Magnetic Resonance Imaging
Introduction
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 shoulder).
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.
Patient Preparation
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 and answer any questions you might have. An MR technologist, also known as a
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.
Post-examination Information
After your films 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 areas of the brain, 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.
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