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Knee CT scan

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

A computed tomography (CT) scan of the knee is a test that uses x-rays to take detailed images of the knee.

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How the Test is Performed

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.

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.

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.

How the Test will Feel

Some people may be uncomfortable lying on the hard table.

Contrast given through an IV may cause:

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:

A CT scan may also be used to guide a surgeon to the right area during a biopsy.

Normal Results

Results are considered normal if no problems are seen.

What Abnormal Results Mean

Abnormal results may be due to:

Risks

Risks of CT scans include:

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 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.

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.

References

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.

Sanders T. Imaging of the knee. In: Miller MD, Thompson SR, eds. DeLee and Drez's Orthopaedic Sports Medicine. 4th ed. Philadelphia, PA: Elsevier Saunders; 2015:chap 93.

Shaw AS, Prokop M. Computed tomography. In: Adam A, Dixon AK, Gillard JH, Schaefer-Prokop CM, eds. Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging. 6th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2015:chap 4.

Thomsen HS, Reimer P. Intravascular contrast media for radiography, CT, MRI and ultrasound. In: Adam A, Dixon AK, Gillard JH, Schaefer-Prokop CM, eds. Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging. 6th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2015:chap 2.

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Review Date: 3/17/2019  

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

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