In preparation for a CT scan, patients are often asked to avoid food, especially when contrast material is to be used. Contrast material may be injected intravenously, or administered by mouth or by an enema in order to increase the distinction between various organs or areas of the body. Therefore, fluids and food may be restricted for several hours prior to the examination. If the patient has a history of allergy to contrast material (such as iodine), the requesting physician and radiology staff should be notified. All metallic materials and certain clothing around the body are removed because they can interfere with the clarity of the images.

Patients are placed on a movable table, and the table is slipped into the center of a large donut-shaped machine which takes the x-ray images around the body. The actual procedure can take from a half an hour to an hour and a half. If specific tests, biopsies, or intervention are performed by the radiologist during CT scanning, additional time and monitoring may be required. It is important during the CT scan procedure that the patient minimize any body movement by remaining as still and quiet as is possible. This significantly increases the clarity of the x-ray images. The CT scan technologist tells the patient when to breathe or hold his/her breath during scans of the chest and abdomen. If any problems are experienced during the CT scan, the technologist should be informed immediately. The technologist directly watches the patient through an observation window during the procedure, and there is an intercom system in the room for added patient safety.

CT scans have vastly improved the ability of doctors to diagnose many diseases earlier in their course and with much less risk than previous methods. Further refinements in CT scan technology continue to evolve which promise even better picture quality and patient safety. Newer CT scans called "spiral" or "helical" CT scans can provide more rapid and accurate visualization of internal organs. For example, many trauma centers are using these scans to more rapidly diagnose internal injuries after serious body trauma. High resolution CT scans (HRCT) are used to accurately assess the lungs for inflammation and scarring.