Katherine David, age 15, of Cleveland, Ohio, for her question:

WHAT IS A CAT SCAN?

CAT scan is a medical diagnostic technique that was developed in England in the early 1970s and used clinically in the United States since 1973. The name comes from a description of the equipment: computerized axial tomography.

The principle of CAT scan is that we can measure how organs and structures of the body change the properties of an X ray beam. The beam is passed through the body at many angles and the alterations of the beam from each angle are measured and stored in the memory of a computer.

By examining the internal structure of the body from enough angles, the scanner builds a picture of the density of the tissue at a particular point.

If enough points are placed on a later construction of the data, an image can be formed.

Because a single picture only demonstrates the cross section of those organs where the body was X rayed, several images are made to visualize the total extent of the area in question. This is based on the same principle as the radiographic tomograms ("tomo" meaning "cut" or "slice" and "graph" meaning "picture.")

The CAT scan was first used to produce images of structures in the brain. The brain was chosen because there was little motion and the anatomical structures are confined to a space that varies little from patient to patient.

From the patient's viewpoint, a CAT scan is no more discomforting than any routine X ray study. If you undergo a scan, you will lie on a small examining table and the X ray tube and detector system will move continously around you. You experience no discomfort.

Because the computed topographic scan is an X ray procedure, patients do receive radiation exposure.

The amount of exposure is about the same as in other X ray studies and less than some.

A scientist anemd Allan Cormack, who was a significant contributor to the development of the basic theories, and another named Godfrey Houndsfield, who fabricated the first working CAT scan instrument, shared the 1979 Noble Prize for medicine.

Today the CAT scan can often eliminate other examinations that are more expensive, time consuming and uncomfortable for the patient. Moreover, a CAT scan can be performed on an outpatient basis, while other tests might require hospitalization. But the cost of a CAT scanner is high, since it runs to more than $1 million dollars.

Recently, a technique has been developed using radio frequency waves rather than X rays to make images in much the same manner as CAT scans. This process is called nuclear magnetic resonance, or NMR.

In NMR, a patient sits inside a huge magnet.

The chemistry of tissues inside his body are analyzed by a camera and a computer that record how hydrogen atoms in the tissues react to powerful magnetic fields.

Ultimately, researchers hope they will be able to distinguish diseased from normal tissue through NMR.