You will lie on a narrow table that slides into the center of the CT scanner.
When 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 makes several images of the body area, called slices. These images can be stored, viewed on a monitor, or printed on film. Models of the body area in 3-D can be created by adding the slices together.
You must stay still during the exam, because movement blurs the pictures. You may have to hold your breath for short periods of time.
The scan should take less than 20 minutes.
How to prepare for the test
Some exams require a special dye, called contrast, to be injected into your body before the test. 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 medicines before the test to avoid this problem.
Before receiving the contrast, tell your health care provider if you take the diabetes medicine metformin (Glucophage). You may need to take extra steps if you are taking this medicine.
Too much weight can cause damage to the scanner's working parts. Talk to your doctor about the weight limit before the test if you weigh more than 300 pounds.
You will need to remove jewelry and wear a hospital gown during the CT exam.
How the test will feel
Some people may be uncomfortable lying on the hard table.
Contrast given through an IV may cause:
Slight burning feeling
Metallic taste in the mouth
Warm flushing of the body
These feelings are normal and usually go away within a few seconds.
Why the test is performed
A CT scan can quickly create more detailed pictures of the knee than standard x-rays. The test may be used to detect:
The cause of pain or other problems in the knee joint (usually when MRI can't be done)
Masses and tumors, including cancer
A CT scan may also be used to guide a surgeon to the right area during a biopsy.
Results are considered normal if no problems are seen.
What abnormal results mean
Abnormal results may be due to:
Abscess (collection of puss)
Bone tumors or cancer
What the risks are
Risks of CT scans include:
Exposure to radiation
Allergy to contrast dye
CT scans give off more radiation than regular x-rays. Many x-rays or CT scans over time may increase your risk for cancer. However, the risk from any one scan is small. You and your doctor should discuss this risk compared with the value of an accurate diagnosis for the problem.
Let your doctor know if you have ever had an allergic reaction to injected contrast dye.
If you need to have this kind 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. You may need extra fluids after the test to help rid your body of the iodine if you have kidney disease or diabetes.
Rarely, the dye may cause a serious allergic response called anaphylaxis. This can be life-threatening. Notify the scanner operator right away if you have any trouble breathing during the test. Scanners have an intercom and speakers so the operator can hear you at all times.
DeLee JC, Drez D Jr, Miller MD, eds. DeLee and Drez’s Orthopaedic Sports Medicine. 3rd ed. Philadelphia, Pa: Saunders Elsevier; 2009:chap 23.
Grainger RG, Thomsen HS, Morcos SK, Koh DM, Roditi G. Intravascular contrast media for radiology, CT, and MRI. In: Adam A, Dixon AK, eds. Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 2.
Shaw AS, Dixon AK. Multidetector computed tomography. In: Adam A, Dixon AK, eds. Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 4.
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, David R. Eltz, Stephanie Slon, and Nissi Wang.