in the opposite direction during the exposure to sharpen structures in the focal plane, while structures in other planes appear blurred.

The tomogram is the picture; the tomograph is the apparatus; and tomography is the process.

The computerized axial tomography (CAT) scan can reveal some soft-tissue and other structures that cannot even be seen in conventional X-rays.

Using the same dosage of radiation as that of an ordinary X-ray machine, an entire slice of the body can be made visible with about 100 times more clarity with the CAT scan.

The "cuts" (tomograms) for the CAT scan are usually made five or ten millimeters (mm) apart. The CAT machine rotates 180 degrees around the patient's body; hence, the term "axial".

The machine sends out a thin X-ray beam at 160 different points. Crystals positioned at the opposite points of the beam pick up and record the absorption rates of the varying thicknesses of tissue and bone. The data are then relayed to a computer that turns the information into a 2-dimensional cross-sectional image.

CAT scanning was invented in 1972 by the British engineer Godfrey N. Hounsfield (later Sir Godfrey) and the South African (later American) physicist Alan Cormack.

CAT scanning was already in general use by 1979, the year Hounsfield and Cormack were awarded the Nobel Prize in Medicine or Physiology for its development.

CT stands for computerized tomography.