MR Imaging (MRI) - Musculoskeletal
What is MRI of the Musculoskeletal System? Back to Top
Magnetic resonance
imaging (MRI) uses radio waves and a strong magnetic field rather than
x-rays to provide clear and detailed pictures of internal organs and
tissues. The parts of the musculoskeletal system that are most
frequently imaged with MRI are the spine, knee and shoulder. However,
MRI has also been used to study almost every joint in the body
including the hips, wrists, and hands. MRI requires specialized
equipment and expertise and allows evaluation of some body structures
that may not be as visible with other imaging methods.
What are some common uses of the MRI procedure? Back to Top
Because MRI can
give such clear pictures of soft tissue structures near and around
bones, it is usually the best choice for examination of the body's
major joints, the spine for disk disease and soft tissues of the
extremities. MRI is widely used to diagnose sports-related injuries, as
well as work-related disorders caused by repeated strain, vibration or
forceful impact.
Using MRI images, physicians can locate and identify the cause of
pain, swelling or bleeding in the tissues in and around the joints and
bones. The images allow the physician to clearly see even very small
tears and injuries to tendons, ligaments and muscles and even some
fractures that cannot be seen on x-rays.
In addition, MRI images can give physicians a clear picture of
degenerative disorders such as arthritis, deterioration of joint
surfaces, or a herniated disk. Neurosurgeons often use MRI to evaluate
the integrity of the spinal cord after trauma.
Finally, MRI is also useful for the diagnosis and characterization
of infections (osteomyelitis for example) and tumors (metastases for
example) involving bones and joints.
How should I prepare for the procedure? Back to Top
Because the strong
magnetic field used for MRI will pull on any ferromagnetic metal object
implanted in the body, MRI staff will ask whether you have a prosthetic
hip, an aneurysm clip in the brain, heart pacemaker (or artificial
heart valve), implanted port, infusion catheter (brand names
Port-o-cath, Infusaport, Lifeport), intrauterine device (IUD) or any
metal plates, pins, screws or surgical staples in your body. In most
cases, surgical staples, plates, pins and screws pose no risk during
MRI if they have been in place for more than four to six weeks. Dyes
used in tattoos and permanent eyeliner may contain metallic iron oxide
and could heat up during MRI; however, this is rare. You will be asked
if you have ever had a bullet or shrapnel in your body or ever worked
with metal. If there is any question of metal fragments, especially in
the eye, you may be asked to have an x-ray that will detect any such
metal objects. Tooth fillings usually are not affected by the magnetic
field but they may distort images of the facial area or brain, so the
radiologist should be aware of them. The same is true of braces, which
may make it hard to "tune" the MRI unit to your body. You will be asked
to remove anything that might degrade MRI images of the head, including
hairpins, jewelry, eyeglasses, hearing aids and any removable dental
work.
The radiologist or technologist may ask if you have any drug
allergies and whether you have undergone any surgery in the past. If
you are or might be pregnant mention it to the MRI staff.
Some patients who undergo MRI in an enclosed unit may feel confined
or claustrophobic. If you are not easily reassured, a sedative may be
administered. Roughly one in 20 patients will require medication.
What does the MRI equipment look like? Back to Top
The conventional MRI
unit is a closed cylindrical magnet in which the patient must lie
totally still for several minutes at a time and consequently may feel
closed-in or truly claustrophobic. However, new patient-friendly
designs are rapidly coming into routine use. These "short-bore" systems
are wider and shorter and do not fully enclose the patient. Some newer
units are open on all sides; however, the images of these systems may
be of poorer quality.
An example of the radiography equipment that may be used is shown at the top of this page and below.
How does the procedure work? Back to Top
MRI is a unique imaging method
because, unlike the usual radiographs (x-rays), radioisotope studies or
even CT, it does not rely on ionizing radiation. Instead, radio waves
are directed at protons, the nuclei of hydrogen atoms, in a strong
magnetic field. The protons are first "excited" and then "relaxed,"
emitting radio signals that can be computer-processed to form an image.
