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Lumbar MRI Scan

Definition

A lumbar magnetic resonance imaging (MRI) scan is a noninvasive way to create detailed pictures of the part of the spine that runs through the lower back. This area is called the lumbar spine. It consists of five vertebrae called L1 to L5.

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.

See also:

Alternative Names

Magnetic resonance imaging - lumbar spine; MRI - lower back

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 inaccurate images.

You will lie on your back on a narrow table, which slides into the middle of the MRI machine.

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

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 currently undergo 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. People 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:

  • Brain aneurysm clips
  • Certain artificial heart valves
  • Inner ear (cochlear) implants
  • Recently placed artificial joints
  • Some older types of vascular stents

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 who 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:

  • 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. 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

Your doctor may order a lumbar MRI if you have signs or symptoms of:

  • Birth defects of the lower spine
  • Infection that involves your lower spine
  • Injury or trauma to the lower spine
  • Multiple sclerosis
  • Low back or leg pain that is severe or does not get better after treatment by your doctor
  • Low back pain and weakness, numbness, or other unusual findings on physical exam
  • Severe scoliosis
  • Syringomyelia
  • Tumor or cancer in the spine
  • Unexplained bowel or bladder incontinence or retention

The test may also be done if you have:

MRI usually works better than CT in evaluating herniated discs, spinal stenosis, abscesses, bone infections, tumors, or other masses near the spinal cord. While CT is better at detecting fractures of the lumbar spine, MRI can detect subtle changes in the bone that may be due to infection or tumor.

It may also be done before lumbar spinal surgery.

Normal Values

Results are considered normal if the spine and surrounding nerves are normal in appearance.

What abnormal results mean

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:

  • Ankylosing spondylitis
  • Cauda equina syndrome
  • Compression fractures of the lower back
  • Disk degeneration
  • Disk inflammation (diskitis)
  • Herniated disk (lumbar radiculopathy)
  • Osteomyelitis
  • Osteoporosis
  • Spinal cord abscess
  • Spinal cord injury
  • Spinal stenosis
  • Spinal tumor
  • Syringomyelia

Consult your health care provider with any questions and concerns.

What the risks are

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.

Special considerations

Tests that may be done instead of an MRI include:

A CT scan may be done in emergency cases, because 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.

Introduction. In: Mettler FA Jr. Essentials of Radiology. 2nd ed. Philadelphia, Pa: Saunders Elsevier; 2005:chap 1.

Katz JN, Harris MB. Clinical practice. Lumbar spinal stenosis. N EnglJMed. 2008;358(8):818-825.

Chou R, Qaseem A, et al. Diagnosis and treatment of low back pain: a joint clinical practice guideline from the American College of Physicians and the American Pain Society. Ann Intern Med. 2007;147(7):478-491.

The Spine. 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 60

Review Date: 3/5/2009

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