November-December 2000 > Features

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.”