You lie on a narrow table that slides into the center of the CT scanner. Only your head is placed inside the CT scanner.
You may be allowed to rest your head on a pillow.
Once you are inside the scanner, the machine's x-ray beam rotates around you.
A computer creates separate images of the body area, called slices. These images can be stored, viewed on a monitor, or printed on film. The computer can create three-dimensional models of the body area by stacking the slices together.
You must lie still during the exam, because movement causes blurred images. You may be told to hold your breath for short periods.
The actual scan takes about 30 seconds. The entire process takes about 15 minutes.
How to Prepare for the Test
You will be asked to remove jewelry and wear a hospital gown during the study.
If you weigh more than 300 pounds, find out if the CT machine has a weight limit. Too much weight can cause damage to the scanner's working parts.
Certain exams require a special dye, called contrast, to be delivered into the body before the test starts. Contrast helps certain areas show up better on the x-rays.
Contrast can be given through a vein (IV) in your hand or forearm. If contrast is used, you may also be asked not to eat or drink anything for 4 to 6 hours before the test.
Let your health care provider know if you have ever had a reaction to contrast. You may need to take medicines before the test in order to safely receive this substance.
Before receiving the contrast, tell your provider if you take the diabetes medicine metformin (Glucophage) because you may need to take extra precautions.
Before the scan, let your provider know if you have poor kidney function. This is because the contrast may not be able to be used.
How the Test will Feel
Some people may have discomfort from lying on the hard table.
Contrast given through an IV may cause a slight burning sensation. You may also have a metallic taste in the mouth and a warm flushing of the body. These sensations are normal and most often go away within a few seconds.
Why the Test is Performed
This test is helpful for diagnosing diseases that affect the following areas:
CT scans and other x-rays are strictly monitored and controlled to make sure they use the least amount of radiation. The risk associated with any individual scan is very low. The risk increases as more studies are performed.
CT scans are done when the benefits greatly outweigh the risks. For example, it can be more risky not to have the exam, especially if your provider thinks you might have cancer.
The most common type of contrast given into a vein contains iodine. If a person with an iodine allergy is given this type of contrast, nausea, sneezing, vomiting, itching, or hives may occur. If you have a known allergy to contrast but need it for a successful exam, you may receive antihistamines (such as Benadryl) or steroids before the test.
The kidneys help filter the iodine out of the body. If you have kidney disease or diabetes, you should be closely monitored for kidney problems after contrast is given. If you have diabetes or have kidney disease, talk to your provider before the test about your risks.
Before receiving the contrast, tell your provider if you take the diabetes medicine metformin (Glucophage) because you may need to take extra precautions and stop the medicine for 48 hours after the test.
In rare cases, the dye can cause a life-threatening allergic response called anaphylaxis. If you have any trouble breathing during the test, you should tell the scanner operator right away. Scanners come with an intercom and speakers, so the operator can hear you at all times.
Thust SC, Miszkiel K, Davagnanam I. Orbit. In: Adam A, Dixon AK, Gillard JH, Schaefer-Prokop CM, eds. Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging. 6th ed. New York, NY: Churchill Livingstone; 2014:chap 66.
Jason Levy, MD, Northside Radiology Associates, Atlanta, GA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.