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Science File / An exploration of issues and trends
affecting medicine and the environment | I Didn't Know
That

April 23, 1998

Q: How does an MRI work and why is it so noisy?

A: Magnetic resonance imaging is simply a fancy way to look at the hydrogen atoms, also called protons, that form part of every water molecule in the body, according to biologist Russ Jacobs of Caltech's Beckman Institute. Protons spin on their axes like tops. When they are exposed to a powerful magnetic field, however, they wobble. When they are given extra energy by beaming a radio wave at them, they emit faintly detectable radio waves themselves, with frequencies depending on how fast they are wobbling.

The heart of an MRI is basically just a strong magnet and a radio transmitter and receiver, plus a lot of electronics to coordinate their operation. Most of the noise comes from the electromagnet. When the magnet is turned on, there is a large outward force all along its coil, causing it to expand slightly, which makes a loud "click." During imaging, the magnet is turned on and off rapidly, making a rapid-fire clicking noise that is amplified by the enclosed space in which the patient lies.

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