Computed tomography (CT) an imaging method that uses x-rays to create cross-sectional pictures of the body.

You will be asked to lie on a narrow table that slides into the center of the CT scanner. Depending on the study being done, you may need to lie on your stomach, back, or side.

Once inside the scanner, the machine's x-ray beam rotates around you. (Modern "spiral" scanners can perform the exam in one continuous motion.)

Small detectors inside the scanner measure the amount of x-rays that make it through the part of the body being studied. A computer takes this information and uses it to create several individual images, called slices. These images can be stored, viewed on a monitor, or printed on film. Three-dimensional models of organs can be created by stacking the individual slices together.

You must be still during the exam, because movement causes blurred images. You may be told to hold your breath for short periods of time.

Generally, complete scans take only a few minutes. The newest multidetector scanners can image your entire body, head to toe, in less than 30 seconds.

Certain exams require a special dye, called contrast, to be delivered into the body before the test starts. Contrast can highlight specific areas inside the body, which creates a clearer image.

Contrast may be delivered using an intravenous line (IV) that is placed in a vein in your hand or forearm. Contrast may also be given through the rectum using an enema, or as a liquid that you drink before the scan. (When you actually drink the contrast depends on the type of exam being done.) The contrast liquid may taste chalky, although some are flavored to make it taste a little better. The contrast eventually passes out of your body through your stools.

If contrast is used, you may also be asked not to eat or drink anything for 4-6 hours before the test.

If you weigh more than 300 pounds, have your doctor contact the scanner operator before the exam. CT scanners have a weight limit. Too much weight can cause damage to the scanner's working parts.

Since x-rays have difficulty passing through metal, you will be asked to remove jewelry and wear a hospital gown during the study.

The x-rays are painless. Some people may have discomfort from lying on the hard table.

Contrast give through an IV may cause a slight burning sensation, a metallic taste in the mouth, and a warm flushing of the body. These sensations are normal and usually go away within a few seconds.