How the Technology Works

Magnetic resonance imaging uses magnets, radio waves and computers to reconstruct images of the body’s soft tissues. The images are clearer than those produced by other techniques that utilize low-level radiation.

The heart of the MRI scanner--the size of a pickup truck and weighing 20,000 pounds--is a giant electromagnetic tube with a magnetic field 30,000 times stronger than that of the Earth. The magnet makes the protons in the body’s hydrogen atoms all spin in one direction. Transmitters send radio waves into the body, making the spinning hydrogen atoms wobble andbounce back a signal to receivers. The signals are interpreted by computers that produce an image of a cross-section of the body. The digitized information can be manipulated, edited and printed.

Here is how MRI is used to help identify the source of a woman’s stroke:

The patient is slid into the scanner’s tube. Working at a terminal, a technician focuses on the woman’s head, generating images on the computer monitor. He vertically slices it from nose to back, revealing the sinuses, the brain stem, the cortex; he cuts horizontally across the tops of her ears, so he can even see the lenses in the eyeballs.

Moving the computer mouse, the technician searches for the source of the woman’s stroke as he slowly spins her head on the computer screen. As her face turns, her arteries jump out in 3-D clarity. It is as if her head were made of crystal, every whorl and convolution clear as etched glass. He edits out the smaller arteries, removing the clutter, draws a yellow line around a piece of tissue and rotates the chosen chunk of brain like a Rubix Cube in someone’s hand.

Because of a new MRI technique that looks at blood flows, the woman was able to avoid an angiogram, a potentially dangerous procedure that requires doctors to thread a catheter into an artery in her groin up to the neck and inject a dye. In contrast, the MRI procedure is not only painless and non-invasive, but also cheaper.