Computerized axial tomography (CAT) scan machines use X-ray beams that pass straight through most of the soft material in the human body. Because a conventional X-ray image is basically a shadow in which a larger structure may block the view of a smaller one behind, in a CAT scan machine, the X-ray beam moves all around the patient, scanning from hundreds of different angles. The computer takes all this information and puts together a 3-D image of the body.

The CAT machine looks like a giant donut standing on its side. The patient lies down on a platform, which slowly moves through the hole in the machine. The X-ray tube is mounted on a movable ring around the edges of the hole with X-ray detectors directly opposite the X-ray tube.

A motor turns the ring so that the X-ray tube and the X-ray detectors revolve around the body. Each full revolution scans a narrow, horizontal "slice" of the body. The control system moves the platform farther into the hole so the tube and detectors can scan the next slice. After the patient passes through the machine, the computer combines all the information from each scan to form a detailed image of the body.

Note: The image shown here is of a CT scan machine with the superimposed image of a head and neck scan (in the upper left-hand corner of the image). This scan shows a detailed view of the nasal cavity and paranasal sinuses.