Magnetic Resonance Imaging
MRI (magnetic resonance imaging) is an imaging technique used to get
cross-sectional images of the body using strong magnetic field and radio
waves (radio frequency pulses) instead of X-rays.
In MRI scanning, patients are placed inside a very large and strong
magnet, so that all the protons in the atoms of the patient's body can
be aligned to a magnetic field. Then, radio waves (called radio frequency
pulses) are directed at the protons (i.e., the nuclei of hydrogen atoms)
to excite the protons. Once the radio waves are stopped, excited atoms
emit radio signals received by an antenna (i.e., a surface coil in the
MRI machine), which are then measured and processed to form an image using
In the human body, protons are most abundant in the hydrogen atoms of
water. Thus, MRI images represent the water content in the area of the
exam. The more water present, the more radio signals emitted, and the
whiter the image.
MRI can provide a clear and detailed picture of soft tissue structures
near and around bones and joints, such as tendons, ligaments, muscles,
joint capsules and mass.
Head and Neck MRI
Cervical spine MRI
Thoracic spine MRI
Lumbosacral spine MRI
Male pelvis MRI
Female pelvis MRI
Cardiovascular MRI (including heart, aorta and blood vessels)
- To provide cross-sectional images in any anatomical plane (upper to
lower, right to left, front to back, oblique).
- To provide clear, detailed images of various soft tissues, such as
internal organs, tumors, and blood vessels.
- No radiation.
- Compared to the iodine-based contrast medium in X-ray or CAT scan,
MRI dye is relatively safe.
- Provides angiographic images without being invasive.
- MRI can evaluate organ function, as well as structure.
You will be asked to lie on the scan table. After proper positioning,
the exam table will slide into the center of the magnet.
During the scan you will be alone in the exam room, but you can talk to
and listen to the technologist using an intercom, who will watch you
through a glass window and video camera.
During actual scanning, you will hear loud tapping noises, but you are
required to remain still until it's done (to get clear pictures).
In the event that a contrast medium is needed to make organs and blood
vessels stand out, it will be injected into your vein during the exam.
The exam usually takes from 15 minutes to an hour and a half.
- Wear comfortable, loose-fitting clothing.
- Disrobe in the examining area and put on a hospital gown. Remove
any metal objects, such as watches, car keys, wallets, beepers, cellular
phones, zippers, snaps, hairpins, jewelry, accessories, eyeglasses,
hearing aids and any removable dental work, because they can be
affected by the huge magnet and degrade quality imaging. The information
on credit cards can be erased by this strong magnetic field.
- Let your radiologist or technologist know when you have a metal
object in your body, such as a cardiac pacemaker, prosthetic heart valve,
prosthetic hip or knee joint, implanted infusion pump, intrauterine device
(IUD), cochlear implant, aneurysm clip or vascular clips, hearing aid,
metal monitoring device, surgical staples, metal plates, pins, screws,
bullets, shrapnel or any metal fragments. This is because the strong
magnetic fields can cause these ferromagnetic metal objects to move,
dislodge, cause burns, or electrical currents.
- Tattoos may degrade image quality.
- For contrast-enhanced MRI, you will be asked if you have any drug allergies.
- Earplugs can be used to protect your ear from loud repetitive noises
A radiologist (a physician specialist trained to interpret MRI images
or other radiology exams, such as CAT scans, X-rays, mammographies, etc.)
reviews the body MRI and reports the results to your personal doctor.
Your physician's office will inform you of the results when they are
complete, and will use the results as a reference in evaluating and
treating your condition.
- The strong magnetic field can cause metal implants to dislodge, burn,
and cause additional injuries. If you have any metal implants in your body,
such as a pacemaker, prosthetic valves, or clips, you should let your
radiologist or technologist know its brand name and model. If it is
not confirmed to be compatible with the magnet, you should not take the
MRI. If you have had bullet injuries or possible metal fragments in your
body, X-rays can be taken, instead.
- Women in the first 12 weeks of pregnancy should avoid MRI and the
contrast medium. There
are no known harmful effects to pregnant women and unborn babies.
However, because it is
a recently developed technology, the long-term effects of MRI are not
- Allergic reactions to the contrast medium are possible, but very rare.
- If you have any history of claustrophobia, you should inform your
radiologist or technologist. Sedatives can be given before scanning.
- Limited ability in imaging bone -- conventional X-ray or CAT scan is
better in demonstrating bone details.
- MRI is less sensitive in demonstrating acute hemorrhage when compared
to CAT scans.
- Hard to depict calcifications.
- MRI does not always distinguish between tumor tissue and edema fluid.
- Less sensitive in detecting small abnormalities compared to CAT scan
(poor spatial resolution).
- Inability to scan critically ill patients requiring life-support
systems and monitoring devices that employ ferromagnetic materials.
- May be dangerous in scanning patients with metal implants and other
- May provoke claustrophobia.
- Longer exam time compared to CAT scans.
- Safety in scanning pregnant women is not known.