A computed tomography (CT) scan of the knee is an imaging method that uses x-rays to create detailed pictures of the knee.

CT rapidly creates more detailed pictures of the knee than can be seen on standard x-rays. The test may be used to:

• Diagnose or evaluate cancer or tumors
• Evaluate a mass that can be felt during a physical exam
• Evaluate an abscess or infection
• Evaluate pain or other problems in the knee joint when MRI cannot be done
• Guide a surgeon to the right area during a biopsy
• Identify masses and tumors, including cancer
• Identify or evaluate a fracture

You will be asked to lie on a narrow table that slides into the center of the CT scanner. You will lie on your back.

Once you are inside the scanner, the machine's x-ray beam rotates around you. (Modern "spiral" scanners can perform the exam in one continuous motion.)

Small detectors inside the scanner measure the amount of x-rays that make it through the knee. A computer takes this information and uses it to create several individual images, called slices. These images can be stored, viewed on a monitor, or printed on film. Three-dimensional models of the knee can be created by stacking the individual slices together.

You must be still during the exam, because movement causes blurred images. You may be told to hold your breath for short periods of time.

The test should take less than 1 hour.

Certain exams require a special dye, called contrast, to be delivered into the body before the test starts. Contrast can highlight specific areas inside the body, which creates a clearer image.

Some people have allergies to IV contrast and may need to take medications before their test in order to safely receive this substance.

Contrast can be given several ways:

• It may be delivered through a vein (IV) in your hand or forearm.
• You might drink the contrast before your scan. The contrast liquid may taste chalky, although some are flavored to make it taste a little better. The contrast eventually passes out of your body through your stools.

If contrast is used, you may also be asked not to eat or drink anything for 4-6 hours before the test.

If you weigh more than 300 pounds, have your doctor contact the scanner operator before the exam. CT scanners have a weight limit. Too much weight can cause damage to the scanner's working parts.

Since x-rays have difficulty passing through metal, you will be asked to remove jewelry and wear a hospital gown during the study.

Some people may have discomfort from lying on the hard table.

Contrast given through an IV may cause a slight burning sensation, a metallic taste in the mouth, and a warm flushing of the body. These sensations are normal and usually go away within a few seconds.