Mayo Clinic Health Library

MRI

Updated: 02-04-2011

Transcript

Magnetic resonance imaging (MRI) uses a magnetic field and radio waves to create clear and detailed cross-sectional images of your head and body.

You don't need to prepare for an MRI. Unless otherwise instructed, eat normally (before the procedure) and if you take medications, continue to do so. Once checked in, you'll likely change into a gown and robe.

Remove all accessories, such as your watch, jewelry and hairpins. Also remove things like wigs, dentures and hearing aids. Tell your MRI technologist if you have metal or electronic devices in your body, because their presence may be a safety hazard. The magnet may disable your electronic device or affect the MR image quality.

Before your exam, the MRI technologist may confirm your health issues and answer any last-minute questions.

Most MRI machines look like a big doughnut. A large magnet is housed within a circular structure. You'll lie down on a table that slides into the opening of the magnet. Depending on where you need magnetic resonance imaging, a small device called a coil may be placed around the body part being examined. The coil receives the magnetic resonance signal.

Your technologist will monitor you from another room, but you can talk with him or her by microphone. In some cases, a friend or family member may stay with you. If you are especially anxious or have claustrophobia, you may be offered a mild sedative.

Magnetic resonance imaging is safe for children, and an adult may stay in the scanning room for reassurance. Young children, toddlers and infants may need sedation since they must remain still throughout the imaging process.

The exam itself is painless, but noisy. You don't feel the magnetic field or radio waves, and there are no moving parts to see. However, the magnet produces repetitive tapping and thumping sounds, so you'll likely be offered earplugs or special metal-free headphones to help block the noise. MRIs that require your head to be in the machine often include a mirror for you to see out.

Here's how an MRI is created. Most machines use tube-shaped magnets. The strong magnetic field is produced by passing an electric current through wire loops inside of the magnet's protective housing. Other coils in the magnet send and receive radio waves.

Once you're positioned inside the magnet, some protons within your body will align with the magnetic field. This is harmless and you won't feel it happening.

Once aligned, these protons respond to low-power radio waves that stimulate signals from your body. The radio waves are generated by specialized radio frequency coils (RF coils), which are antennas designed for medical imaging.

The signal that your body returns is detected by the coils surrounding the specific body part targeted for imaging. A computer processes all the signals and reconstructs a highly detailed image.

The final picture is a composite, three-dimensional representation of your body. Any two-dimensional plane — or slice — can be electronically created and displayed on a computer for interpretation. These images can also be converted from the screen into photographic film for further study.

An MRI exam lasts between 30 and 90 minutes. Because movement can blur the resulting images, remain still and breathe quietly without moving your head or body.

Although not shown in this video, contrast agents are sometimes injected into your veins to enhance the appearance of certain tissues or blood vessels.

Once your MRI is complete, you may be asked to wait until the images are reviewed to make sure that no additional imaging is necessary. If no further studies are required, you'll be released and can resume your regular activities.