A magnetic resonance imaging (MRI) scan of the knee is a noninvasive way to take pictures of the knee joint and surrounding muscles and other tissues.

Unlike x-rays and computed tomographic (CT) scans, which use radiation, MRI uses powerful magnets and radio waves. The MRI scanner contains the magnet. The magnetic field produced by an MRI is about 10 thousand times greater than the earth's.

The magnetic field forces hydrogen atoms in the body to line up in a certain way (similar to how the needle on a compass moves when you hold it near a magnet). When radio waves are sent toward the lined-up hydrogen atoms, they bounce back, and a computer records the signal. Different types of tissues send back different signals.

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.

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 inaccurate images.

You will lie on your back on a narrow table, which slides into the middle of the MRI machine. Small devices, called coils, may be placed around your knee. These devices help send and receive the radio waves, and improve the quality of the images.

Some exams require a special dye (contrast). The dye is usually given before the test through a vein (IV) in your hand or forearm. 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. Several sets of images are usually needed, each taking 2 - 15 minutes. The exam may take 1 hour or longer.

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

Before the test, tell the radiologist if you are currently receiving dialysis, as this may affect whether you can have IV contrast.

If you fear confined spaces (have claustrophobia), tell your doctor before the exam. You may be given a medicine to help you feel sleepy and less anxious, or your doctor may recommend an "open" MRI, in which the machine is not as close to the body.

The strong magnetic fields created during an MRI can interfere with certain implants, particularly pacemakers. Persons with cardiac pacemakers cannot have an MRI and should not enter an MRI area.

You may not be able to have an MRI if you have any of the following metallic objects in your body:

• Brain aneurysm clips
• Certain artificial heart valves
• Inner ear (cochlear) implants
• Recently placed artificial joints
• Some older types of vascular stents

Tell your health care provider if you have one of these devices when scheduling the test, so the exact type of metal can be determined.

Before an MRI, sheet metal workers or any person that may have been exposed to small metal fragments should receive a skull x-ray to check for metal in the eyes.

Because the MRI contains a magnet, metal-containing objects such as pens, pocketknives, and eyeglasses may fly across the room. This can be dangerous, so they are not allowed into the scanner area.

Other metallic objects are also not allowed into 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.

An MRI exam causes no pain. Some people may become anxious inside the scanner. If you have difficulty lying still or are very anxious, you may be given a mild sedative. Excessive 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 the person operating the scanner 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 need sedation. After an MRI scan, you can resume your normal diet, activity, and medications.

MRI is a very useful tool for evaluating a number of different knee injuries and diseases.

Your doctor may order this test if you have:

An abnormal finding on a knee x-ray or bone scan

• Anterior cruciate ligament injury
• A feeling that your knee is giving away in the knee joint
• Arthritis of the knee
• Bursitis of the knee
• Buildup of joint fluid behind the knee (Baker's cyst)
• Fluid collecting in the knee joint
• Infection of the knee joint
• Knee pain with fever
• Knee cartilage tear (meniscus tear)
• Knee locking when you walk or moving
• Signs of a knee fracture
• Signs of damage to the knee muscle, cartilage, or ligaments
• Unexplained knee pain that does not get better with treatment

Your doctor may also order this test to determine if you need arthroscopy of the knee, or to monitor your progress after knee surgery.

Results are considered normal if the knee joint and surrounding structures look normal in size and shape, and are in the proper position.

Results depend on the nature of the problem. Different types of tissues send back different MRI signals. For example, healthy tissue sends back a slightly different signal than cancerous tissue.

Abnormal results may be due to:

• Anterior cruciate ligament injury
• Arthritis of the knee
• Avascular necrosis (also called osteonecrosis)
• Bone tumor or cancer
• Broken bone
• Bursitis of the knee
• Buildup of joint fluid behind the knee (Baker's cyst)
• Fluid collecting in the knee joint
• Infection in the bone (osteomyelitis)
• Inflammation
• Knee cartilage tear (meniscus tear)
• Medial or lateral collateral ligament injury
• Muscle damage
• Osteochondral dissecans of the knee (a rare condition in which there is no blood flow to the end of knee bone)
• Posterior cruciate ligament injury

Consult your health care provider with any questions and concerns.

MRI contains no ionizing radiation. To date, there have been no documented significant side effects of the magnetic fields and radio waves used on the human body.

The most common type of contrast (dye) used is gadolinium. It is very safe. Allergic reactions to the substance rarely occur. The person operating the machine will monitor your heart rate and breathing as needed.

MRI is usually not recommended for acute trauma situations, because traction and life-support equipment cannot safely enter the scanner area and the exam can take quite a bit of time.

People have been harmed in MRI machines when they did not remove metal objects from their clothes or when metal objects were left in the room by others.

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

• CT scan of the knee
• Knee x-ray

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.