In the body, protons are most abundant in the hydrogen atoms of
water—the "H" of H2O—so that an MRI image shows differences in the
water content and distribution in various body tissues. Even different
types of tissue within the same organ, such as the gray and white
matter of the brain, can easily be distinguished. Typically an MRI exam
consists of four to six imaging sequences, each lasting two to 15
minutes. Each sequence has its own degree of contrast and shows a cross
section of the body in one of several planes (right to left, front to
back, upper to lower).
How is the procedure performed? Back to Top
The patient is comfortably
positioned on a special table that slides into the MRI system opening
where the magnetic field is created. Then the radiologist and
technologist leave the room and the individual MRI sequences are
performed. The patient will hear tapping noises during the exam. The
tapping is created when magnetic field gradient coils are switched on
and off to measure the MRI signal reflecting back out of the patient's
body. The patient is able to communicate with the radiologist or
technologist at any time using an intercom. Also, many MRI centers
allow a friend or, if a child is being examined, a parent to stay in
the room.
Depending on how many images are needed the exam will generally take
15 to 45 minutes, although a very detailed study may take longer. You
will be asked not to move during the actual imaging process, but
between sequences some movement is allowed. Patients are generally
required to remain still for only a few seconds to a few minutes at a
time.
Depending on the part of the body being examined a contrast material
may be used to enhance the visibility of certain tissues or blood
vessels. A small needle connected to an intravenous line is placed in
an arm or hand vein. The contrast material is injected about two-thirds
of the way through the exam.
When the exam is over the patient is asked to wait until the images are examined to determine if more images are needed.
What will I experience during the MRI procedure? Back to Top
MRI causes no
pain but some patients can find it uncomfortable to remain still during
the examination. Others experience a sense of being "closed in" though
the more open construction of newer MRI systems has done much to reduce
that reaction. You may notice a warm feeling in the area under
examination; this is normal but if it bothers you, the radiologist or
technologist should be told.
Most bothersome to many patients are the loud tapping or knocking noises heard at certain phases of imaging. Ear plugs may help.
Who interprets the results and how do I get them? Back to Top
A radiologist,
who is a physician experienced in MRI and other radiology examinations,
will analyze the images and send a signed report with his or her
interpretation to the patient's personal physician. The personal
physician's office will inform the patient on how to obtain their
results. New technology also allows for distribution of diagnostic
reports and referral images over the Internet at some facilities.
What are the benefits vs. risks? Back to Top
Benefits
- MR images of the soft tissue structures of the body such as the
muscles, tendons, joints and blood vessels, are clearer and more
detailed than with other imaging methods.
- MR images can help identify the causes of pain, swelling and bleeding so that appropriate treatment can begin.
- MRI
contrast material is less likely to produce an allergic reaction than
the iodine-based materials used for conventional x-rays and CT
scanning.
- MRI enables the detection of abnormalities, injuries and
diseases that might be obscured by bone tissue with other imaging
methods.
- MRI provides a fast, noninvasive way to assess a variety of muscle and joint injuries and disorders.
- The detail of MR images makes them an invaluable tool in early diagnosis and evaluation of tumors.
- Exposure to radiation is avoided.
Risks
- An undetected metal implant may be affected by the strong magnetic
field. It may cause some distortion of the images or the MRI system may
have difficulty "tuning" to your body if the implant is in the area
being imaged.
- MRI is generally avoided in the first 12 weeks of pregnancy although there are no proven hazards.
What are the limitations of a Musculoskeletal MRI? Back to Top
Bone is better
imaged by conventional x-rays and CT is preferred for unstable patients
with severe bleeding. MRI may not always distinguish between tumor
tissue and edema fluid, and does not detect calcium when this is
present within a tumor. In most cases the exam is safe for patients
with metal implants with the exception of a few types of implants, so
patients should inform the technician of an implant prior to the test.
The exam must be used cautiously in early pregnancy. The presence of a
pacemaker precludes the use of MRI. Back to Top |
