In a first, scientists have used ultrasound waves to peer inside a person's brain. The man's brain activity was recorded as he completed tasks outside a medical facility, including playing a video game.
To achieve this feat, researchers implanted a material into the man's skull that allowed ultrasound waves to pass into his brain.
After entering through this "acoustically transparent" window, these waves bounced off boundaries between tissues. Some of the bouncing waves then returned to the ultrasound probe, which was connected to a scanner. The data allowed scientists to build a picture of what was going on in the man's brain, similar to how ultrasound scans can visualize a fetus in the womb.
The team monitored changes in blood volume in the brain over time, specifically zooming in on brain regions called the posterior parietal cortex and the motor cortex. Both of these regions help to coordinate movement.
Assessing changes in blood volume is one way to indirectly track the activity of brain cells. That's because when neurons are more active, they require more oxygen and nutrients, which are delivered by blood vessels.
The new study built upon prior research in nonhuman primates. Now working with a person, scientists were able to use ultrasound imaging to monitor the precise neural activity unfolding in a man's brain as he conducted various tasks, such as playing a simple connect-the-dots video game and strumming a guitar. The team described their findings in a paper published May 29 in the journal Science Translational Medicine.
"Just as had been the case with nonhuman primates, the patient's ultrasound data indicated intentions — move this joystick, strum this guitar — while the actions themselves were taken," Dr. Charles Liu, co-senior study author and a neurosurgeon at the University of Southern California, said in a statement.
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Functional ultrasound imaging — meaning ultrasound that tracks changes in blood volume in the brain — is considered a promising alternative to conventional brain imaging techniques, such as functional magnetic resonance imaging (fMRI). This is because it's thought to more sensitive to changes in brain activity. In addition, the resulting images have higher resolution, and the method doesn't require patients to lie still in a machine for extended periods of time in a hospital.
As such, you could theoretically track the brain activity of patients in real-life settings. This is currently possible with ambulatory EEG, but EEG tracks electrical activity, rather than blood flow, and it does so through the skin of the head and the skull, so it's not super precise.
Similarly, the human skull has historically been a barrier to ultrasound waves, preventing them from entering the brain. That's why, in the new study, Liu and colleagues overcame this obstacle by testing their approach in a patient who'd had some of his skull removed. This had been done to relieve pressure in his brain after a severe traumatic brain injury (TBI).
Normally, TBI patients who undergo this procedure are given a titanium mesh or custom-built implant to replace the missing part of their skull. In this case, the team built the acoustically transparent implant. In the future, the new technique may not be limited to patients with TBI, the study authors said.
"There are a number of interventions that require removal of a portion of the skull," Mikhail Shapiro, co-senior study author and a professor of chemical engineering and medical engineering at Caltech, said in the statement. "So, there are many patients who could potentially benefit from a cranial implant that is transparent to the acoustic signals that ultrasound uses."
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