Magnetic resonance imaging (MRI) 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. It is particularly helpful in providing details about soft tissues, including the spinal cord, nerves and the discs between the bones of the spine.
Magnetic resonance imaging can be used to:
- Diagnose disorders of the central nervous system such as multiple sclerosis
- Identify brain or spinal tumors or other chronic disorders of the nervous system
- Diagnose pituitary gland diseases
- Find eye or inner ear tissue abnormalities
- Identify bone and joint damage from injuries, degenerative disorders and tumors
- Identify infections of the bones and joints
Before an MRI, it is important to remove any clothing, wigs, appliances such as hearing aids or dentures or jewelry that may contain metal or electronics. If you have metal or electronic devices in your body such as artificial joints or heart valves, a pacemaker or rods, plates or screws holding bones in place, be sure to tell the technician. Metal may interfere with the magnetic field used to create an MRI image and can cause a safety hazard. The magnetic field may damage electronic items.
Do not have an MRI scan if you have an implantable cardioverter defibrillator or pacemaker. The strong magnetic field created by the MRI unit may interfere with how these devices work. If you are or think you may be pregnant, be sure to the tell the technician before having an MRI. The effects of magnetic fields on fetuses aren't well understood. It may be necessary to use a different type of test or postpone an MRI if you are pregnant.
Most MRI machines are large, cylindrical-shaped magnets. The strong magnetic field is produced by sending an electric current through wire loops or coils inside the unit. Other coils in the unit send and receive radio waves. While you are inside the machine, radio waves are sent to your body which responds by sending faint signals back. The MRI machine senses the signals, sends them to a computer, which then generates an image. The image is a composite, three-dimensional picture of your body. A cross-sectional slice through your body at any point can be electronically generated from the picture. This image can then be seen on a video monitor. The images can also be put on photographic film.
When you are having an MRI, you will lie down on a table that slides into the opening of the magnet. A technician monitors you from another room and can talk to you through a microphone or intercom. You won't feel anything as the MRI generates a strong magnetic field around you and directs radio waves at your body. Nothing moves around you. You may however hear a tapping or thumping or other sounds from inside the machine. You may be given earplugs or music to listen to.
It takes 30 to 90 minutes to do an MRI. It is important to lie still and breathe quietly as movement can blur the images. If being in an enclosed space makes you anxious, talk to your doctor before the MRI.
Sometimes a dye or contrast agent is injected into your veins to enhance how certain tissues or blood vessels look in the images. The dye or contrast agent used in an MRI is different than those used in computerized tomography (CT) scanning. They don't contain iodine and aren't as likely to cause an allergic reaction.
As with X-rays or CT scans, a radiologist who trained to read the images will review the results of your MRI and make a report to your doctor or surgeon.
While MRIs are themselves relatively new technology, advances are continuing to be made. Some of these advances include functional MRIs, which make it possible to measure split-second nerve cell activity in parts of the brain; spectroscopic MRIs, which measure certain metabolites in the body, helping doctors to diagnose and treat conditions such as cancer or infections; diffusion MRIs, which create an image based on the microscopic movement of water in the spaces outside the cells; and stronger magnets that allow for more detail and faster imaging than conventional MRI machines.