Magnetic resonance imaging is a painless, non-invasive test that can detect the presence of blockages in the arteries of the brain and heart.

An MRI (or magnetic resonance imaging) scan is a radiology technique which uses magnetism, radiowaves, and a computer to produce images of body structures. The MRI scanner is a tube surrounded by a giant circular magnet. The patient is placed on a moveable bed which is inserted into the magnet. The magnet creates a strong magnetic field which aligns the protons of hydrogen atoms, which are then exposed to a beam of radio waves. This spins the various protons of the body, and they produce an faint signal which is detected by the receiver portion of the MRI scanner. The receiver information is processed by a computer, and an image is produced. The image and resolution is quite detailed and can detect tiny changes of structures within the body.

An MRI images the spine by using a magnet that goes around the body to excite hydrogen atoms. After the atoms return to their normal level of excitation, they emit energy that is picked up on a scanner. Since humans are composed primarily of water (which is two parts hydrogen), MRI's provide highly refined detail of the spine's anatomy (see Figure 1 below).

Fig 1. MRI of Lumbar Disc Herniation (larger view)

Unlike an x-ray beam, there is no radiation with the magnet so the scans may be done for pregnant women. However, patients with a pacemaker implanted in their heart should not have a scan because the magnetic field will cause the pacemaker to malfunction. Also, anyone who works around metal should first have an x-ray of their eye sockets to ensure that they do no have any metal filings in their eyes, which the magnetic field may cause to migrate and damage the eye.

Because most scanners are fairly tight, certain patients may feel uncomfortable, or may not tolerate, lying in a tight tunnel for 45 to 60 minutes while the scan is being performed. To address this issue, newer generation scanners are designed with more open space, although a more open tube does sacrifice the excellent detail provided by the tight tubes.
 

An MRI is a completely different technology from an x-ray and CT scan and represents the single most useful imaging study available for spine surgery.

It is particularly useful as an aid in the assessment of certain conditions by providing detail of the disc (such as for degenerative disc disease, isthmic spondylolisthesis) and nerve roots (such as for lumbar disc herniation, lumbar spinal stenosis). MRI scans are also useful to rule out tumors or spinal infections.

An MRI scan can be used as an extremely accurate method of disease detection throughout the body. In the head, trauma to the brain can be seen as bleeding or swelling. Other abnormalities often found include brain aneurysms, stroke, tumors of the brain, as well as tumors or inflammation of the spine. Neurosurgeons use an MRI scan not only in defining brain anatomy but in evaluating the integrity of the spinal cord after trauma. It is also used when considering problems associated with the vertebrae or intervertebral discs of the spine. An MRI scan can evaluate the structure of the heart and aorta, where it can detect aneurysms or tears. It provides valuable information on glands and organs within the abdomen, and accurate information about the structure of the joints, soft tissues, and bones of the body. Often, surgery can be deferred or more accurately directed after knowing the results of an MRI scan.

Physicians usually have a good idea of what they are looking for on the MRI scan before one is performed. The scans are most commonly used for pre-surgical planning, such as for a decompression or a lumbar spinal fusion. MRI scans are extremely sensitive to picking up information about the health of the discs, as well as the presence of any tumors or a lumbar disc herniation (see figure 1) pinching the nerve roots.

An MRI scan is not the best diagnostic study to assess a fracture. A computerized Tomography (CT scan) is generally done for patients who have suffered some trauma (such as a fall) if a fracture is suspected.

If you have low back pain and/or leg pain and are wondering if you should get an MRI scan to see what is causing the pain, the following general rules explain when a scan is useful and when it is not useful:

There are a number of important factors to take into consideration for an MRI scan, including limitations with interpretation of findings and the timing of when an MRI scan should be performed.

First, the difficulty with the results of an MRI scan, as with many other diagnostic studies, is that the "abnormality" that shows up on the MRI scan may not actually be the cause of pain. Numerous clinical studies have shown that approximately 30% of individuals in their thirties and forties have a lumbar disc herniation on their MRI scan, although they do not have any pain.

Therefore, an MRI scan cannot be interpreted on its own. Everything seen on an MRI needs to be well-correlated to the individual patient’s situation, including:

• Symptoms (such as the duration, location, and severity of pain)

• Any neurological deficits on their physical examination

Another important consideration with MRI scans is the timing of when the scan is done. The only time an MRI scan is needed immediately is when a patient has either:

• Bowel or bladder incontinence

• Progressive weakness in the legs due to nerve damage.

Fortunately, both of the above situations are rare.

When patients have predominantly leg pain and a lumbar disc herniation is suspected, MRI scans are usually recommended early in a patient’s course of pain. This is because surgery for a lumbar disc herniation generally carries few unwanted side effects (morbidity) and leads to an early return to normal function for the patient.

