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Computed tomography (CT)

Medically reviewed by Last updated on Sep 21, 2023.

What is Computed tomography (CT)?

Harvard Health Publishing

CT scans are pictures taken by a specialized x-ray machine. The machine circles your body and scans an area from every angle within that circle. The machine measures how much the x-ray beams change as they pass through your body. It then relays that information to a computer, which generates a collection of black-and-white pictures, each showing a slightly different "slice" or cross-section of your internal organs.

Because these "slices" are spaced only about a quarter-inch apart, they give a very good representation of your internal organs and other structures. Doctors use CT scans to evaluate all major parts of the body, including the abdomen, back, chest, and head.

Computed tomography (CT)

You can have a CT scan in an outpatient facility or in a hospital. The procedure is painless and takes about 15 to 20 minutes, but can be longer or shorter depending on the area of the body being scanned.

What it's used for

CT can reveal abnormal masses that may be cancerous tumors. CT scans show the size and shape of the tumor, its precise location in the body, and whether the tumor is solid or hollow. Sometimes, a CT scan can tell the difference between a noncancerous and cancerous tumor, although a biopsy or other test is needed to make the final diagnosis. In a biopsy, a small piece of tissue is removed to be examined in a laboratory. During a CT-guided biopsy, the physician will use the CT scan as a guide as he or she inserts the needle into the right location to remove a sample of the tumor.

In addition to detecting cancer, CT scans have many other uses. They can show abscesses and other infections, strokes, head injuries and bleeding inside the skull, as well as a variety of other medical conditions.

For obese patients, CT scanning may be a more useful diagnostic tool than ultrasound, because large amounts of body fat can interfere with ultrasound waves, producing poor images.


CT scanning usually does not require any special preparation. Remove jewelry in the area being scanned. If you are a woman and there is any chance that you may be pregnant, tell your doctor before your scan.

You may need to drink a liquid dye to make organs or blood vessels stand out more clearly on the CT scan. Sometimes, this dye, called a "contrast medium," is injected into your vein. Tell your doctor if you have ever had an allergic reaction to this sort of dye or if you have any allergies to medicines. If you take a diabetes medication called metformin (Glucophage), ask your doctor if you should stop taking it 48 hours before your test, because it may interact with the contrast dye.

Treatment options

The following list of medications are in some way related to or used in the treatment of this condition.

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How it's done

The test is done in the radiology department of a hospital or in a diagnostic clinic. You wear a hospital gown and lie on your back on a table that can slide back and forth through the donut-shaped CT machine. If your test requires contrast dye, a technician or other health care professional may insert an IV and inject contrast dye through it. This dye outlines blood vessels and soft tissue to help them show up clearly on the pictures.

The technologist moves the table with a remote control to enable the CT machine to scan your body from all of the desired angles. You will be asked to hold your breath for a few seconds each time a new level is scanned. The technologist usually works the controls from an adjoining room, watching through a window and sometimes speaking to you through a microphone.


A radiologist (a doctor who specializes in imaging) reads and interprets your CT images. The personnel at the CT scan facility will tell you when to check with your doctor for the results of your scanning.


Although CT scanning requires a slightly higher amount of X-ray exposure than conventional X-rays, it gives images that are much clearer.

When to call a professional

If you received contrast dye by an injection, and the injection site becomes red or painful, call your doctor.

Additional info

National Library of Medicine (NLM)

American College of Radiology

Learn more about Computed tomography

Treatment options

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.