MRI of lumbosacral spine

Alternative names
NMR - lumbosacral spine; Magnetic resonance imaging - lumbosacral spine; Nuclear magnetic resonance - lumbosacral spine; Lumbosacral spine MRI

Definition

A lumbosacral spine MRI uses powerful magnets and radio waves to construct pictures of the structures that make up the spine, the spinal cord, and the spaces between the vertebrae through which the nerves travel.

Unlike conventional radiography and Computed Tomographic (CT) imaging, which make use of potentially harmful radiation (X-rays) passing through a patient to generate images, magnetic resonance imaging (MRI) is based on the magnetic properties of atoms.

A powerful magnet generates a magnetic field roughly 10,000 times stronger than the Earth’s. A small percentage of hydrogen atoms within the body align with this field. Radio wave pulses are broadcast towards the hydrogen atoms in tissues of interest, returning a signal. The subtle differing characteristics of that signal from different tissues enables MRI to differentiate between various organs, and potentially, provide contrast between benign and malignant tissue.

Any imaging plane, or “slice”, can be projected, and then stored in a computer or printed on film. MRI can easily be performed through clothing and bones, however, certain types of metal in or around the area of interest can cause significant errors in the reconstructed images.

How the test is performed

Since MRI makes use of radio waves very close in frequency to those of ordinary FM radio stations, the scanner must be located within a specially shielded room to avoid outside interference. The patient will be asked to lie on a narrow table which slides into a large tunnel-like tube within the scanner.

If a contrast dye is to be used, it will usually be injected into a small vein of the hand or forearm. A technologist will operate the machine and observe you during the entire study from an adjacent room.

Several sets of images are usually required, each taking from 2 to 15 minutes. A complete scan may take up to one hour. Newer scanners with more powerful magnets utilizing updated software and advanced sequences may complete the process in less time.

How to prepare for the test

Because the strong magnetic fields can displace or disrupt the action of implanted metallic objects, people with cardiac pacemakers cannot be scanned and should not enter the MRI area.

MRI also should not be used for people with metallic objects in their bodies such as inner ear (cochlear) implants, brain aneurysm clips, some artificial heart valves, older vascular stents, and recently placed artificial joints.

Sheet metal workers, or persons with similar potential exposure to small metal fragments, will first be screened for metal shards within the eyes with X-rays of the skull. The patient will be asked to sign a consent form confirming that none of the above issues apply before the study will be performed.

A hospital gown may be recommended, or the patient may be allowed to wear “sweats” or similar clothing without metal fasteners.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child’s age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child’s age:

     
  • infant test or procedure preparation (birth to 1 year)  
  • toddler test or procedure preparation (1 to 3 years)  
  • preschooler test or procedure preparation (3 to 6 years)  
  • schoolage test or procedure preparation (6 to 12 years)  
  • adolescent test or procedure preparation (12 to 18 years)

How the test will feel

There is no pain. The magnetic field and radio waves are not felt. Some people may experience a claustrophobic feeling from being inside the scanner. The table may be hard or cold, but you can request a blanket or pillow.

The machine produces loud thumping and humming noises during normal operation. Ear plugs are usually given to the patient to reduce the noise. A technologist observes the patient during the entire procedure and may be spoken to through an intercom in the scanner. Some MRI scanners are equipped with televisions and special headphones to help the examination time pass.
Excessive movement can blur MRI images and cause errors in the image. If the patient has difficulty lying still or is very anxious, an oral or intravenous sedative may be given. There is no recovery, unless sedation was necessary. After an MRI scan, you can resume normal diet, activity, and medications.

Why the test is performed

After plain x-rays of the lower spine, MRI is usually the next non-invasive imaging test a doctor will use to investigate back pain and radiating leg pain. MRI is excellent at showing degenerative changes, such as from arthritis, which may be narrowing the spaces through which the spinal nerves travel.

In addition, MRI can demonstrate disease of the lumbar discs between vertebral levels which may be bulging and compression the spinal cord.

An MRI may be performed in a trauma setting to rule out acute compression of the spinal cord in the setting of weakness or paralysis. MRI is also superior to Computed Tomography (CT) in the evaluation of abscesses, tumors, or other masses near the spinal cord. While CT is better at detecting fractures of the vertebrae, MRI can detect subtle changes in the bone which may be due to infection or tumor.

What the risks are

There is no ionizing radiation involved in MRI, and there have been no documented significant side effects of the magnetic fields and radio waves used on the human body to date.

However, because the effects of strong magnetic fields on a fetus are not well documented at this time, pregnant women are usually advised to avoid MRI scans.

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.

The most common MR intravenous contrast agent, gadolinium, is very safe, and although there have been documented allergic reactions to it, it is an extremely rare occurrence.

If sedation is used, there are associated risks of over-sedation. The technologist monitors the patient’s vital signs, including heart rate and respiration as needed.

Johns Hopkins patient information

Last revised: December 2, 2012
by Arthur A. Poghosian, M.D.

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