What Is a Knee MRI?
A magnetic resonance imaging (MRI) test uses magnets and radio waves to capture images inside your body without making a surgical incision. An MRI can be performed on any part of your body, but a knee MRI looks specifically at your knee and its surrounding areas.
An MRI allows your doctor to see the soft tissues in your body along with the bones. In this way, he or she can inspect the elements of the knee that might have been injured during physical activity or simply from wear and tear.
An MRI can provide detailed images of various sections of the knee—such as the bones, cartilage, tendons, muscles, blood vessels, and ligaments—in better contrast than other tests.
Your doctor may want you to undergo a special kind of MRI called an MRI arthrogram. For this procedure, a doctor will inject a contrast fluid into your knee, providing a better view of its structure.
Why a Knee MRI Is Done
Your doctor may have you undergo an MRI scan if he or she suspects there are any abnormalities within your knee joint. The test helps your doctors to visualize the anatomy of your knee to determine the possible cause of your pain, inflammation or weakness, without having to do surgery.
According to the Radiological Society of North America, an MRI is usually ordered to help doctors diagnose and treat many types of conditions (RSNA, 2011). These include:
- arthritis and other degenerative joint disorders
- bone fractures
- damaged cartilage, ligaments, tendons, or meniscus
- decreased motion of the knee joint
- fluid buildup in the knee
- problems with implanted medical devices
- sports or trauma-related injury
Your doctor may order other imaging tests, such as an X-ray, along with your knee MRI. He or she may also order an MRI before performing a knee arthroscopy, which is a minor surgery where a doctor views the inside of your knee by inserting a scope with a camera through a small incision.
The Risks of a Knee MRI
Unlike X-rays and computed tomography (CT) scans, an MRI does not use radiation. It is considered a safer alternative for everyone, especially pregnant women. While the radiation levels in CT scans are safe for adults, they are not safe for developing fetuses.
There are risks for those who have implants containing metal. The magnets used in an MRI can cause problems with pacemakers or cause implanted screws or pins to shift in the body.
One complication that could arise is an allergic reaction to the contrast dye. The most common type of contrast dye is gadolinium. However, the Radiological Society of North America states that these allergic reactions are often mild and easily controlled by medication (RSNA, 2011).
How to Prepare for a Knee MRI
Preparations for an MRI vary between testing facilities. Your doctor or attending technician will give you complete instructions on how to prepare for your specific test.
Before your MRI, your doctor will explain the test and do a complete physical and medical history. During this time, be sure to tell your doctor about any medication you may be taking, including over-the-counter drugs and herbal supplements, or any known allergies. Also, let him or her know if you have any implanted medical devices, as these can be affected by the test.
Alert your doctor if you have had allergic reactions to contrast dye in the past or if you have been diagnosed with kidney problems.
If you are pregnant, concerned you may be pregnant, or nursing, you may need to take special precautions. MRIs performed with radioactive contrast dye are not considered safe for pregnant women. Also, nursing mothers should stop breastfeeding for about two days after the test. Be sure to tell your doctor if you are pregnant or breastfeeding.
The MRI machine is a tight, enclosed space and you may be worried if you are claustrophobic or scared of small spaces. Talk to your doctor about your concerns. He or she may give you a sedative to help you relax. In extreme cases, your doctor may opt for an “open” MRI where the machine is not as close to your body. Your doctor can best explain your options.
How a Knee MRI Is Performed
An MRI machine looks like a giant skateboard wheel. The center is open so a flat table can slide in and out of the machine. The rounded, wheel-like part sends out the magnetic and radio waves used to produce images of your body.
Before the scan, you will change into a hospital gown and remove all jewelry and body piercings.
If you are using a contrast dye, an intravenous (IV) line will be inserted into your arm. This will inject dye into your bloodstream.
In the MRI room, you will lie on your back or side on a padded table. The technician may use pillows or straps to make your knee more comfortable during the test. This will also help keep your leg still so the clearest images can be taken.
The technician will then slide you into the machine feet first. He or she will give you instructions on when to hold still, including holding your breath. These instructions will be given over microphone (the technician will be in a separate room, watching the monitors as they collect images).
You will not feel the machine working, but there may be some loud noises, such as clacks or thuds, and possibly a whirring noise. The technician may give you earplugs or provide music.
The test typically takes between 30 minutes and an hour. Once the right images have been recorded, you will be free to change back into your regular clothes and go about your day.
After a Knee MRI
A radiologist will review your knee MRI scans and give them to your doctor, who will usually review them later.
MRI images are black-and-white images. Abnormalities may appear as bright white spots. These indicate areas where the contrast dye has collected due to enhanced cell activity.
When your doctor reviews the results, he or she will inform you of the problem and help decide on the next steps for treatment. Depending on your condition, treatment may require more tests, medication, physical rehabilitation, or surgery. It could also require some combination of these options. Your doctor will go over your choices and pick the methods that are best for you.