A knee magnetic resonance imaging (MRI) is a detailed image of the inside of your knee. The MRI machine scans the outside of your leg without using radiation. Strong magnets, radio waves, and a computer make the MRI image. A knee MRI allows your doctor to see the soft tissues inside your knee joint as a whole image or in slices.
Your knee is the largest and easiest joint to injure in your body. Your thigh bone (femur), shinbone (tibia), and kneecap (patella) form your knee joint. But the most common knee problems are soft tissue tears, kneecap dislocations, and fractures. An X-ray only looks at broken bones, but a knee MRI can detect bone, cartilage, tendon, and ligament damage.
Cartilage, ligaments, and tendons connect to bones and muscles inside your knee. Your meniscus are two wedge-shaped cartilage pieces that cushion the joint. A knee MRI can also help your doctor know if surgery is the best treatment. Find out what happens during a knee MRI, why you might need one, and how to prepare for your scan.
What Happens During an MRI?
A typical MRI machine looks like a large, hollow tube.
During your knee MRI, you may:
- Wear a hospital gown or loose-fitting clothes
- Remove any metal such as jewelry, glasses, or watches to avoid a blurry image
- Lie on an exam table that slides into the hollow tube
- Sometimes, get a special dye called a contrast agent given through your vein (IV) and a shot; it can help your knee images be even clearer. You may feel a warm sensation after the shot.
- Go in feet first, only your lower body will be inside the scanner
- Hold still for around 20-25 minutes as the machine makes images of your knee
You’re usually alone in the room during your scan. Your MRI technologist usually steps outside. They’ll control the MRI machine from a computer in a small room. They can see you the whole time. And they’ll talk to you via a two-way intercom.
You won’t feel any discomfort during the scan. But if it’s your first MRI, you may be surprised by loud sounds. But only some machines make thumping, knocking, and humming sounds. The technologist may offer you headphones or earplugs. If they don’t, you can ask for them.
After the exam, your knee images are sent to a radiologist. It’s a doctor who knows how to read imaging scans. They’ll send a report to your doctor with details about your knee. You’ll be able to drive yourself home and continue your day as you normally would.
Why You Might Need a Knee MRI
You may need a knee MRI if you have a soft tissue knee injury. Along with X-ray, your doctor may order a “knee MRI to assess ongoing pain, weakness, or swelling,” says Kara Gaetke-Udager, MD, a clinical associate professor and director of the Division of Musculoskeletal Radiology at Michigan Medicine.
A knee MRI can diagnose or check for:
- Damaged cartilage or muscles
- Torn meniscus (half-circles of cartilage that cushion your knee joint), tendons, or ligaments
- Any bone fractures with ligament tears
- Trabecular fractures, a break in the spongy part at the end of your bone
- Bone bruises
- Kneecap dislocation, injury, or pain
- Fluid buildup in the joint
- An infection in the knee (osteomyelitis)
- Locking, catching when you walk, or bone fragments not showing on an X-ray
- Knee instability (feeling like it might give out)
- Pain that doesn’t get better with other treatments
- Arthritis-related damage
- Bone tumors in the knee joint
- Ongoing pain or issues after knee surgery
How to Prepare for a Knee MRI
A knee MRI is a fairly quick and easy test. Your doctor may ask you not to eat or drink four to six hours before your MRI scan. In most cases, you’ll be able to go about your day as usual afterward.
Remove all metal
The MRI machine is made of powerful magnets. This makes the machine very sensitive to any type of metal. The MRI scan can cause pieces of metal, even inside your body, to heat up or move. Any metal you wear can also blur your MRI image.
Be sure to take off any metal items before your exam, such as:
- Jewelry
- Hairpins
- Zippers
- Body piercings
- Watches
- Hearing aids
- Pocketknives
- Eyeglasses
Report any metal inside your body
The strong magnets inside the MRI machine can cause certain medical devices to stop working. If you have metal inside your body, tell your doctor or the technologist before your scan. Many people can still safely have an MRI, but in some cases, your doctor may suggest another test.
Tell your doctor if you have:
- Metal implants, plates, or pieces of metal in your body (including metal clips)
- A heart valve, stent, defibrillator, or pacemaker
- An inner ear (cochlear) implant
- Kidney disease or dialysis (you may not be able to get contrast)
Although some pacemakers are safe for MRI testing, many are not. Always ask your doctor to discuss the best imaging test for your health.
Ask your doctor about options if you have back pain and “it’s hard for you to lie flat and still for extended periods,” Gaetke-Udager says. “They can find ways to make you feel more comfortable.”
How Long Does a Knee MRI Take?
A knee MRI usually takes about 20-25 minutes, but it can also take longer. How long your knee MRI takes can depend on your injury and whether you need contrast dye or sedation. With sedation, the scan can take closer to an hour.
