MRI: Viewing the body's hidden structure

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MRI: Viewing the body's hidden structure

Magnetic resonance imaging (MRI), a diagnostic test, creates cross-sectional pictures of your body. Knowing what to expect during the test can help you feel more comfortable.

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.

Who is it for?

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

Head MRI

Head MRI

A tumor appears as a white spot in this MRI image of a head.

How do you prepare?

Before an MRI exam, eat normally and continue to take your usual medications, unless otherwise instructed. You may be given a gown and robe to wear or told to wear clothing without metal fasteners, such as sweats.

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 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.

Don't receive an MRI scan if you have an implantable cardioverter defibrillator or 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.

If you have any questions, ask your doctor or MRI technologist.

How is it done?

Most MRI machines are large, cylindrical-shaped magnets. The strong magnetic field is produced by passing an electric current through wire loops or coils, which are located inside a protective housing. Other coils in the housing send and receive radio waves. When you're in the machine, your body produces very faint signals in response to the radio waves. These signals are detected by coils within the machine, or by additional coils designed to surround a specific body part needing examination. A computer then processes the signals and generates an image.

The collected signals 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.

What can you expect during the test?

The MRI machine is a large magnet with a central opening. You lie down on a moveable table that slides into the opening of the magnet. Depending on the part of your body that needs examination, a small coil may be placed around the portion being examined to receive the MR signal.

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.

The MRI machine creates a strong magnetic field around you, and radio waves are directed at your body. The exam is painless. You don't feel the magnetic field or radio waves, and there are no moving parts around you.

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.

An MRI exam lasts between 30 and 90 minutes. Because movement can blur the resulting images, breathe quietly but comfortably, without moving your head or body. After several minutes of imaging you may become relaxed and have few problems lying still for the duration of the exam. If you're anxious about the enclosed space, talk with your doctor before the exam.

In some cases contrast agents are injected into your veins to enhance the appearance of certain tissues or blood vessels in the images. These will be inserted into an arm or hand vein with the use of a needle connected to an intravenous line. These contrast agents are different from those used in kidney tests or computerized tomography (CT) scanning. They don't contain iodine and are less likely to cause an allergic reaction.

Results

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.

Risks

There are no known harmful effects from exposure to the magnetic field or radio waves used in making MRI images.

Looking ahead

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.
  • Spectroscopic MRI. Magnetic resonance spectroscopy measures the amount of certain metabolites in the body. Different diseases produce different amounts of these metabolites. Spectroscopic MRI can help your doctor diagnose and follow the treatment of conditions including cancer and infections.
  • 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 or less magnet. In comparison, the strength of the large electromagnets at junkyards range from about 1.5T to 2T. Newer MRI machines contain 3T magnets. In some cases the stronger magnetic field provides better detail and faster imaging than conventional machines.
  • Diffusion MRI. Diffusion imaging creates an image based on the microscopic movement or diffusion of water in the spaces outside of cells. This is particularly useful in the early detection of strokes, within minutes of onset. It's also being used to characterize brain tumors and to map out fiber tracts in the brain.

    Last Updated: 04/22/2005
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