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Alternative NamesMRI - lower extremity; Magnetic resonance imaging - leg; Magnetic resonance imaging - lower extremity; MRI - ankle; Magnetic resonance imaging - ankle; MRI - femur; MRI - leg
Definition Return to top
A magnetic resonance imaging (MRI) scan of the leg is a noninvasive method to create detailed 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.
Unlike x-rays and computed tomographic (CT) scans, which use radiation, MRI uses powerful magnets and radio waves. The MRI scanner contains the magnet. The magnetic field produced by an MRI is about 10 thousand times greater than the earth's.
The magnetic field forces hydrogen atoms in the body to line up in a certain way (similar to how the needle on a compass moves when you hold it near a magnet). When radio waves are sent toward the lined-up hydrogen atoms, they bounce back, and a computer records the signal. Different types of tissues send back different signals.
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.
How the Test is Performed Return to top
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 inaccurate images.
You will lie on a narrow table, which slides into the middle of the MRI machine. If you fear confined spaces (have claustrophobia), tell your doctor before the exam.
Small devices, called coils, are placed around the leg. These devices help send and receive the radio waves, and improve the quality of the images.
Some exams require a special dye (contrast). The dye is usually given before the test through a vein (IV) in your hand or forearm. 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. Several sets of images are usually needed, each taking 2 - 15 minutes. Depending on the type of equipment, the exam may take 1 hour or longer.
How to Prepare for the Test Return to top
You may be asked not to eat or drink anything for 4 - 6 hours before the scan.
Before the test, tell the radiologist if you are currently receiving dialysis, as this may affect whether you can have IV contrast.
If you fear confined spaces (have claustrophobia), tell your doctor before the exam. You may be given a medicine to help you feel sleepy and less anxious, or your doctor may recommend an "open" MRI, in which the machine is not as close to the body.
The strong magnetic fields created during an MRI can interfere with certain implants, particularly pacemakers. Persons with cardiac pacemakers cannot have an MRI and should not enter an MRI area.
You may not be able to have an MRI if you have any of the following metallic objects in your body:
Tell your health care provider if you have one of these devices when scheduling the test, so the exact type of metal can be determined.
Before an MRI, sheet metal workers or any person that may have been exposed to small metal fragments should receive a skull x-ray to check for metal in the eyes.
Because the MRI contains a magnet, metal-containing objects such as pens, pocketknives, and eyeglasses may fly across the room. This can be dangerous, so they are not allowed into the scanner area.
Other metallic objects are also not allowed into the room:
How the Test Will Feel Return to top
An MRI of the leg causes no pain. Some people may become anxious inside the scanner. If you have difficulty lying still or are very anxious, you may be given a mild sedative. Excessive 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 the person operating the scanner 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 need sedation. After an MRI scan, you can resume your normal diet, activity, and medications.
Why the Test is Performed Return to top
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:
Your doctor may also order a leg MRI to:
Normal Results Return to top
Results are considered normal if the leg structures being examined are normal in appearance.
What Abnormal Results Mean Return to top
Results depend on the nature of the problem. Different types of tissues send back different MRI signals. For example, healthy tissue sends back a slightly different signal than cancerous tissue.
Abnormal results may be due to:
Consult your health care provider with any questions and concerns.
Risks Return to top
MRI contains no ionizing radiation. To date, there have been no documented significant side effects of the magnetic fields and radio waves used on the human body.
The most common type of contrast (dye) used is gadolinium. It is very safe. Allergic reactions to the substance rarely occur. The person operating the machine will monitor your heart rate and breathing.
MRI is usually not recommended for acute trauma situations, because traction and life-support equipment cannot safely enter the scanner area and the exam can take quite a bit of time.
People have been harmed in MRI machines when they did not remove metal objects from their clothes or when metal objects were left in the room by others.
Considerations Return to top
Tests that may be done instead of an MRI include:
A CT scan may be preferred in emergency cases, since it is faster and usually available right in the emergency room.
References Return to top
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.
Diagnosis of Venous Thromboembolism. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics - Normal and Problem Pregnancies. 5th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2007:chap 41.
Lyn E, Pallin D, Antosia RE. Knee and Lower Leg. In: Marx J, ed. Rosen’s Emergency Medicine: Concepts and Clinical Practice. 6th ed. St Philadelphia, Pa: Mosby Elsevier; 2006:chap 54.
Ho K, Abu-Laban RB. Ankle and Foot. In: Marx J, ed. Rosen’s Emergency Medicine: Concepts and Clinical Practice. 6th ed. St Philadelphia, Pa: Mosby Elsevier; 2006:chap 55.
Bearcroft PPW. Joint Disease. In: Grainger RC, Allison D, Adam, Dixon AK, eds. Diagnostic Radiology: A Textbook of Medical Imaging. 4th ed. New York, NY: Churchill Livingstone; 2001:chap 50.
Grainger AJ, Davies M. Techniques and Imaging of Soft Tissues. In: Grainger RC, Allison D, Adam, Dixon AK, eds. Diagnostic Radiology: A Textbook of Medical Imaging. 4th ed. New York, NY: Churchill Livingstone; 2001:chap 45.
Sanders, TG. Imaging Techniques. In: DeLee JC, Drez D Jr, Miller MD, eds. DeLee and Drez’s Orthopaedic Sports Medicine. 2nd ed. Philadelphia, Pa: Saunders Elsevier; 2003:chap 16.Update Date: 3/19/2009 Updated by: Benjamin Taragin M.D., Department of Radiology, Montefiore Medical Center Bronx, N.Y. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.