You will lie on a narrow table that slides into the center of the CT scanner.
Once you are inside the scanner, the machine's x-ray beam rotates around you. (Modern "spiral" scanners can perform the exam without stopping.)
A computer creates separate images of the body area, called slices. These images can be stored, viewed on a monitor, or printed on film. Three-dimensional models of the leg can be created by adding the slices together.
You will need to lie still during the exam. Movement can cause blurred images. You may need to hold your breath for short periods of time.
The scan should take only 10-15 minutes.
How to prepare for the test
Some exams use a special dye, called contrast, that is put into your 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 - 6 hours before the test.
Let your doctor know if you have ever had a reaction to contrast. You may need to take medications before the test in order to avoid this problem.
Before having the contrast, tell your health care provider if you take the diabetes medicine metformin (Glucophage). You may need to take extra steps before the test if your take this drug.
Too much weight can cause damage to the scanner's working parts. Find out if the CT machine has a weight limit if you weigh more than 300 pounds.
You will wear a hospital gown during the study. You will need to take off all jewelry.
How the test will feel
Some people may be uncomfortable lying on the hard table.
Contrast given through an IV may cause a slight burning feeling, a metal taste in the mouth, and a warm flushing of the body. These feelings are normal and go away in a few seconds.
Why the test is performed
CT makes detailed pictures of the body very quickly. The test may help look for:
CT scans expose you to more radiation than regular x-rays. Having many x-rays or CT scans over time may raise your risk for cancer, but the risk from any one scan is small. Talk to your doctor about this risk against the benefits of the test.
Some people have allergies to contrast dye. Let your doctor know if you have ever had this type of reaction.
The most common type of contrast given into a vein contains iodine. A person with an iodine allergy may have nausea or vomiting, sneezing, itching, or hives from this type of contrast.
If you must have this type of contrast, your doctor may give you antihistamines (such as Benadryl) or steroids before the test.
The kidneys help remove iodine out of the body. People with kidney disease or diabetes may need to get extra fluids after the test to help flush the iodine out of the body.
The dye may cause a life-threatening allergic response called anaphylaxis. This is rare. Tell the scanner operator right away if you have any trouble breathing during the test. Scanners come with an intercom and speakers, so the operator can hear you at all times.
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C. Benjamin Ma, MD, Assistant Professor, Chief, Sports Medicine and Shoulder Service, UCSF Department of Orthopaedic Surgery. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, Bethanne Black, Stephanie Slon, and Nissi Wang.