During a CT procedure, the person lies very still on a table, and the table passes slowly through the center of a large donut-shaped 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 dye highlights 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 cause kidney problems for certain patients, such as those with impaired kidney function. Patient kidney function can be checked with a simple blood test before the contrast agent is injected.
CT is a noninvasive procedure and 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. Depending on the part of the body to be scanned, the provider may reduce the radiation dose or use an alternative imaging method. However, the level of radiation exposure in a CT scan is believed to be too low to harm a growing fetus.
Patient undergoing CT of the abdomen. Drawing shows the patient on a table that slides through the CT machine, which takes x-ray pictures of the inside of the body.
Credit:  Terese Winslow