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Magnetic resonance imaging is a painless, non-invasive test that
can detect the presence of blockages in the arteries of the
brain and heart. |
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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).
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:
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Symptoms (such as the duration, location, and
severity of pain)
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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:
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:
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After 4 to 6 weeks of leg pain, if the pain is
severe enough to warrant surgery.
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After 3 to 6 months of low back pain, if the
pain is severe enough to warrant surgery.
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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.
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For patients who may have lumbar spinal stenosis
and are considering an epidural injection to alleviate painful
symptoms.
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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:
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Patients who have a heart pacemaker may not have
an MRI scan
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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.
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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.
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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.
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MRI scanning uses magnetism, radiowaves, and a
computer to produce images of body structures.
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MRI scanning is painless and does not involve
x-ray radiation.
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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.
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Claustrophobic sensation can occur with MRI
scanning.
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