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How Does MRI Work?


How does the MRI magnet produce detailed cross-sections of the brain? "Typically MRI causes the body to emit a signal," explains Jim Rosato, head interventional MRI technologist at Brigham and Women's Hospital. "As the protons--the nuclei--of the hydrogen atoms in our body spin, they form north and south poles, but they can be facing in different directions. Once the patient goes into the magnet, it causes all the protons to align in one direction." With a magnetic field strength 10 thousand times greater than the earth's, the electromagnet in the Signa SP has plenty of power to perform this feat.

"The banging, jackhammer sound people hear during MRI is actually caused by the electromagnetic gradients, or radio waves, switching on and off extremely fast," Rosato continues. "This causes the protons, which are now lined up with the main magnetic field, to flip 90 degrees. Each time the radio pulse switches off, the protons go back to their original position. When they do that, they emit an electromagnetic signal--a radio frequency signal, like FM radio--and that's picked up by the 'antenna,' or the coil," hence magnetic "resonance."

Rosato next points out a "Mayfield radiolucent headrest," a less-than-restful-looking clamp that stabilizes the patient's head during surgery, and a coil like the one he wraps preoperatively around the patient's head. As the coil sends and receives radio frequency waves, "a computer figures out where exactly in the patient's body a signal is coming from," he explains. The computer also determines the gray scale for different elements in the image, depending on the strength of the signal, creating crisp two-dimensional pictures with great detail. The signal from the tumor lasts longer than the signal from normal tissue, which is why it shows up so distinctly.

Given the strength of the Signa SP electromagnet, intraoperative MR imaging has required the design and manufacture of entirely new tools and equipment out of special metals--such as nonmagnetic grades of stainless steel or titanium--and plastic. Nonmagnetic scalpels, needles, drills, IV-drips, microscopes, endoscopes, furniture, surgical drapes, and anesthesia machines are just a few of the MRI-compatible devices needed. Neurosurgeon Peter Black, for example, uses an MRI-compatible "ultrasound aspirator" to pulverize the tumor area and suck it away, notes Angela Kanan, one of the operating-room nurses.

Any magnetic metallic objects that slip past the tight monitoring at the door of the OR can create havoc--and injury. Kanan's fellow nurse Dennis Sullivan recalls that someone once brought in a non-MRI compatible metal cart. "It flew across the room and pinned me against the magnet," he remembers. "It was a good thing it was me. If it had been Angela, that cart would have broken her in two."