Magnetic Resonance Imaging (MRI) of the Knee
Magnetic resonance imaging (MRI) is a test done with a large machine that uses a magnetic field and pulses of radio wave energy to make pictures of the knee. Muscles, ligaments, cartilage, and other joint structures are often best seen with an MRI. In many cases MRI gives information about structures in the body that cannot be seen as well with an X-ray, ultrasound, or CT scan.
For an MRI test, you are placed inside the magnet so that your knee is inside the strong magnetic field. MRI can find changes in the structure of organs or other tissues. It also can find tissue damage or disease, such as infection or a tumor. Pictures from an MRI scan are digital images that can be saved and stored on a computer for further study. The images also can be reviewed remotely, such as in a clinic or an operating room. Photographs or films of selected pictures can also be made.
In some cases, a contrast material may be used during the MRI scan to show certain structures more clearly in the pictures. The contrast material may be used to check blood flow, find some types of tumors, and show areas of inflammation or infection. The contrast material may be put in a vein (IV) in your arm or directly into your knee.
Why It Is Done
Magnetic resonance imaging (MRI) of the knee is done to:
- Check for the cause of unexplained knee pain or the knee giving out for no reason.
- Find problems in the knee joint, such as arthritis, bone tumors, or infection, or damaged cartilage, meniscus, ligaments, or tendons.
- Find out if a knee arthroscopy is needed.
MRI may also find a bone fracture when X-rays and other tests do not give a clear answer. MRI is done more commonly than other tests to check for certain bone and joint problems.
How To Prepare
Before your MRI test, tell your doctor and the MRI technologist if you:
- Are allergic to any medicines. The contrast material used for MRI does not contain iodine. If you know that you are allergic to the contrast material used for the MRI, tell your doctor before having another test.
- Are or might be pregnant.
- Have metal screws in your knee from a past knee surgery.
- Have any metal implanted in your body. This helps your doctor know if the test is safe for you. Tell your doctor if you have:
- Heart and blood vessel devices such as a coronary artery stent, pacemaker, ICD (implantable cardioverter-defibrillator), or metal heart valve.
- Metal pins, clips, or metal parts in your body, including artificial limbs and dental work or braces.
- Any other implanted medical device, such as a medicine infusion pump or a cochlear implant.
- Cosmetic metal implants, such as in your ears, or tattooed eyeliner.
- Had recent surgery on a blood vessel. In some cases, you may not be able to have the MRI test.
- Have an intrauterine device (IUD) in place. An IUD may prevent you from having the MRI test done.
- Become very nervous in confined spaces. You need to lie very still inside the MRI magnet, so you may need medicine to help you relax. Or you may be able to have the test done with open MRI equipment. It is not as confining as standard MRI machines. You may need medicine to help you relax.
- Have any other health conditions, such as kidney problems or sickle cell anemia, that may prevent you from having an MRI using contrast material.
- Wear any medicine patches. The MRI may cause a burn at the patch site.
You may need to arrange for someone to drive you home after the test, if you are given a medicine (sedative) to help you relax.
You will be asked to sign a consent form that says you understand the risks of the test and agree to have it done.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form( What is a PDF document? ).
How It Is Done
A magnetic resonance imaging (MRI) test is usually done by an MRI technologist. The resulting pictures are usually interpreted by a radiologist. But some other types of doctors, such as an orthopedic surgeon, can also interpret a knee MRI scan.
You will need to remove all metal objects (such as hearing aids, dentures, jewelry, watches, and hairpins) from your body because these objects may be attracted to the powerful magnet used for the test.
You will need to take off all or most of your clothes, depending on which area is examined (you may be allowed to keep on your underwear if it is not in the way). You will be given a gown to use during the test. If you are allowed to keep some of your clothes on, you should empty your pockets of any coins and cards (such as credit cards or ATM cards) with scanner strips on them because the MRI magnet may erase the information on the cards.
During the test
During the test, you will lie on your back on a table that is part of the MRI scanner. The table will slide into the space that contains the magnet. A device called a coil may be placed over or wrapped around the area to be scanned.
Some people feel nervous (claustrophobic) inside the standard MRI machine. If feeling nervous keeps you from lying still, you can be given a medicine (sedative) to help you relax.
