Focused ultrasound works by directing sound waves toward a point in space. Individually, the waves are not powerful enough to affect the tissue, but when targeted, their collective intensity is much greater. High-intensity focused ultrasound (HIFU), which applies more intense sound waves, has been used to destroy tumors through heating, a process known as ablation.

When targeting the brain, though, Konofagou's team used much lower-intensity levels, similar to those applied in diagnostic ultrasound, the technology used during a pregnancy sonogram. While researchers don't know exactly how this technique is able to open the barrier, they say it's not through heating.

Unlike tumor ablation --and this distinction is key -- Konofagou's technique appears to be reversible. Using an MRI contrast agent, she was able to show that the barrier closed up after about four hours. This is important, explains Pierre Mourad, principal physicist and research associate professor in the department of neurological surgery at the University of Washington, because "the longer the blood-brain barrier is open, the longer you let nasty stuff in the brain."

"It's an exciting and very viable field," says Mourad. It is important to start applying this technique to animal models that simulate specific diseases, he says, just as Konofagou is doing, although he adds that the skulls of mice are extremely thin, unlike those of humans.

Konofagou says she's now working on using higher-frequency ultrasound waves, which she believes will be able to penetrate human skulls.