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. These are 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.

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). 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 to avoid this problem.
• Before receiving the contrast, tell your 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. Ask about the weight limit before the test if you weigh more than 300 pounds (135 kilograms).

You will need to remove jewelry and wear a hospital gown during the CT exam as metal can affect the CT images.

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.

A CT scan can quickly create more detailed pictures of the knee than standard x-rays. The test may be used to detect:

• Abscess or infection
• Broken bone
• Examine fractures and pattern of fractures
• The cause of pain or other problems in the knee joint (usually when MRI can't be done)
• Masses and tumors, including cancer
• Healing problems or scar tissue following injuries or surgery

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.

Abnormal results may be due to:

• Abscess (collection of pus)
• Arthritis
• Broken bone
• Bone tumors or cancer
• Healing problems or scar tissue after surgery

Risks of CT scans include:

• Exposure to radiation
• Allergy to contrast dye
• Birth defect if done during pregnancy

CT scans give off more radiation than regular x-rays. Having 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 provider should discuss this risk compared with the value of an accurate diagnosis for the problem.

Let your provider know if you have ever had an allergic reaction to injected contrast dye.

• The most common type of contrast contains iodine. You may have nausea or vomiting, sneezing, itching, or hives if you have this an iodine allergy.
• If you need to have this kind of contrast, you may need 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.

CAT scan - knee; Computed axial tomography scan - knee; Computed tomography scan - knee

Damasena I, Spalding T. Computed tomography of the knee joint: indications and significance. In: LaPrade RF, Chahla J, eds. Evidence-based Management of Complex Knee Injuries. St Louis, MO: Elsevier; 2021:chap 4.

Kapoor G, Toms AP. Current status of imaging of the musculoskeletal system. In: Adam A, Dixon AK, Gillard JH, Schaefer-Prokop CM, eds. Grainger & Allison's Diagnostic Radiology. 7th ed. Philadelphia, PA: Elsevier; 2021:chap 38.

Madoff SD, Burak JS, Math KR, Walz DM. Knee imaging techniques and normal anatomy. In: Scott WN, ed. Insall & Scott Surgery of the Knee. 6th ed. Philadelphia, PA: Elsevier; 2018:chap 5.

Major N, Colemen C. Imaging of the knee. In: Miller MD, Thompson SR, eds. DeLee, Drez, & Miller's Orthopaedic Sports Medicine. 5th ed. Philadelphia, PA: Elsevier; 2020:chap 91.

Updated by: C. Benjamin Ma, MD, Professor, Chief, Sports Medicine and Shoulder Service, UCSF Department of Orthopaedic Surgery, San Francisco, CA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.