DefinitionA computed tomography (CT) scan of the knee is test that uses x-rays to make detailedimages of the knee.Alternative NamesCAT scan – knee; Computed axial tomography scan – knee; Computed tomography scan – kneeHow the test is performedYou will lie on a narrow table that slides into the center of the CT scanner.When you are inside the scanner, the machines 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 byadding the slices together.You must stay still during the exam, because movement blurs the pictures. You mayhave tohold your breath for short periods of time.The scan should take less than 20 minutes.How to prepare for the testSome exams require a special dye, called contrast, to be injected intoyour 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 scanners working parts. Talk to your doctor about the weight limit before the test if youweigh more than 300 pounds.advertisementYou willneed toremove jewelry and wear a hospital gown during the CT exam.How the test will feelSome people may be uncomfortablelying on the hard table.Contrast given through an IV may cause:Slight burning feelingMetallic taste in the mouthWarm flushing of the bodyThesefeelings are normal and usually go away within a few seconds.Why the test is performedA CT scan can quickly createmore detailed pictures of the knee than standard x-rays. The test may be used to detect:Abscess or infectionBroken boneExamine fracturs and pattern of fracturesThe cause of pain or other problems in the knee joint (usually when MRI cant be done)Masses and tumors, including cancerA CT scan may also be used to guide a surgeon to the right area during a biopsy.Normal ValuesResults are considered normal if no problems are seen.What abnormal results meanAbnormal results may be due to:Abscess (collection of puss)ArthritisBroken boneBone tumors or cancerWhat the risks areRisks of CT scans include:Exposureto radiationAllergyto contrast dyeCT scans give offmore 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 ofan accurate diagnosis for the problem.Let your doctor know if you have ever had an allergic reaction to injected contrast dye.The most common type of contrast contains iodine. You may havenausea or vomiting,sneezing, itching,or hivesif you have this an iodine allergy.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 havekidney 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. Scannershave an intercom and speakers so the operator can hear you at all times.ReferencesDeLee 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 & Allisons 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 & Allisons Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 4.Review Date:1/17/2013Reviewed By:advertisementC. 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.