During the scan, you’ll need to lie still. This helps the images come out clear. Your radiologist should let you know how long it will take to read your report.
Are There Any Risks or Side Effects?
For most people, a knee MRI is very safe and carries little risk. The scan doesn’t use radiation and most people don’t have any side effects. That said, there are a few extra things to consider.
Metal implants and medical devices
The strong MRI magnets can cause your implanted medical device to not work the same. If you have an implanted medical device — like a pacemaker or insulin pump — the strong magnets in the MRI machine could interfere with how it works. And the scan may cause metal fragments in your body, such as a bullet, to heat up or move during the scan.
To avoid problems, always tell your doctor about any metal such as medical devices, implants, or fragments in your body. They’ll check to see if its safe for you to have an MRI. If not, they’ll suggest another type of test for your knee.
Allergic reactions to the contrast dye
If you need contrast dye, there’s a small chance for an allergic reaction. But most reactions are mild and go away with medication.
With contrast dye allergic reactions, you may get:
- Hives
- Itchy eyes
- Nausea
- Headache
- Pain or tenderness around your injection site
Rarely, a type of contrast called gadolinium can react in people with kidney disease. Gadolinium is a metal used to make the contrast. Normally, your kidneys rid your body of the rare metal. But if you have kidney disease, a reaction called nephrogenic systemic fibrosis (NSF) can lead to thicker skin, organs, and tissues.
If you’ve had a reaction to contrast dye before, tell your doctor. They can give you medicine ahead of time to keep an allergic reaction from happening.
When Will You Get Your MRI Results?
After the scan, a radiologist looks at your knee images. They’ll usually send a report to your doctor after a few days.
“Oftentimes, those reports are being released quickly through a portal. When you get these results in a vacuum, that can be confusing for people — especially if you don’t know what it means or see something that is potentially scary,” Gaetke-Udager says. Instead of trying to interpret the results yourself, she suggests waiting to discuss them with your doctor.
Other Knee MRI Tips
Knee MRIs are safe for most people. Here are a few knee MRI tips if you have claustrophobia, are pregnancy, or have an allergic reaction:
Claustrophobia
Yes, if you have any problems in closed spaces, tell your doctor ahead of time. They can give you medication to help you relax and feel sleepy during the scan. Or your doctor can send you to a place with an “open” MRI machine. This machine is not as close to your body.
“I think knowing yourself and how you might react to [an MRI] is important,” she says. “If you have any concerns about it, talk to your doctor ahead of time so they can try to set you up for success.”
Tell your doctor if you have a fear of tight spaces. You may need to take medicine before the test to calm your anxiety. The MRI technologist does not offer this medicine, so you’ll need to mention it to your doctor beforehand. Depending on availability, they may also suggest an “open” MRI — a scan that happens in a machine that’s open on both sides, instead of being fully enclosed.
Pregnancy
Sometimes, MRI is used in pregnancy for certain types of fractures if radiation is a concern. Before you get an MRI or a contrast dye, let your doctor know if there’s any chance you’re pregnant. Although MRIs are considered safe, your doctor may only suggest an MRI scan for serious or life-threatening problems. And MRI scans aren’t usually done during the first trimester.
Allergic reaction
If you’re at risk for an allergic reaction to contrast dye, your doctor may give you medicine to keep it from happening.
To help avoid a reaction, before your exam, your doctor may have you take:
- Steroids such as methylprednisolone or prednisone
- Cetirizine (Zyrtec) or diphenhydramine (Benadryl)
Always tell your doctor and your MRI technologist about your contrast dye allergy before the MRI. This can help lower your risk for complications.
Takeaways
A knee MRI uses strong magnets and radio waves to create detailed pictures of the inside of your knee without radiation. Unlike an X-ray, an MRI can view soft tissues to help your doctor diagnose cartilage, tendon, ligament, and kneecap injuries. A knee MRI can help your doctor decide on the best treatment for your knee pain, weakness, or swelling.
Knee MRI FAQs
Does your whole body go in for a knee MRI?
No, usually it’s only the lower half of your body that goes into the scanner. Some machines are open on the sides. These open MRI machines only surround your knee.
Is a knee MRI painful?
No, an MRI scan doesn’t cause any pain. If your doctor orders contrast dye for a clearer image, you may feel a quick pinch from the IV needle. And the area may be a little sore for a few days, until it heals.
Who reads a knee MRI?
A radiologist — a doctor trained to read images from X-rays, CT, and MRI scans — views your knee MRI and sends the report to your doctor.
How much does a knee MRI cost?
The cost of an MRI can range from $400 to $12,000. But a knee MRI can range between $480 and $650, depending on your insurance and where you get the scan. Adding a contrast dye can make the cost higher. The contrast dye may cost $110 to $310 more.
