Brain surgery is fraught with huge risks and uncertainty. Parts of the skull (and sometimes most of it) need to be removed, a lengthy and harrowing procedure that could expose the brain to infection and almost always results in significant postoperative pain. Once the surgeon makes the first incision, the smallest error could have devastating consequences—seizure, loss of sensory or motor function, stroke, or even coma. But what if you could slice through the brain without removing any of the skull—create incisions inside the brain from the outside?
Through “transcranial focused ultrasound,” physicians can now use high frequencies of ultrasound (typically from 650 to 710 kilohertz) to create discrete lesions in brain tissue without making direct physical cuts. Patients and doctors alike hope this could be a transformative tool for treating many different psychiatric and neurological disorders more easily and more effectively.
In fact, focused ultrasound has already proved successful in treating patients with the condition known as essential tremor. Neurosurgeons at the University of Virginia’s Focused Ultrasound Center, in Charlottesville, saw tremor rates fall by more than 50 percent in patients treated with ultrasound. The surgeons are currently finishing the final round of patient testing before seeking U.S. government approval as a prescribed treatment of essential tremor. The same clinical team is conducting trials for Parkinson’s patients as well.
“Ultrasound is able to move through obstructions, like the skull,” explains psychiatrist Alexander Bystritsky, of the University of California, Los Angeles. “It’s noninvasive. You simply have to focus ultrasound much like you would focus a light.”
Actually, it’s a bit more complicated than that. Ordinarily ultrasound passes through body tissue without any effect. But in the same way that rays of sunlight can be focused by a magnifying glass to start a fire, beams of ultrasound can be focused to converge on a specific target, raising temperatures and destroying tissue. This three-dimensional “thermal ablation” can be anywhere between 1 and 15 millimeters in diameter, and it can be localized to a specific target deep in the brain without affecting the surrounding tissue. It’s like performing surgery with a heat ray instead of a knife, but with the same precision and deep brain penetration—all without opening up a person’s skull.
The initial problem with the transcranial delivery of ultrasound was that sound waves would lose focus and reflect off the skull. A section of skull would have to be removed to deliver the ultrasound deep into the brain. If part of the skull had to be removed, there was little real advantage to ultrasound over conventional surgery.
Robert Dallapiazza, a neurosurgeon at the University of Virginia, says phased-array systems developed decades later meant that “instead of just two or three ultrasound sources, they started using 200, 500, and 1,000 elements, all focused into one discrete spot.” Combined with a machine-mediated correction algorithm that informs doctors how the energy would be reflected or how its transmission would be altered through the skull, there is now no need to carve a window into the cranium.
Of the multitude of ultrasound applications being researched, one of the most promising is the treatment of motor disorders. Dallapiazza and his colleagues are currently researching the surgical applications of MRI-guided high-frequency focused ultrasound. A patient enters an MRI machine, and an ultrasound transducer is placed over his head. The whole setup, Dallapiazza says, “kind of looks like a hair dryer at a beauty salon.” Within the MRI, low-energy ultrasound is applied to verify the machine’s alignment with the targeted brain region. Amplitudes are then increased to generate a more intense beam that results in lesions. Patients are awake during the entire procedure and can usually feel the effects instantaneously. “They can really tell when they got the treatment,” he says. “People are really brimming with excitement over focused ultrasound, because it doesn’t require awake surgery or electrode implantation for potentially the rest of their lives.”
Dallapiazza is optimistic about using ultrasound as a way to treat other diseases too, such as cancer. In March 2014, neurosurgeons at the University Children’s Hospital Zurich used ultrasound to treat a brain tumor for the first time. Much more research will be needed to assess the efficacy of such a procedure, but the surgery was a milestone nonetheless.
“In the next five or 10 years, I think there are going to be a lot of breakthroughs of what we’re actually able to do with this technology,” says Dallapiazza. “It’s an exciting time.”