A knee MRI (magnetic resonance imaging) scan uses energy from strong magnets to create pictures of the knee joint and muscles and tissues.
An MRI does not use radiation (x-rays). Single MRI images are called slices. The images can be stored on a computer or printed on film. One exam produces many images.
How the Test is Performed
You will wear a hospital gown or clothes without metal zippers or snaps (such as sweatpants and a t-shirt). Please remove your watches, glasses, jewelry, and wallet. Certain types of metal can cause blurry images.
You will lie on a narrow table that slides into a large tunnel-like scanner.
Some exams use a special dye (contrast). Most of the time, you will get the dye through a vein (IV) in your arm or hand before the test. Sometimes, the dye is injected into a joint. 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. The test most often lasts 30 to 60 minutes, but may take longer. It can be loud. The technician can give you some ear plugs if needed.
How to Prepare for the Test
You may be asked not to eat or drink anything for 4 to 6 hours before the scan.
Tell your health care provider if you are afraid of closed spaces (have claustrophobia). You may be given a medicine to help you feel sleepy and less anxious. Your provider may suggest an "open" MRI, in which the machine is not as close to the body.
Before the test, tell your provider if you have:
- Brain aneurysm clips
- Certain types of artificial heart valves
- Heart defibrillator or pacemaker
- Inner ear (cochlear) implants
- Kidney disease or dialysis (you may not be able to receive contrast)
- Recently placed artificial joints
- Certain types of vascular stents
- Worked with sheet metal in the past (you may need tests to check for metal pieces in your eyes)
Because the MRI contains strong magnets, metal objects are not allowed into the room with the MRI scanner:
- Pens, pocketknives, and eyeglasses may fly across 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.
How the Test will Feel
An MRI exam causes no pain. You will need to lie still. Too much movement can blur MRI images and cause errors.
The table may be hard or cold, but you can ask for a blanket or pillow. The machine makes loud thumping and humming noises when turned on. You can wear ear plugs to help block out the noise.
An intercom in the room allows you to speak to someone at any time. Some MRIs have televisions and special headphones to help the time pass.
There is no recovery time, unless you were given a medicine to relax. After an MRI scan, you can return to your normal diet, activity, and medicines.
Why the Test is Performed
Your provider may order this test if you have:
- An abnormal result on a knee x-ray or bone scan
- A feeling that your knee is giving away in the knee joint
- Buildup of joint fluid behind the knee (Baker cyst)
- Fluid collecting in the knee joint
- Infection of the knee joint
- Knee cap injury
- Knee pain with fever
- Knee locking when you walk or moving
- Signs of damage to the knee muscle, cartilage, or ligaments
- Knee pain that does not get better with treatment
- Instability of the knee
You may also have this test to check your progress after knee surgery.
A normal result means your knee looks OK.
What Abnormal Results Mean
Abnormal results may be due to a sprain or tear of the ligaments in the knee area.
Abnormal results may also be due to:
- Degeneration or changes that occur with age
- Meniscus or cartilage injuries
- Arthritis of the knee
- Avascular necrosis (also called osteonecrosis)
- Bone tumor or cancer
- Broken bone
- Buildup of joint fluid behind the knee (Baker cyst)
- Infection in the bone (osteomyelitis)
- Injury of the knee cap
Talk to your provider if you have questions or concerns.
MRI contains no radiation. There have been no reported side effects from the magnetic fields and radio waves.
The most common type of contrast (dye) used is gadolinium. It is very safe. Allergic reactions to the substance are rare. However, gadolinium can be harmful to people with kidney problems that need dialysis. If you have kidney problems, please tell your provider before the test.
The strong magnetic fields created during an MRI can cause heart pacemakers and other implants to not work as well. It can also cause small pieces of metal inside your body to move or shift. For safety reasons, please do not bring anything that contains metal into the scanner room.
Tests that may be done instead of a knee MRI include:
- CT scan of the knee
- Knee x-ray
MRI - knee; Magnetic resonance imaging - knee
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Helms CA. Magnetic resonance imaging of the knee. In: Helms CA, ed. Fundamentals of Skeletal Radiology. 5th ed. Philadelphia, PA: Elsevier; 2020:chap 9.
Thomsen HS, Reimer P. Intravascular contrast media for radiography, CT, MRI and ultrasound. In: Adam A, Dixon AK, Gillard JH, Schaefer-Prokop CM, eds. Grainger & Allison's Diagnostic Radiology. 6th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2015:chap 2.
Wilkinson ID, Graves MJ. Magnetic resonance imaging. In: Adam A, Dixon AK, Gillard JH, Schaefer-Prokop CM, eds. Grainger & Allison's Diagnostic Radiology. 6th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2015:chap 5.
Review Date 3/17/2019
Updated by: C. Benjamin Ma, MD, Professor, Chief, Sports Medicine and Shoulder Service, UCSF Department of Orthopaedic Surgery, San Francisco, CA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.