When patients have primarily low back pain, generally the only surgical treatment available is a lumbar spinal fusion. This type of surgery does carry a reasonable amount of unwanted aftereffects (morbidity) and a longer healing time. Therefore, physicians often recommend waiting 3 to 6 months (after the onset of pain) before having an MRI scan done in order to see if the pain will get better with conservative treatments.

As a very general rule, if the results of the MRI scan are not going to affect a patient’s further treatment – and patient will continue with non-surgical treatments such as chiropractic treatments, physical therapy and medications – waiting to obtain a scan in most situations is a reasonable option.

An MRI scan is a painless radiology technique which has the advantage of avoiding x-ray radiation exposure. There are no known side effects of an MRI scan. The benefits of an MRI scan relate to its precise accuracy in detecting structural abnormalities of the body. Patients who have any metallic materials within the body must notify their physician prior to the examination or inform the MRI staff. Metallic chips, materials, surgical clips, or foreign material (artificial joints, metallic bone plates, or prosthetic devices, etc.) can significantly distort the images obtained by the MRI scanner. Patients who have heart pacemakers, metal implants, or metal chips or clips in or around the eyeballs cannot be scanned with an MRI because of the risk of the magnet affecting these metallic areas. Similarly, patients with artificial heart valves, metallic ear implants, bullet fragments, and chemotherapy or insulin pumps should not have MRI scanning.

During the MRI scan, patients lie in a closed area inside the magnetic tube. Some patients can experience a claustrophobic sensation during the procedure. Therefore, patients with any history of claustrophobia should relate this to the practitioner who is requesting the test, as well as the radiology staff. A mild sedative can be given prior to the MRI scan to help alleviate this feeling. It is customary that the MRI staff will be nearby during MRI scan. Furthermore, there is usually a means of communication with the staff (such as a buzzer held by the patient) which can be used for contact if the patient cannot tolerate the scan.

The following general rules are usually considered by a physician before ordering an MRI scan for a patient.

Indications include:

• After 4 to 6 weeks of leg pain, if the pain is severe enough to warrant surgery.

• After 3 to 6 months of low back pain, if the pain is severe enough to warrant surgery.

• If the pain is accompanied by constitutional symptoms (such as loss of appetite, weight loss, fever, chills, shakes, or severe pain when at rest). These symptoms may indicate that the pain is due to a tumor or an infection.

• For patients who may have lumbar spinal stenosis and are considering an epidural injection to alleviate painful symptoms.

• For patients who have not done well after having surgery, if their pain symptoms do not get better after 4 to 6 weeks.

Contraindications include:

• Patients who have a heart pacemaker may not have an MRI scan

• Patients who have a metallic foreign body (metal sliver) in their eye, or who have an aneurysm clip in their brain, cannot have an MRI scan since the magnetic field may dislodge the metal.

• Patients with severe claustrophobia may not be able to tolerate an MRI scan, although more open scanners are now available, and medical sedation is available to make the test easier to tolerate.

• Patients who have had metallic devices placed in their back (such as pedicle screws or anterior interbody cages) can have an MRI scan, but the resolution of the scan is often severely hampered by the metal device and the spine is not well imaged.

If a patient's symptoms match the indications for an MRI scan, and there are no known risk factors (contraindications), then an MRI scan can potentially be very beneficial in helping plan further treatment.

All metallic objects on the body are removed prior to obtaining an MRI scan. Occasionally, patients will be given a sedative medication to decrease anxiety and relax the patient during the MRI scan. MRI scanning requires that the patient lie still for best accuracy. Patients lie within a closed environment inside the magnetic machine. Relaxation is important during the procedure and patients are asked to breathe normally. Interaction with the MRI technologist is maintained throughout the test. There are loud, repetitive clicking noises which occur during the test as the scanning proceeds. Occasionally, patients require injections of liquid intravenously to enhance the images which are obtained. The MRI scanning time depends on the exact area of the body studied, but ranges from half an hour to an hour and a half.

After the MRI scanning is completed, the computer generates visual images of the area of the body that was scanned and these images are transferred to film (hard copy). This film is given to a radiologist, a physician who is specially trained to interpret images of the body reproduced on film. The interpretation is transmitted in the form of a report to the practitioner who requested the MRI scan. The practitioner can then discuss the results with the patient and/or family.

• MRI scanning uses magnetism, radiowaves, and a computer to produce images of body structures.

• MRI scanning is painless and does not involve x-ray radiation.

• Patients with heart pacemakers, metal implants, or metal chips or clips in or around the eyes cannot be scanned with MRI because of the effect of the magnet.

• Claustrophobic sensation can occur with MRI scanning.