Magnetic Resonance Imaging (MRI) - Spine
What is MRI of the Spine? Back to Top
Magnetic resonance imaging (MRI) is
a non-invasive imaging method that uses radio waves and a powerful
magnetic field rather than x-rays to produce clear and detailed images
of virtually any part of the body. This procedure has greatly improved
the ability to visualize normal and diseased tissue in the spine (also
called the "vertebral column" or "backbone"), the bony structure that
encloses the spinal cord. MRI frequently makes it possible to determine
the cause of back pain. The examination shows the anatomy of the
vertebrae that make up the spine, as well as the spinal cord and the
spaces between the vertebrae through which nerves pass. MRI requires
specialized equipment and expertise to perform it properly and to
correctly interpret the findings.
What are some common uses of the MRI procedure? Back to Top
Perhaps
the most common reason for spinal MRI is to detect a bulging,
degenerated or herniated intervertebral disk—a frequent cause of severe
lower back pain and sciatica.
Compressed (or pinched) and inflamed
nerves are visible on MRI. In some cases the cause of nerve
compression, whether from a herniated disk, arthritis or some other
abnormality, also is demonstrated. MRI generally is the next
non-invasive imaging examination used after x-rays of the lower spine;
however, in cases of suspected disk herniation, MR may be the first
study ordered.
MRI is frequently done to help plan surgeries on
the spine such as the decompression of a pinched nerve or spinal
fusion. MRI performed after spinal surgery will show whether anything
has changed and whether post-operative scarring or infection is
present. If a patient has failed to improve as expected, MRI may show
why.
The exam can help to diagnose—or rule out—spinal infection or
tumors that arise in, or have spread to, the spine; the common tumors
include prostate, lung and breast cancers.
When done before
steroids are injected to relieve spinal pain, MRI can ensure that the
medication will not be mistakenly injected into a tumor or site of
infection.
If your child has trouble with daytime wetting and does
not fully empty the bladder, imaging the spinal cord may show why this
is happening.
How should I prepare for the procedure? Back to Top
The
strong magnetic field used in MRI can disrupt the operation of an
implanted cardiac pacemaker or other medical device. Staff will ask
whether you have a pacemaker, artificial heart valve, implanted drug
infusion port, cochlear implant, artificial limb, aneurysm clip,
intrauterine device or any metal pins, screws, plates or surgical
staples. In most cases, metal objects used in orthopedic surgery pose
no risk during MRI. Objects that may prohibit MRI are an inner ear
implant, clips used on brain aneurysms and a recently placed artificial
joint. Sheet metal workers and others who might have metal objects such
as a bullet or shrapnel in their body should have an x-ray performed
before MRI.
You will be asked to remove all metal objects
including jewelry, watches and hairpins. Dyes used in tattoos may
contain iron and could heat up during MRI, but this is rarely a
problem. You may wear a hospital gown during the exam, or may be
allowed to keep your own clothing if it is loose-fitting and has no
metal fasteners. If you might be pregnant you should mention this to
MRI staff.
Because some MRI studies use oral or injected contrast
material to obtain the clearest images, the radiologist or technologist
will ask you about allergy of any kind such as hay fever, hives,
allergic asthma or allergy to food or medications. The radiologist
should know of any serious health problems because some of them—such as
kidney disease or sickle cell anemia—may prevent you from having MRI
with contrast material. People who tend to be claustrophobic might
become uncomfortable if examined by an enclosed MRI unit, and may find
it hard to lie still during the exam. A sedative will relieve anxiety
in this situation, but probably only one in twenty patients will
require such medication. Another option is to use an open MRI unit.
What does the MRI equipment look like? Back to Top
Most MRI
units have a closed, cylinder-shaped compartment in which the patient
must lie still for periods lasting several minutes. Some MRI machines
that are less confining are made in such a way that the magnet does not
completely surround the patient. Some of them are open on all sides.
These "open" MRI units are especially useful for patients who are
claustrophobic or obese. However, the strength of these open magnets is
generally less than that of closed magnets, often resulting in
lower-quality images.
How does the procedure work? Back to Top
Among imaging
methods, MRI is unique in that it does not depend on ionizing
radiation, as does the conventional x-ray examination. The basis of MRI
is to direct radio waves at protons, particles that help make up the
nucleus of hydrogen atoms. When this is done in a strong magnetic field
generated by a large magnet that surrounds the patient's body, the
protons are alternately "excited" and "relaxed," emitting signals that
are processed by a computer program to form images. Because protons are
most abundant in the hydrogen atoms of water (the "H" in H20), MRI
images depict differences in the content and distribution of water in
various tissues. With MRI, different types of tissue within the same
body structure are clearly displayed in fine anatomic detail. In the
spine, for instance, fatty tissue, cerebrospinal fluid and the central
portion of the material making up the intervertebral disks contain
considerable water, more than is found in bone, cartilage and nerve
tissue. MRI is well suited to detecting conditions that increase the
amount of fluid, such as tumors, inflammation and infection.
