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MRI

What Is Magnetic Resonance Imaging (MRI)?

Last modified: 
17/04/2013 - 13:44

Contributing Author: Guy Slowik FRCS

Magnetic Resonance Imaging, or MRI, is a painless and safe diagnostic procedure that uses a powerful magnet and radio waves to produce detailed images of the body's organs and structures, without the use of X-rays or other radiation.

A computer converts signals from the MRI scan into extremely clear, cross-sectional images of the part of the body that has been scanned. Each image is a slice of the body area scanned, and numerous images are created that clearly show all the features of that particular part of the body.

The images produced by MRI can be compared to a sliced loaf of bread. Just as you can lift each individual slice from the loaf and see both the slice and the inside of the bread, so too the image "slices" produced by the MRI show the exact details of the inside of the body.

The computer is able to reconstruct all the images into a single image resembling an X-ray. This reconstruction also can be made into three-dimensional images, allowing complete and remarkable visualization of the body area scanned from all angles

MRI is arguably the greatest advance in diagnostic medical techniques over the past century.

How Does Magnetic Resonance Imaging Work?

Unlike CT, or computed tomography-another type of imaging-MRI uses no radiation. Instead, MRI uses a powerful magnet and radio waves to produce high-quality, cross-sectional pictures of the part of the body being studied. Each picture represents a virtual slice through the part of the body being imaged.

  • The MRI scanning machine is a large donut-shaped magnet with a sliding scanning table. A person lies on this table, which then slides into the desired position in the MRI magnet. The machine produces loud, repetitive noises, like banging, during the procedure. But these noises, while unpleasant at times, aren't dangerous or indicative of a problem.
  • In our bodies, the nuclei of hydrogen atoms (called protons) normally point randomly in different directions. However, when exposed to the magnetic field in an MRI chamber, the nuclei line up in parallel formation, like rows of tiny magnets. Nearly two-thirds of the body's hydrogen atoms are found in water and fat molecules.
  • When the nuclei are subjected to a strong but harmless (and painless) pulse of radio waves from the MRI machine, they are knocked out of their parallel alignment. As they fall back into alignment, they produce a detectable radio signal.
  • The signal is recorded by the machine and transferred to a computer.
  • The computer uses these signals to calculate an image that is based on the strength of signal produced by different types of tissue. For example, tissues that contain little or no hydrogen (such as bone) appear black. Those that contain large amounts of hydrogen (such as the brain) produce a bright image.
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From Andrew Maynard - Chair of the University of Michigan Department of Environmental Health Sciences, with help from David Faulkner - 2013 Master of Public Health graduate.