Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is a sophisticated diagnostic technique that
uses a magnetic field, radiowaves and a computer to generate detailed,
cross-sectional images of human anatomy. Because it produces better soft-tissue
images than x-rays can, MRI is most commonly used to image the brain, spine,
thorax, vascular system and musculoskeletal system (including the knee and
During an MRI exam, the patient is placed inside a scanner that produces a
static magnetic field up to 8,000 times stronger than the earth's own magnetic
field. Exposure to this force causes the hydrogen protons within the patient's
body to align with the magnetic field. When a radiofrequency pulse is applied,
the protons spin perpendicular to the magnetic field. As the protons relax back
into alignment with the magnetic field, a signal is sent to a radiofrequency
coil that acts as an antenna. This signal then is processed by a computer.
Different tissues produce different signals. For example, protons in water relax
more slowly than those in fat. This differentiation can be detected, measured
and converted into a cross-sectional image of the patient's anatomy.
MRI is a safe procedure for most patients, although it generally is not
recommended for pregnant women. If you are pregnant, let your physician know.
Also, because the body is exposed to a strong magnetic field, patients who have
a pacemaker, cochlear implants or aneurysm clips should check with a physician
before undergoing an MRI examination. Patients who have other types of metal
implants and patients who have been exposed to shrapnel or whose eyes have been
exposed to metal shavings also might not be candidates for MRI; it's important
to let your physician know if these conditions apply to you. For similar
reasons, women undergoing an MRI exam should not wear eyeshadow, because it
sometimes contains metallic substances.
If you are claustrophobic or experience pain when lying on your back for more
than 30 minutes, let your doctor know. He or she may be able to prescribe a
relaxant or pain medication. If you are sedated for the examination, a friend
will have to drive you home afterward. In some facilities, you can arrange for
your scan to be performed in an "open" magnet. Open MR units are less confining
than traditional MRI machines. Instead of sliding the patient into a long metal
tube, the magnet is suspended above the patient. Keep in mind, however, that
open magnets are a new technology and not all facilities have them.
Before your examination, an MR technologist will explain the procedure to you
radiologic technologist, is a skilled medical professional who has received
specialized education in the areas of anatomy, patient positioning, patient
care, imaging techniques and MR procedures.
During the Examination
Examination time depends upon the part of the body being examined, but
typically ranges from 30 minutes to an hour. You will be asked to undress,
remove all jewelry and put on a hospital gown. Remember, the magnet will damage
wristwatches and erase credit cards and bank cards, so don't take them into the
exam room with you. You will be provided a secure place to store these items
during your examination.
For most types of exams, the MR technologist will wrap a special coil around
the body part that is being examined. This coil helps concentrate the
radiofrequency pulses. The MR technologist then will position you on a padded,
movable table that will slide into the opening of the scanner.You may be given a
contrast agent to highlight internal organs and structures. The contrast
changes the relaxation rate of protons in the body, illuminating organs and
tissues and making tumors, vessels and scar tissue appear brighter.
You won't feel anything during the scan, but you may hear intermittent
humming, thumping, clicking and knocking sounds. These are the sounds of the
magnetic gradients turning on and off. Some MR centers provide patients with
headphones or earplugs to help mask the noise.
The MR technologist will not be in the room during the scan, but will be able
to observe you through a window from a room next door and will be able to hear
you and talk to you through a two-way microphone system. The technologist will
tell you when each scan sequence is beginning and how long it will last. You
will be asked to remain as still as possible throughout the sequence.
When the exam is complete, your MR scans obtained will be given to a radiologist - a physician who specializes in the diagnostic interpretation
of medical images.
After your images have been reviewed by a radiologist, your personal
physician will receive a report of the findings. Your physician then will advise
you of the results and discuss what further procedures, if any, are needed.
Magnetic resonance imaging is a noninvasive procedure, and there are no known
side effects or after effects. If a contrast agent was administered, you may
experience nausea, headache or dizziness following your examination. It's
important to increase your water consumption in the days following the
examination. If these symptoms persist, contact your physician.
Please remember that the material presented here is for informational
purposes only. If you have specific questions about a medical imaging procedure,
contact your physician or the radiology department of the institution where your
test will be performed.
About Magnetic Resonance Angiography and Other MR Procedures
In addition to MRI, magnetic resonance has other diagnostic applications. One
rapidly advancing technique is magnetic resonance angiography, or MRA.
Angiography is the imaging of blood vessels. Usually, it is performed to
evaluate aneurysms or to determine whether vessels in the brain, neck, legs or
other areas have become narrowed due to atherosclerosis ("hardening of the
arteries"). During conventional x-ray angiography, a catheter is inserted into
the body through the groin and an iodine-based contrast agent is injected into
the blood vessel while a series of x-rays are taken. MRA allows physicians to
view blood vessels and the flow of blood through arteries without the need to
introduce a catheter or a contrast agent into the patient's artery.
Neurological and intracranial applications for magnetic resonance technology
include functional MR and MR spectroscopy. Functional MR uses magnetic resonance
to map the brain, matching motor and sensory activities with corresponding areas
of brain activation. Functional MR detects changes in the blood supply to
specific areas of the brain. Using it, researchers can "watch" changes that
occur in the brain when a patient speaks, recalls a past event or moves a body
part. Functional MR is still in its early stages of development, but it shows
promise as a method to study stroke, multiple sclerosis, epilepsy, Alzheimer
disease and Parkinson disease.
MR spectroscopy uses magnetic resonance to measure
metabolites, creating a chemical spectrum (map) of the brain. Metabolites are
byproducts of metabolism that are present in the brain matter. By measuring
their concentrations in different area researchers can determine how the brain
works at an unprecedented level of detail. Applications include measurement of
the volume and flow of blood to brain tumors, assessment of brain tissue and
study of sleep disorders and Alzheimer disease.
The above patient information provided by the American Society of Radiologic
Technologist (ASRT) with permission. www.asrt.org