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In computed tomography (CT), 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 images taken from multiple angles all around the person. However, the images 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. CT used to be called CAT (computed axial tomography).
For CT, a person lies on a motorized table that is moved through the opening of a doughnut-shaped scanner. The person is moved continuously 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 images appear to be taken in a spiral fashion around the person, hence the term spiral (helical) CT.
People should wear clothing that has no 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, they 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 contrast agent (see see Overview of Imaging Tests : Contrast Agents). Contrast agents are substances that can be seen on x-rays (called radiopaque dyes) and help distinguish one tissue from another. The dye may be injected into a vein, taken by mouth, or inserted through the anus. The dye used depends on what type of test is done and which body part is being evaluated.
CT can be done as an outpatient procedure. People can resume their usual activities immediately after the test.
Imaging the Interior: Computed Tomography
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 often 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).
Some Disorders Detected by Computed Tomography
This procedure uses CT and a radiopaque dye to produce 2- and 3-dimensional images of blood vessels, including the arteries that supply the heart (coronary arteries). The dye 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 dye flowing through the blood vessels being evaluated. The computer digitally removes all tissues except blood vessels from the images.
CT angiography is used to detect the following:
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). 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.
Typically, CT of the abdomen uses about 500 times the amount of radiation used for a chest x-ray. CT now 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 see Overview of Imaging Tests : 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 dye used in CT angiography contains iodine. A few people have a mild to severe allergic reaction or kidney damage after such dyes are injected (see see Side Effects). People who have had reactions to radiopaque dyes 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.
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