Inside the scanner, you will hear a fan and feel air moving. You may also hear tapping or snapping noises as the MRI scans are taken. You may be given earplugs or headphones with music to reduce the noise. It is very important to hold completely still while the scan is being done. You may be asked to hold your breath for short periods of time.
During the test, you may be alone in the scanner room. But the technologist will watch you through a window. You will be able to talk with the technologist through a two-way intercom.
If contrast material is needed, the technologist will usually put it in through an IV in your arm or hand. The injection may be given over 1 to 2 minutes.
An MRI test usually takes 30 to 60 minutes but can take as long as 2 hours.
How It Feels
You won't have pain from the magnetic field or radio waves used for the MRI test. The table you lie on may feel hard and the room may be cool. You may be tired or sore from lying in one position for a long time.
If a contrast material is used, you may feel some coolness when it is put into your IV.
In rare cases, you may feel:
- A tingling feeling in the mouth if you have metal dental fillings.
- Warmth in the area being examined. This is normal. Tell the technologist if you have nausea, vomiting, headache, dizziness, pain, burning, or breathing problems.
There are no known harmful effects from the strong magnetic field used for MRI. But the magnet is very powerful. The magnet may affect pacemakers, artificial limbs, and other medical devices that contain iron. The magnet will stop a watch that is close to the magnet. Any loose metal object has the risk of causing damage if it gets pulled toward the strong magnet.
Metal parts in the eyes can damage the retina. If you may have metal fragments in the eye, an X-ray of the eyes may be done before the MRI. If metal is found, the MRI will not be done.
Iron pigments in tattoos or tattooed eyeliner can cause skin or eye irritation.
An MRI can cause a burn with some medicine patches. Be sure to tell your health professional if you are wearing a patch.
There is a slight risk of an allergic reaction if contrast material is used during the MRI. But most reactions are mild and can be treated using medicine. There also is a slight risk of an infection at the IV site.
A dye (contrast material) that contains gadolinium may be used in this test. Be sure to tell your doctor if:
- You are pregnant or think you may be pregnant.
- You have kidney problems.
- You've had more than one test that used gadolinium.
The U.S. Food and Drug Administration (FDA) has safety warnings about gadolinium. But for most people, the benefit of its use in this test outweighs the risk.
If you breastfeed and are concerned about whether the dye used in this test is safe, talk to your doctor. Most experts believe that very little dye passes into breast milk and even less is passed on to the baby. But if you prefer, you can store some of your breast milk ahead of time and use it for a day or two after the test.
Magnetic resonance imaging (MRI) is a test done with a large machine that uses a magnetic field and pulses of radio wave energy to make pictures of the knee.
The radiologist may discuss preliminary results of the MRI with you right after the test. Complete results are usually available for your doctor in 1 to 2 days.
No growths, such as tumors, are present.
No broken bones ( fractures), extra fluid, or loose bodies are present.
No signs of inflammation or infection in bones, joints, or soft tissues are present.
Bones show an injury or a fracture. The MRI also may show a collection of fluid, which could mean an infection is present.
Ligament or meniscus tears are present.
Tendon tears are present. The MRI may also show a thickening, meaning surgery or a tear you had in the past or repeated stress.
Growths, such as tumors, are present.
Changes common to arthritis are present.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Metal devices in your leg from previous surgery. These may make the MRI pictures blurry and prevent your doctor from seeing what is wrong with your knee.
- Medical devices that use electronics, such as a pacemaker or medicine infusion pump. The MRI magnet may cause problems with these devices.
- If you are not able to remain still during the test.
- Obesity. A person who is very overweight may not fit into the opening of some standard MRI machines.
What To Think About
- You may be able to have an MRI with an open MRI machine that doesn't enclose your entire body. But open MRI machines aren't available everywhere. The pictures from an open MRI may not be as good as those from a standard MRI machine.
- Sometimes your MRI test results may be different than those from CT, ultrasound, or X-ray tests because the MRI scan is more specific.
- In some people, a knee MRI has given enough information about the knee joint that those people do not need an arthroscopy. To learn more, see the topic Arthroscopy.
- Contrast material put directly in the knee (arthrogram) may be done in people who can't have an MRI or where MRI is not available. An arthrogram may also be done in people with total knee joint replacements to check for loose parts. To learn more, see the topic Arthrogram.
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.