Knee CT scan

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

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.

• 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.

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:

• Abscess or infection

  Abscess

  An abscess is a collection of pus in any part of the body. In most cases, the area around an abscess is swollen and inflamed.

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• 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)

  MRI

  A magnetic resonance imaging (MRI) scan is an imaging test that uses powerful magnets and radio waves to create pictures of the body. It does not us...

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• 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.

Normal Results

Results are considered normal if no problems are seen.

What Abnormal Results Mean

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

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.

  Nausea or vomiting

  Nausea is feeling an urge to vomit. It is often called "being sick to your stomach. "Vomiting or throwing-up forces the contents of the stomach up t...

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  Sneezing

  A sneeze is a sudden, forceful, uncontrolled burst of air through the nose and mouth.

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  Itching

  Itching is a tingling or irritation of the skin that makes you want to scratch the area. Itching may occur all over the body or only in one location...

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  Hives

  Hives are raised, often itchy, red bumps (welts) on the surface of the skin. They can be an allergic reaction to food or medicine. They can also ap...

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• 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.

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