06/01/1996 07:00:00

Question of the Month: How does an MRI work, and why is it so noisy?

Question of the Month Answered by: Russ Jacobs, Ph.D., Member of the Beckman Institute, Biology

Magnetic resonance imaging machines, or MRIs, use strong magnetic fields and radio waves to look inside a patient without the need for surgery or the use of damaging radiation such as X-rays. MRIs have become standard equipment in many hospitals over the last decade.

When an MRI machine looks inside the body, what it really sees are water molecules. Because all parts of the body contain some water, MRIs can examine any part of a patient's body. Water molecules consist of oxygen and hydrogen atoms, and the core of a hydrogen atom—its nucleus—is a single proton.

These protons have a basic, inherent property called nuclear spin. One way to think about this nuclear spin is that the protons actually spin like a gyroscope or a top. Because the protons also have an electrical charge, the spin makes them act like tiny magnets.

Scientists have found that a magnetic field will make the spinning protons wobble, just like a spinning top that isn't quite vertical will wobble. And the stronger the magnetic field, the faster the wobble. But while a top might wobble around a few times in a second, nuclei wobble about 50 million times per second.

Scientists also know that if a proton is excited—that is, given some extra energy—by a radio wave, it will send that energy back out as a radio signal that is faintly detectable. The frequency of the proton's radio wave is determined by its wobbling rate; so the protons in water emit a radio frequency of about 50 million cycles per second.

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. The magnet creates a strong magnetic field, hundreds of thousands of times stronger than the earth's magnetic field; the radio transmitter beams an intense burst of radio waves into the patient to excite the wobbling protons; and the receiver detects the protons' faint radio signal.

To create an image, MRIs must determine which radio signals are coming from which protons and plot these protons in their proper locations. To distinguish protons from one another, MRIs manipulate both the magnetic field and the burst of radio energy so that protons in different parts of the patient emit slightly different radio signals. The MRI detects these different signals, figures out automatically where they came from, and builds up a three-dimensional image.

MRIs also notice the strength of these radio signals. Where there is a lot of water, for example in muscle, the signal is strong, and where there is less water, for example in bone, the signal is weak. The different strengths of the radio waves enable the MRI to fill in the image with the proper shades of gray.

An MRI is noisy because its magnetic field is created by running electrical current through a coiled wire—an electromagnet. When the current is switched on, there is an outward force all along the coil. And because the magnetic field is so strong, the force on the coil is very large.

When the current is switched on, the force on the coil goes from zero to huge in just milliseconds, causing the coil to expand slightly, which makes a loud "click." When the MRI is making an image, the current is switched on and off rapidly. The result is a rapid-fire clicking noise, which is amplified by the enclosed space in which the patient lies.