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Knee MRI scan

Definition

A knee MRI (magnetic resonance imaging) scan is a imaging method that uses powerful magnets and radio waves to create pictures of the knee joint and surrounding muscles and other tissues.

See also: Leg MRI

An MRI does not use radiation. Single MRI images are called slices. The images can be stored on a computer or printed on film. One exam produces dozens or sometimes hundreds of images.

Alternative Names

MRI - knee

How the test is performed

You may be asked to wear a hospital gown or clothing without metal fasteners (such as sweatpants and a t-shirt). Certain types of metal can cause blurry images.

You will lie on a narrow table, which slides into a large tunnel-like tube.

Some exams require a special dye (contrast). The dye is usually given before the test through a vein (IV) in your hand or forearm. Sometimes, the dye is given directly into a joint. The dye helps the radiologist see certain areas more clearly.

During the MRI, the person who operates the machine will watch you from another room. The test most often lasts 30-60 minutes, but may take longer.

How to prepare for the test

You may be asked not to eat or drink anything for 4 - 6 hours before the scan.

Tell your doctor if you are afraid of close spaces (have claustrophobia). You may be given a medicine to help you feel sleepy and less anxious, or your doctor may suggest an "open" MRI, in which the machine is not as close to the body.

Before the test, tell your health care provider if you have:

• Brain aneurysm clips
• Certain types of artificial heart valves
• Heart defibrillator or pacemaker
• Inner ear (cochlear) implants
• Kidney disease or dialysis (you may not be able to receive contrast)
• Recently placed artificial joints
• Certain types of vascular stents
• Worked with sheet metal in the past (you may need tests to check for metal pieces in your eyes)

Because the MRI contains strong magnets, metal objects are not allowed into the room with the MRI scanner:

• Pens, pocketknives, and eyeglasses may fly across the room.
• Items such as jewelry, watches, credit cards, and hearing aids can be damaged.
• Pins, hairpins, metal zippers, and similar metallic items can distort the images.
• Removable dental work should be taken out just before the scan.

How the test will feel

An MRI exam causes no pain. If you have difficulty lying still or are very nervous, you may be given a medicine to relax you. Too much movement can blur MRI images and cause errors.

The table may be hard or cold, but you can request a blanket or pillow. The machine produces loud thumping and humming noises when turned on. You can wear ear plugs to help reduce the noise.

An intercom in the room allows you to speak to someone at any time. Some MRIs have televisions and special headphones that you can use to help the time pass.

There is no recovery time, unless you were given a medicine to relax. After an MRI scan, you can resume your normal diet, activity, and medications.

Why the test is performed

Your doctor may order this test if you have:

• An abnormal finding on a knee x-ray or bone scan
• A feeling that your knee is giving away in the knee joint
• Buildup of joint fluid behind the knee (Baker's cyst)
• Fluid collecting in the knee joint
• Infection of the knee joint
• Knee cap injury
• Knee pain with fever
• Knee locking when you walk or moving
• Signs of damage to the knee muscle, cartilage, or ligaments
• Knee pain that does not get better with treatment

Your doctor may also order this test to monitor your progress after knee surgery.

Normal Values

A normal result means your knee appears normal.

What abnormal results mean

Abnormal results may be due to a sprain or tear of the ligaments in the knee area, including:

• Anterior cruciate ligament injury
• Medial or lateral collateral ligament injury
• Knee cartilage tear (meniscus tear)
• Posterior cruciate ligament injury

Abnormal results may also be due to:

• Arthritis of the knee
• Avascular necrosis (also called osteonecrosis)
• Bone tumor or cancer
• Broken bone
• Buildup of joint fluid behind the knee (Baker's cyst)
• Infection in the bone (osteomyelitis)
• Inflammation
• Injury of the knee cap

Consult your health care provider with any questions and concerns.

What the risks are

MRI contains no radiation. To date, no side effects from the magnetic fields and radio waves have been reported.

The most common type of contrast (dye) used is gadolinium. It is very safe. Allergic reactions to the substance rarely occur. However, gadolinium can be harmful to patients with kidney problems who require dialysis. If you have kidney problems, please tell your health care provider before the test.

The strong magnetic fields created during an MRI can cause heart pacemakers and other implants to not work as well. It can also cause a piece of metal inside your body to move or shift.

Special considerations

Tests that may be done instead of a knee MRI include:

• CT scan of the knee
• Knee x-ray

References

Wilkinson ID, Paley MNJ. Magnetic resonance imaging: basic principles. In: Grainger RC, Allison D, Adam, Dixon AK, eds. Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 5.

DeLee 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 & Allison's Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 2.