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.
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).
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. 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.
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