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Computed Tomography (CT)

By

Mehmet Kocak

, MD, Rush University Medical Center

Last full review/revision Jun 2019| Content last modified Jul 2019
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In computed tomography (CT), which used to be called computed axial tomography (CAT), an x-ray source and x-ray detector rotate around a person. In modern scanners, the x-ray detector usually has 4 to 64 or more rows of sensors that record the x-rays that pass through the body. Data from the sensors represent a series of x-ray measurements taken from multiple angles all around the person. However, the measurements are not viewed directly but are sent to a computer. The computer converts them into images that resemble 2-dimensional slices (cross-sections) of the body. (Tomo means slice in Greek.) The computer can also construct 3-dimensional images from the recorded images.

Procedure for CT

For CT, a person lies on a motorized table that is moved through the opening of a doughnut-shaped scanner. The person is moved through the scanner as these devices rotate around the person. For some CT scans, the table moves incrementally and stops when each scan (slice) is taken. For other CT scans, the table moves continuously during scanning. Because the person is moving in a straight line and the detectors are moving in a circle, the series of measurements appear to be taken in a spiral fashion around the person—hence the term spiral (helical) CT.

People should wear clothing that has no metallic buttons, snaps, zippers, or other metal in it over the area to be scanned and should remove any jewelry. Such items are not dangerous but may block x-rays and distort the image. During the test, people must remain still and periodically hold their breath when the x-rays are taken so that the images are not blurred. People may hear whirring sounds during the procedure.

The procedure, depending on the area examined and how modern the scanner is, usually takes from only a few seconds to a few minutes. CT of the chest takes less than a minute, and people have to hold their breath only once and only for a few seconds.

For CT, people may be given a radiopaque contrast agent. Contrast agents are substances that can be seen on x-rays and help distinguish one tissue from another. The contrast agent may be injected into a vein, taken by mouth, or inserted through the anus. The contrast agents used depends on what type of test is done and which body part is being evaluated.

CT is usually done as an outpatient procedure. People can resume their usual activities immediately after the test.

Imaging the Interior: Computed Tomography

In computed tomography, a scanner produces and records x-rays as it rotates around a person, who is moved through the scanner on a motorized table. On one side of the scanner is an x-ray tube, which produces x-rays, and on the other side is an x-ray detector.

Imaging the Interior: Computed Tomography

Uses of CT

The highly detailed images provide more detail about tissue density and location of abnormalities than plain x-rays, so doctors can precisely locate structures and abnormalities. CT enables the examiner to distinguish between different types of tissue, such as muscle, fat, and connective tissues. Thus, CT can provide detailed images of specific organs not visible on plain x-rays and is more useful for imaging most structures in the brain, head, neck, chest, and abdomen.

CT can detect and provide information about disorders in almost every part of the body. For example, doctors can use CT to detect a tumor, measure its size, precisely locate it, and determine how far it has spread into nearby tissues. CT can also help doctors monitor the effectiveness of treatment (such as antibiotics for a brain abscess or radiation therapy for a tumor).

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Some Disorders Detected by Computed Tomography

Body System

Disorders

Brain and spinal cord

Bleeding within the brain (intracerebral hemorrhage)

Digestive tract

Eyes

Infections in the eyeball and infections around the eye socket (orbit)

Tumors of the eye socket (orbit) or optic nerve

Heart and blood vessels

Kidneys and urinary tract

Bleeding in or around the kidneys

Stones in the kidneys or urinary tract

Tumors in or around the kidneys

Liver

Fatty liver (excess fat in the liver)

Lungs

Bronchiectasis (widened airways)

Muscles and bones

Fractures and other problems with bone and soft tissues

Variations of CT

CT angiography

CT angiography uses CT and a radiopaque contrast agent to produce 2- and 3-dimensional images of blood vessels, including the arteries that supply the heart (coronary arteries). The contrast agent is injected into a vein (not an artery as in conventional angiography), usually in the arm. Images are taken rapidly and are timed so that they show the radiopaque contrast agent flowing through the blood vessels being evaluated. The computer digitally removes all tissues except blood vessels from the images. (See also Coronary Angiography.)

CT angiography is used to detect the following:

  • Narrowing or blockages (such as blood clots) in arteries

  • Bulges (aneurysms) and tears (dissections) in large arteries

  • Abnormal blood vessels that carry blood to tumors

CT angiography is commonly used instead of conventional angiography because it is safe and less invasive (it does not require insertion of a catheter in an artery, which has slightly more risk than insertion of a catheter into a vein). CT angiography shows abnormalities in blood vessels about as accurately as magnetic resonance angiography, but slightly less accurately than conventional angiography.

CT angiography usually takes only 1 to 2 minutes.

Other variations

CT can be used to provide images of the

  • Stomach or small intestine (called CT enterography)

  • Colon (called virtual colonoscopy, or CT colonography)

  • Kidneys, ureters, and bladder (called CT intravenous urography or pyelography)

  • Arteries of the lungs (called CT pulmonary angiography)

Disadvantages of CT

Typically, CT of the abdomen uses about 300 to 400 times the amount of radiation used for a single-view plain x-ray of the chest. Even though newer CT techniques use much lower radiation doses than those previously used, CT accounts for most exposure to man-made radiation in the general population and for about 70% of radiation exposure in medical practice. Therefore, the doctor and person should carefully weigh the benefit of each CT procedure against the risks (see Risks of Radiation). Generally, CT is avoided when possible in pregnant women unless there is no good alternative. Use of CT in children should be limited as much as possible.

The radiopaque contrast agents used in CT angiography contain iodine—called iodinated contrast agents. A few people have a mild to severe allergic reaction or kidney damage after such agents are injected. People who have had reactions to these agents should let their doctor know before CT angiography is done.

In some countries and in some areas of the United States, CT is not readily available.

Did You Know

  • Most radiation exposure in medicine comes from computed tomography (CT).

  • Before CT, people should let their doctor know whether they have ever had a reaction to a contrast agent used in CT.

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NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
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