What is computed tomography?
Computed tomography (CT) is an imaging procedure that uses special x-ray equipment to create detailed pictures, or scans, of areas inside the body. It is also called computerized tomography and computerized axial tomography (CAT).
The term tomography comes from the Greek words tomos (a cut, a slice, or a section) and graphein (to write or record). Each picture created during a CT procedure shows the organs, bones, and other tissues in a thin “slice” of the body. The entire series of pictures produced in CT is like a loaf of sliced bread—you can look at each slice individually (2-dimensional pictures), or you can look at the whole loaf (a 3-dimensional picture). Computer programs are used to create both types of pictures.
Most modern CT machines take continuous pictures in a helical (or spiral) fashion rather than taking a series of pictures of individual slices of the body, as the original CT machines did. Helical CT has several advantages over older CT techniques: it is faster, produces better 3-D pictures of areas inside the body, and may detect small abnormalities better. The newest CT scanners, called multislice CT or multidetector CT scanners, allow more slices to be imaged in a shorter period of time.
In addition to its use in cancer, CT is widely used to help diagnose circulatory (blood) system diseases and conditions, such as coronary artery disease (atherosclerosis), blood vessel aneurysms, and blood clots; spinal conditions; kidney and bladder stones; abscesses; inflammatory diseases, such as ulcerative colitis and sinusitis; and injuries to the head, skeletal system, and internal organs. CT can be a life-saving tool for diagnosing illness and injury in both children and adults.
What can a person expect during a CT procedure?
During a CT procedure, the person lies very still on a table, and the table passes slowly through the center of a large x-ray machine. With some types of CT scanners, the table stays still and the machine moves around the person. The person might hear whirring sounds during the procedure. At times during a CT procedure, the person may be asked to hold their breath to prevent blurring of the images.
Sometimes, CT involves the use of a contrast (imaging) agent, or “dye.” The dye may be given by mouth, injected into a vein, given by enema, or given in all three ways before the procedure. The contrast dyehighlights specific areas inside the body, resulting in clearer pictures. Iodine and barium are two dyes commonly used in CT.
In very rare cases, the contrast agents used in CT can cause allergic reactions. Some people experience mild itching or hives (small bumps on the skin). Symptoms of a more serious allergic reaction include shortness of breath and swelling of the throat or other parts of the body. People should tell the technologist immediately if they experience any of these symptoms, so they can be treated promptly. Very rarely, the contrast agents used in CT can also cause kidney problems in certain patients. These kidney problems usually do not have any symptoms, but they can be detected by running a simple test on a blood sample.
CT does not cause any pain. However, lying in one position during the procedure may be slightly uncomfortable. The length of a CT procedure depends on the size of the area being scanned, but it usually lasts only a few minutes to half an hour. For most people, the CT is performed on an outpatient basis at a hospital or a radiology center, without an overnight hospital stay.
Some people are concerned about experiencing claustrophobia during a CT procedure. However, most CT scanners surround only portions of the body, not the whole body. Therefore, people are not enclosed in a machine and are unlikely to feel claustrophobic.
Women should let their health care provider and the technologist know if there is any possibility that they are pregnant, because radiation from CT can harm a growing fetus.