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:
How To Prepare
Before your MRI test, tell your doctor and the MRI technologist if you:
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 and flushing as it is put into your IV.
In rare cases, you may feel:
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 doctor if you are wearing a patch.
There is a slight chance of an allergic reaction if contrast material is used during the MRI. But most reactions are mild and can be treated using medicine. Contrast material that contains gadolinium may cause a serious problem (called nephrogenic systemic fibrosis) in people with kidney failure. If you have decreased kidney function or serious kidney disease, tell your doctor before having an MRI scan.
There also is a slight risk of an infection at the IV site if contrast material was used.
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.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
What To Think About
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