Patient Basics: Computed Tomography (CT)

Originally published by Harvard Health.

What Is It?

Computed tomography, also called CT or CT scan, is a process that uses X-rays and computer technology to make cross-sectional images of the body. A series of X-ray pictures, each a thin slice, are put together in a computer to form a three-dimensional view of the inside of your body. If an X-ray is like looking at a photo of a heart, a CT scan is like looking at a model that you can pick up and examine from any angle.

Computed Tomography (CT)

In a CT scan, X-rays pass through the body and are analyzed by a computer. The computer builds an image based on the amount of X-rays passing through tissues of different thickness. For example, bone appears white on a CT scan, and gas bubbles in the stomach and intestines appear black.

Computed Tomography (CT) 1

You can have a CT scan in an outpatient facility or in a hospital. The procedure is painless and takes about 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.

How It’s Done

You will lie down on a special scanning table. The scanner rotates 360° around you. The scanning table also may be moved.

Computed Tomography (CT) 2


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)
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Bethesda, MD 20894
Phone: (301) 594-5983
Toll-Free: (888) FIND-NLM (346-3656)
Fax: (301) 496-4450

American College of Radiology
1891 Preston White Drive
Reston, VA 20191-4397
Toll-Free: (800) ACR-LINE (227-5463)