Sidebar to Bloodless Revolution
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."
~H.F.H.
