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Leg MRI scan
Definition
A leg MRI (magnetic resonance imaging) scan of the leg is a imaging test that uses powerful magnets and radio waves to create pictures of the leg, including the ankle, foot, and surrounding tissues.
A leg MRI also creates pictures of the knee. However, the knee is discussed in a separate article. See also: MRI of the knee
MRI does not use radiation. Single MRI images are called slices. The images can be stored on a computer or printed on film. One exam produces dozens or sometimes hundreds of images.
Alternative Names
MRI - lower extremity; Magnetic resonance imaging - leg; Magnetic resonance imaging - lower extremity; MRI - ankle; Magnetic resonance imaging - ankle; MRI - femur; MRI - leg
How the Test is Performed
You may be asked to wear a hospital gown or clothing without metal fasteners (such as sweatpants and a t-shirt). Certain types of metal can cause blurry images.
You will lie on a narrow table, which slides into a tunnel-shaped scanner.
Some exams require a special dye (contrast). The dye is usually given before the test through a vein (IV) in your hand or forearm. Sometimes, the dye is given 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 30-60 minutes, but may take longer.
How to Prepare for the Test
You may be asked not to eat or drink anything for 4 - 6 hours before the scan.
Tell your doctor if you are afraid of close spaces (have claustrophobia). You may be given a medicine to help you feel sleepy and less anxious, or your doctor may suggest an "open" MRI, in which the machine is not as close to the body.
Before the test, tell your health care 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 (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. If you have difficulty lying still or are very nervous, you may be given a medicine to relax you. Too much movement can blur MRI images and cause errors.
The table may be hard or cold, but you can request a blanket or pillow. The machine produces loud thumping and humming noises when turned on. You can wear ear plugs to help reduce the noise.
An intercom in the room allows you to speak to someone at any time. Some MRIs have televisions and special headphones that you can use to help the time pass.
There is no recovery time, unless you were given a medicine to relax. After an MRI scan, you can resume your normal diet, activity, and medications.
Why the Test is Performed
This test provides clear pictures of parts of the leg that are difficult to see clearly on CT scans.
Your doctor may order an MRI of the leg if you have:
- A mass that can be felt on a physical exam
- An abnormal finding on an x-ray or bone scan
- Birth defects of the leg, ankle, or foot
- Bone pain and fever
- Broken bone
- Decreased motion of the ankle joint
- Pain, swelling, or redness in a leg
- Redness or swelling of the ankle joint
- Leg pain and a history of cancer
- Leg, foot, or ankle pain that does not get better with treatment
Normal Results
A normal result means your leg appears normal.
What Abnormal Results Mean
Abnormal results may be due to:
- Abscess
- Achilles tendonitis
- Arthritis
- Broken bone or fracture
- Infection in the bone
- Ligament, tendon, or cartilage injury
- Muscle damage
- Osteonecrosis (avascular necrosis)
- Plantar fascia rupture (See: Plantar fascitis)
- Posterior tibial tendon dysfunction
- Tear or rupture of the Achilles tendon in the ankle area
- Tumor or cancer in the bone, muscle, or soft tissue
Consult your health care provider with any questions and concerns.
Risks
MRI contains no radiation. To date, no side effects from the magnetic fields and radio waves have been reported.
The most common type of contrast (dye) used is gadolinium. It is very safe. Allergic reactions to the substance rarely occur. However, gadolinium can be harmful to patients with kidney problems who require dialysis. If you have kidney problems, please tell your health care 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 a piece of metal inside your body to move or shift.
Considerations
Tests that may be done instead of an MRI include:
- Bone scan
- CT scan of the leg
- Positron emission tomography (PET) scan
- X-ray of the leg
A CT scan may be preferred in emergency cases, since it is faster and usually available right in the emergency room.
References
Wilkinson ID, Paley MNJ. Magnetic resonance imaging: basic principles. In: Grainger RC, Allison D, Adam, Dixon AK, eds. Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 5.
DeLee JC, Drez D Jr, Miller MD, eds. DeLee and Drez’s Orthopaedic Sports Medicine. 3rd ed. Philadelphia, Pa: Saunders Elsevier; 2009:chap 24.
Grainger RG, Thomsen HS, Morcos SK, Koh DM, Roditi G. Intravascular contrast media for radiology, CT, and MRI. In: Adam A, Dixon AK, eds. Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 2.
Reviewed By: Linda J. Vorvick, MD, Medical Director, MEDEX Northwest Division of Physician Assistant Studies, University of Washington, School of Medicine; C. Benjamin Ma, MD, Assistant Professor, Chief, Sports Medicine and Shoulder Service, UCSF Department of Orthopaedic Surgery. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