A
typical MRI exam includes two to six imaging sequences, which produce
sectional views or "slices" of the spine in different planes: left to
right, front to back, upper to lower. The sections are often about a
quarter-inch apart, providing a detailed look at the tissues making up
the spinal column. The images may be stored in a computer and
subsequently viewed on screen, or they may be printed on film much like
a conventional x-ray. Depending on the location of symptoms, you
probably will have only part of your spine imaged: the cervical (neck)
portion, the thoracic (chest) spine or the lumbar (lower) spine.
How is the procedure performed? Back to Top
You will lie on
your back on a narrow table that can be moved back and forth, into and
out of the MRI tunnel. The table is moved by an automatic mechanism
operated by the technologist. Bolsters or cushions are used to maintain
proper positioning and to make you as comfortable as possible. When all
is ready, the radiologist (a physician specially trained to obtain and
interpret medical imaging) and the radiologic technologist will leave
the examining room, but you will be able to communicate with the
technologist at any time using a two-way intercom installed in the MRI
unit. The technologist will watch you through a glass window next to
the examining room. Many imaging centers permit a friend to remain near
the patient, or a parent if a child is being examined. An MRI study
generally takes 15 to 45 minutes, although only a fraction of that time
is needed for the actual imaging. You will be instructed not to move
during imaging because motion will blur the images. You will know when
images are being recorded because you will hear tapping or thumping
sounds whenever the coils creating a secondary magnetic field are
turned on. Each imaging sequence takes from a few seconds to a few
minutes, so you will be able to relax at intervals during the
examination. When it is completed you will wait a short time on the
table while the images are examined; occasionally additional images are
needed.
What will I experience during the MRI procedure? Back to Top
MRI
causes no pain, although some patients find it uncomfortable to stay
still while imaging is taking place. This is sometimes the case for
those who are considerably overweight or for those who are bothered by
a closed-in feeling similar to claustrophobia. These discomforts
usually can be relieved by having the exam in an open MRI unit or by
taking a sedative. It is rare that a patient is unable to tolerate MRI
of any type and requires a different form of imaging.
When first
entering the MR unit, you may hear a fan and feel air moving. Some
centers provide earplugs or a stereo headset to block the noises heard
during the imaging procedure. If contrast material is to be injected,
you may feel brief pain from the needle stick and a sense of warmth
when the material is injected—which takes one to two minutes.
No
period of recovery is needed after MRI of the spine unless you have
received a sedative. You may resume normal activities, your usual diet
and needed medications immediately.
Who interprets the results and how do I get them? Back to Top
The
radiologist will need time to examine and interpret the pictures. A
preliminary interpretation may be available shortly after the exam and
a formal report of the complete results will be sent to the referring
physician within a day or two. Your primary care physician or the
radiologist may discuss the findings of the MRI examination with you.
Some facilities today are able to transmit a diagnostic report
accompanied by some of the images over the Internet.
What are the benefits vs. risks? Back to Top
Benefits
- MRI is an excellent method of obtaining clear, detailed images of
the bony structures and soft tissues of the spine, including the spinal
cord. It demonstrates abnormalities, injuries and diseases in the
spinal region that may not be visualized with other imaging methods.
- MRI is a noninvasive study that—unlike conventional x-rays, CT scanning and myelography—does not require exposure to radiation.
- This
method takes little time to carry out, making it very useful for
evaluating people who have been injured. It is especially helpful for
diagnosing or ruling out acute compression of the spinal cord when
clinical examination shows muscle weakness or paralysis.
- The
contrast material sometimes used for MRI does not contain iodine, and
therefore is far less likely to produce an allergic reaction than the
contrast materials used for conventional x-rays and CT scanning.
- MRI
is able to detect subtle changes in the vertebral column that may be an
early stage of infection or tumor. The procedure may be better than CT
scanning for evaluating tumors, abscesses and other masses near the
spinal cord.
Risks
- An iron-containing implant or cardiac pacemaker may be affected by the strong pull of the magnetic field.
What are the limitations of MRI of the Spine? Back to Top
MRI
generally is not done in the first 12 weeks of pregnancy; however, in
urgent situations it may be performed. Radiologists prefer to use other
methods such as ultrasound imaging in pregnant women unless no
examination other than MRI will do. It may not be possible to perform
MRI in a very obese patient, although an open unit will work in most
cases. The presence of an implant or other metallic object often makes
it difficult to obtain clear images, and patient movement can have the
same effect. A patient with severe pain may not be able to lie still
during imaging.
The MRI findings by themselves do not establish an
absolute diagnosis, but in most situations the findings will suggest
the correct diagnosis. The images must be interpreted along with the
patient's history, physical findings and information from other tests. Vertebral fractures may be better detected by CT scanning. MRI may be more costly than other imaging methods, including CT scanning. Back to Top |
