CAT (computed axial tomography) scans are generally performed by a radiology technician in a hospital or outpatient clinic. Patients are urged follow all preparatory steps recommended by their physician prior to the test. Immediately before the test, patients may be asked to change into a hospital gown. All metallic objects, such as watches and jewelry, should be removed so they do not interfere with test results. Infants or children receiving the test may be given a sedative to ensure they are relaxed and still during the test.

If the CAT scan is being performed without the use of a dye (contrast medium), no special preparation is needed. If a contrast medium will be used, patients are instructed not to eat or drink for about four to six hours before the test. A contrast medium contains an iodine liquid that makes certain organs and tissues in the body more visible on a CAT scan. This highlights the structure of a specific organ or vessel to better show the presence of disease or injury.

Contrast mediums may be administered orally (by mouth), through an enema or intravenously (injected into a vein). Although CAT scans are typically painless, patients may experience a stinging sensation or slight discomfort if a contrast medium is administered intravenously. Additionally, injection of the dye may produce a warm, flushed feeling for several minutes or a metallic taste in the mouth. Both sensations are normal and disappear quickly.

During the test, the patient is asked to lie down on a scanning table, which slides slowly into a short, open-ended tunnel. The table can be raised, lowered or tilted. Straps and pillows may be used to keep the patient in a fixed position. During a CAT scan of the head, the table may be fitted with a special cradle that holds the head.

Parents of infants or children receiving a CAT scan are generally permitted to remain with the child during the test. Parents accompanying children may be asked to wear a lead apron to prevent exposure to radiation.

Speakers and a microphone are present to allow the patient to communicate with the technician conducting the test. Patients will be asked to lie very still, and may be asked to hold their breath for short periods of time as the machine captures the images.

In a conventional CAT scan, the x-ray sensing unit and detectors rotate around the body part being scanned while the table moves by a fraction of an inch between scans. Patients are urged to speak with the technician if they are feeling uncomfortable about the sounds made by the machine or any other aspect of the test. Depending on the type of CAT scan performed, the procedure may take anywhere between 10 minutes to an hour.

If a sedative is given, patients are generally advised not to drive themselves from the testing site. Otherwise, patients can resume their regular daily routines immediately following the test. If contrast mediums were administered through an injection and the injection site remains sore, patients should notify their physician.