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November-December 2000
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Sidebar to Bloodless Revolution |
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| Alternatives to the scalpel include freezing and heating diseased tissue. Here, MRI-guided cryosurgery destroys cancer, in this case in the liver. |
In addition to designing and perfecting FUS for breast-cancer ablations, Hynynen is at work creating his particular dream technology. Medical physicists had long assumed that using ultrasound through the skull was impossible, but Hynynen always "knew it was possible, and we had a breakthrough a few years ago," he says.
The skull poses two major challenges. Low-power ultrasound is harmless to soft tissue, but the same power, if used by an FUS transducer on a brain tumor, would cause the skull itself to overheat and burn because bones absorb 10 to 20 times as much ultrasound energy as soft tissue. In addition, although soft tissue creates no obstacle to ultrasound, allowing for precise and unobstructed focus on the tumor focal point, "the skull destroys the focus," says Hynynen. His breakthrough for fixing both problems--heat absorption and wave distortion--was to devise a "phased array" ultrasound transducer. (Transducers convert electrical energy into ultrasound waves, much as stereo speakers convert electrical signals into sound.)
Instead of the small "spherical transducers" he had designed
for breast applications, Hynynen created a number of experimental
"hemispherical transducers" to deal with brain tumors. The concave
interiors of these plate-sized half-globe transducers are dotted
with anywhere from 64 to 501 electrical elements. "By creating
a 64-[or more]-element array that goes around the head, the magnification
becomes so large that you eliminate the heating of the skull,"
says Hynynen. In other words, the ultrasound power is spread across
the whole surface of the skull, yet converges into focus at the
point of the tumor. The skull still distorts the ultrasound beams,
"but with phased arrays you can correct for that and get very
sharp focus," he explains. "This allows us to [cook] small tumors.
And since we can do this in the MRI, we can see the hot spot before
it reaches the critical temperature. We can see the spot and move
it wherever we want." Ultimately, he says, "this would allow you
to do brain surgery, deep in the brain, without opening the brain
up."
 A hemispherical "phased array" ultrasound transducer, used for nonsurgical heating treatment of brain tumors. |
| Medical imagery courtesy of the office of Ferenc Jolesz, M.D., unless noted. |
Such noninvasive neurosurgery is a far cry from the skull-drilling, bone-sawing process Joe underwent for his five-hour craniotomy. Without blood, without incision, without the side effects of ionized radiation or chemotherapy, MR-monitored focused ultrasound would destroy a brain tumor and the brain tumor alone. Thus far, Hynynen's team have tested their tumor-killing ultrasound helmets on various tissues placed inside a collection of human skulls and have successfully coagulated or "cooked" the samples at predetermined points and sizes without overheating the skull. Late this year they will begin testing on animals, and next year, on humans.
