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Magnetic resonance imaging (MRI) is a technique that uses a magnetic field and radio waves to create cross-sectional images of your head and body. Your doctor uses these detailed, clear images to identify and diagnose a wide range of conditions.
MRI is a noninvasive way for your doctor to examine your body, in particular your brain, neck, spinal cord and soft tissues. MRI often helps with the diagnosis of central nervous system disorders, such as multiple sclerosis, because it produces such high-resolution images of the brain and spinal cord.
MRI is also used to:
- Identify brain tumors, strokes and chronic disorders of the nervous system
- Reveal brain abnormalities in people with dementia
- Diagnose diseases of the pituitary gland
- Locate eye or inner ear tissue abnormalities
- Identify damage caused by heart attack or heart disease
- Detect blood vessel plaques and blockages
- Identify and diagnose bone and joint damage
- Identify bone and joint infections, injuries, degenerative disorders and tumors
- Reveal tumors and functional disorders in organs such as the lungs, liver, pancreas, kidney and spleen
- Detect breast cancer
- Detect reproductive system and bladder problems
Before an MRI exam, eat normally and continue to take your usual medications, unless otherwise instructed. When you report for your exam, you will be asked to complete a screening form. Don't receive an MRI scan if you have an implantable cardioverter defibrillator or a pacemaker. The strong magnetic field produced by the MRI unit may interfere with the function of these devices.
Before undergoing MRI imaging, also tell the technologist if you think you're pregnant, because the effects of magnetic fields on fetuses aren't well understood. Your doctor may recommend postponing the exam or choose an alternative exam.
You may be given a gown and robe to wear or told to wear clothing without metal fasteners, such as sweats. At this time you'll be asked to remove all accessories — watch, jewelry, hairpins. Also remove wigs, dentures and hearing aids.
It's important that you remove all such objects, which may contain metal or electronics, from your body before the exam. Metal objects may interfere with the magnetic field used during the exam, affecting the quality of the MRI images. And the magnetic field may damage electronic items.
Tell the technologist at the time of your exam if you have any metal or electronic devices in your body, such as metallic joint prostheses, artificial heart valves, implanted electronic devices, cochlear implants or magnets in your dentures. The presence of metal in your body may be a safety hazard or affect a portion of the MRI image.
Most MRI machines contain a large magnet shaped like a doughnut or tunnel. You lie on a movable table that slides into the tunnel. The magnetic field aligns atomic particles in some of your cells. When radio waves are broadcast toward these aligned particles, they produce signals that vary according to the type of tissue they are.
The collected signals are interpreted by a computer to create a composite, three-dimensional representation of your body. Any two-dimensional plane (slice) can be electronically created from this representation and displayed on a video monitor for examination. These images can be converted from the screen into photographic film for further viewing and analysis.
The exam is painless. You don't feel the magnetic field or radio waves. Depending on the part of your body that needs examination, a small device may be placed around the portion being examined to receive the MR signal. Some types of MRI scans can be enhanced by the injection of a contrast material via an intravenous line placed in your hand or arm.
During the MRI scan, the internal part of the magnet produces repetitive tapping, thumping sounds and other noises. Earplugs or music may be provided to help block the noise. A technologist monitors you from another room. You can talk with him or her by microphone. In some cases, a friend or family member may stay in the room with you.
Several sets of images are usually required, each taking from two to 15 minutes to complete. If many sets of images are needed, the entire procedure can last more than an hour. Because movement can blur the resulting images, breathe quietly but comfortably, without moving your head or body.
Some people feel anxious inside a conventional MRI machine. Discuss this with your doctor if you feel this way. He or she can arrange a sedative to help you relax, if necessary. Short-bore MRI machines are shorter and wider, so that more of your body is outside the machine during scans. Some newer MRI machines are open on all sides, but this may reduce image quality.
When the exam is complete, you may be asked to wait until the images are reviewed to make sure that no additional imaging is necessary. You then leave and resume your regular activities. A radiologist reviews the images from your examination and reports the findings to your doctor. Your doctor then discusses any important findings and next steps with you.
There are no known harmful effects from exposure to the magnetic field or radio waves used in making MRI images. If your scan requires the injection of a contrast material, there is a very small risk of an allergic reaction.
If you are a metalworker, you may need to have an X-ray of your head before undergoing MRI. This will check to see if there are any tiny slivers of metal in your eyes, which could move during the MRI and possibly damage your eyes.
Metal objects are supposed to be kept out of the rooms that house MRI machines. However, accidents can happen and there's a slight chance that you could be injured by a metal object being drawn forcefully toward the strong magnetic field.
With advancing technology, MRI exams continue to improve. Advances in this area include:
- Magnetic resonance angiography. Vascular imaging is one of the newer uses of MRI. Magnetic resonance angiography (MRA) is a noninvasive way to evaluate the arteries and veins throughout your body. This procedure doesn't require threading a catheter into your arteries, as does traditional angiography.
- Functional MRI. A development in MRI technology, known as functional MRI, enables researchers to measure split-second nerve cell activity in parts of the brain. Functional MRI can locate areas of the brain that control movement, speech, vision and memory.
- Stronger magnets. The strength of the magnetic field of an MRI machine is described by the term "tesla" (T). Most clinical MRI machines contain a 1.5T magnet and some contain 3T magnets. The stronger magnetic field may provide better detail than do conventional machines for some exams.