Magnetic Resonance Imaging
Also known as
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 a computer.
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.
Types of MRI
# Head MRI Brain MRI
# Sellar MRI
# Head and Neck MRI
# Orbit MRI
# Temporal MRI
# Cervical spine MRI
# Thoracic spine MRI
# Lumbosacral spine MRI
# Abdominal MRI
# Male pelvis MRI
# Female pelvis MRI
# Thorax MRI
Cardiovascular MRI (including heart, aorta and blood vessels)
# Ankle MRI
# Hip MRI
# Knee MRI
# Shoulder MRI
# Wrist MRI
Advantages of this procedure
* 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.
How this procedure is performed
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.
Preparation for this procedure
* 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 during scanning.
Results of this procedure
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.
Risks of this procedure
* 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 known.
* 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.
Limitations of this procedure
* 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 metal objects.
* May provoke claustrophobia.
* Longer exam time compared to CAT scans.
* Safety in scanning pregnant women is not known.