Computed tomography (CT) an imaging method that uses x-rays to create cross-sectional pictures of the body. CT rapidly creates detailed pictures of the body, including the brain, chest, and abdomen. The test may be used to study blood vessels, identify masses or tumors (including cancer), or guide a surgeon to the right area during a biopsy.

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. 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. 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 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 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.

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.

CT scans do create low levels of radiation, which has the potential to cause cancer and other defects. However, the risk associated with any individual scan is small. The risk increases as numerous additional studies are performed. In some cases, a CT scan may still be done if the benefits greatly out weigh the risks. For example, it can be more risky not to have the exam, especially if your health care provider thinks you might have cancer. Rarely, the dye may cause anaphylaxis (a life-threatening allergic response). If you have any trouble breathing during the test, you should notify the scanner operator immediately. Scanners come with an intercom and speakers, so the operator can hear you at all times.