Computed Axial Tomography, known as CT or CAT scanning, is similar to conventional X ray. In both procedures, X rays are passed through the body to produce an image. Scans and X rays are two-dimensional pictures with the tissues, organs and bones represented in various shades of gray. The darkest areas of an image denote the greatest penetration of the rays. Translucent areas usually indicate dense matter such as bone or muscle tissue that radiation cannot fully penetrate.

Images produced by Computed Tomography capture a much higher level of detail than plain film X ray. Conventional X-ray technique can produce 20 to 30 shades of gray. Sensitive electronic CT detector and computers can recognize and register up to 200 shades of gray. Plain film X ray depends on a single exposure to produce an image. Computed Tomography employs a series of images taken as an X-ray tube rapidly circles the body. An electronic detector collects the data and sends it to a computer that processes and assembles the information. A cross-sectional image of the scanned area is reconstructed from this data.

27. CT scans of two masses in chest. Courtesy of the the Department of Radiology, UIHC.