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Bloodless Brain Surgery

11.1.00

The scientist behind focused ultrasound (FUS), the most noninvasive treatment of all, is a medical physicist from Finland. Kullervo Hynynen (pronounced HIN-i-nin), associate professor of radiology at Harvard Medical School, began developing FUS at the University of Arizona, then joined Jolesz's group in 1993 to work on the integration of MRI with FUS technology. "This is a good place to do things, a very good environment, a very good group," he says, referring to his ongoing collaborations with MIT and the Dana-Farber Cancer Institute, as well as with GE Medical Systems and other companies. Fifteen years ago, he recalls, "I proposed FUS to my colleagues, and they said, 'Why do it? If you can do a surgery, what's the point?'" Although focused ultrasound is an old idea--"People knew it could be done 50 years ago"--little progress was made without the technology to monitor temperature changes in tissue, he says. "The big step here has been to use the MRI to see the hot spots, which has finally allowed us to think about FUS applications."

Alternatives to the scalpel include freezing and heating diseased tissue. Here, MRI-guided cryosurgery destroys cancer, in this case in the liver.
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 Òphased arrayÓ ultrasound transducer, used for nonsurgical heating treatment of brain tumors.

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.

~H.F.H.