MR Imaging (MRI) - Body
What is MRI of the Body? Back to Top
Magnetic resonance imaging (MRI) uses radio frequency waves and a strong magnetic field rather than x-rays to
provide remarkably clear and detailed pictures of internal organs and
tissues. The technique has proven very valuable for the diagnosis of a
broad range of pathologic conditions in all parts of the body including
cancer, heart and vascular disease, stroke, and joint and
musculoskeletal disorders. 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 the most sensitive exam for spinal and joint problems. MRI
is widely used to diagnose sports-related injuries, especially those
affecting the knee, shoulder, hip, elbow and wrist. The images allow
the physician to see even very small tears and injuries to ligaments
and muscles.
In addition, MRI of the heart, aorta, coronary arteries and blood
vessels is a fast, noninvasive tool for diagnosing coronary artery
disease and heart problems. Physicians can examine the size and
thickness of the chambers of the heart and determine the extent of
damage caused by a heart attack or progressive heart disease.
Organs of the chest and abdomen—including the lungs, liver, kidney,
spleen, pancreas and abdominal vessels—can also be examined in high
detail with MRI, enabling the diagnosis and evaluation of tumors and
functional disorders. MRI is growing in popularity as an alternative to
traditional x-ray mammography in the early diagnosis of breast cancer.
Because no radiation exposure is involved, MRI is often the preferred
diagnostic tool for examination of the male and female reproductive
systems, pelvis and hips and the bladder.
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, 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. Tattoos and permanent eyeliner may also
create a problem. 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, 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 about drug allergies and
whether head surgery has been done in the past. If you might be
pregnant, this should be mentioned. 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 to reduce the anxiety associated with
claustrophobia.
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 seconds at a time and consequently may feel
"closed-in" or truly claustrophobic. However, new "patient-friendly"
designs are rapidly coming into routine use.
The "short-bore" systems are wider and shorter and do not fully
enclose the patient. Some newer units are open on all sides, however
the image quality may vary.
Examples of the radiography equipment that may be used are shown at the top of this page.
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 Computed Tomography (CT) scanning, it does not rely on ionizing
radiation. Instead radiofrequency 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 MR
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 examination consists of two 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 placed on a
sliding table and positioned comfortably for the MRI examination. Then
the radiologist and technologist leave the room and the individual MRI
sequences are performed. 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 (usually gadolinium) 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. A saline solution
will drip through the intravenous line to prevent clotting until 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. A radiologist
experienced in MRI will analyze the images and send a report with his
or her interpretation to the patient's personal physician. This should
take only a few days or less.
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 notified.
If a contrast injection is needed, there may be discomfort at the
injection site and you may have a cool sensation at the site during the
injection. 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 patient
receives MRI results from the referring physician who ordered the test.
New technology also allows for distribution of diagnostic reports and
referral images over the Internet at many facilities.
What are the benefits vs. risks? Back to Top
Benefits
- Images of the soft-tissue structures of the body—such as the heart,
lungs, liver and other organs—are clearer and more detailed than with
other imaging methods.
- MRI can help physicians evaluate the function as well as the structure of many organs.
- The detail makes MRI an invaluable tool in early diagnosis and evaluation of tumors.
- 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 that might be obscured by bone with other imaging methods.
- MRI
provides a fast, noninvasive alternative to x-ray angiography for
diagnosing problems of the heart and cardiovascular system.
- Exposure to radiation is avoided.
Risks
- An undetected metal implant may be affected by the strong magnetic field.
- MRI
is generally avoided in the first 12 weeks of pregnancy. Doctors
usually use other methods of imaging, such as ultrasound, on pregnant
women unless there is a strong medical reason to use MRI.
What are the limitations of MRI of the Body? Back to Top
Bone is better
imaged by conventional x-rays in some cases and CT is preferred for
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 examination 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 examination must be used cautiously in early
pregnancy. MRI typically costs more than CT scanning. Back to Top |
