Computed tomography (CT) is 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.

Some people have allergies to IV contrast and may need to take medications before their test in order to safely receive this substance.

Contrast can be given several ways, and depends on the type of CT being performed.

• It may be delivered through a vein (IV) in your hand or forearm.
• It may be given through the rectum using an enema.
• You might drink the contrast before your 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.

CT rapidly creates detailed pictures of the body, including the brain, chest, spine, and abdomen. The test may be used to:

• Guide a surgeon to the right area during a biopsy
• Identify masses and tumors, including cancer
• Study blood vessels

Results are considered normal if the organs and structures being examined are normal in appearance.

The significance of abnormal results depends on the part of the body being studied and the nature of the problem. Consult your health care provider with any questions and concerns.

CT scans and other x-rays are strictly monitored and controlled to make sure they use the least amount of radiation. CT scans do create low levels of ionizing 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.

An abdominal CT scan is usually not recommended for pregnant women, because it may harm the unborn child. Women who are or may be pregnant should speak with their health care provider to determine if ultrasound can be used instead.

The most common type of contrast given into a vein contains iodine. If a person with an iodine allergy is given this type of contrast, nausea, sneezing, vomiting, itching, or hives may occur.

If you absolutely must be given such contrast, your doctor may choose to treat you with antihistamines (such as Benadryl) or steroids before the test.

The kidneys help filter the iodine out of the body. Therefore, those with kidney disease or diabetes should receive plenty of fluids after the test, and be closely monitored for kidney problems. If you have diabetes or are on kidney dialysis, talk to your health care provider before the test about your risks.

Rarely, the dye may cause a life-threatening allergic response called anaphylaxis . 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.

For additional information, please see the specific CT topics:

• Cranial CT scan
• Lumbosacral spine CT scan
• Thoracic CT scan
• Orbit CT scan

Shaw AS, Dixon AK. Multidetector computed tomography. In: Grainger RC, Allison D, Adam, Dixon AK, eds.Diagnostic Radiology: A Textbook of Medical Imaging. 4th ed. New York, NY: Churchill Livingstone; 2001:chap 4.