Recording brain activity with laser light
A Caltech professor, working with researchers at the University of Southern California, has demonstrated for the first time a new technology for imaging the human brain using laser light and ultrasonic sound waves.
The technology, known as Computerized Photoacoustic Tomography, or PACT, was developed by Lihong Wang, Bren Professor of Medical Engineering and Electrical Engineering, as a method of imaging tissues and organs. Previous versions of PACT technology have been shown to be able to image the internal structures of a rat’s body; PACT is also able to detect tumors in human breasts, making it a possible alternative to mammograms.
Now Wang has made new improvements to the technology that make it so precise and sensitive that it can detect even tiny changes in the amount of blood flowing through very small blood vessels as well as the level of oxygenation in that blood. Since blood flow increases in specific areas of the brain during cognitive tasks – blood flow will increase to the visual cortex while you watch a movie, for example – a device that shows changes in blood concentration and oxygenation can help researchers and healthcare professionals monitor brain activity. This is called functional imaging.
“In breast imaging, you just want to see the blood vessels because they can reveal the presence of a tumor. [tumors secrete chemicals that stimulate blood vessel formation]Wang said. “But the functional change in imaged brain activity is only a few percent change in baseline signal. It’s more than an order of magnitude harder to measure. “
Previously, this type of imaging was performed only with functional magnetic resonance imaging (fMRI) machines, which use radio waves and magnetic fields 100,000 times stronger than the Earth’s magnetic field to monitor oxygen levels. in the blood. The machines work well and are mature technology, but they have some drawbacks. On the one hand, they are very expensive, costing up to a few million dollars each. Another disadvantage is that the strong magnetic fields created by the machine require special care, since objects containing iron like some medical tools, as well as surgical implants, can be pulled with great force by the machine. to be placed inside a narrow tube during imaging, which may be uncomfortable for people with claustrophobia.
In contrast, Wang’s technology is much simpler, inexpensive and compact, and does not require the patient to be placed inside the machine.
It works by projecting a pulse of laser light into the head. As the light passes through the scalp and skull, it is diffused into the brain and absorbed by hemoglobin molecules carrying oxygen in the patient’s red blood cells. The energy that hemoglobin molecules pick up from light causes them to vibrate ultrasonically. These vibrations return through the tissue and are picked up by an array of 1,024 tiny ultrasonic sensors placed around the outside of the head. The data from these sensors is then assembled by a computer algorithm into a 3D map of blood flow and oxygenation throughout the brain.
To test the technology in humans, Wang worked with Jonathan Russin, assistant professor of clinical neurological surgery at the Keck School and associate director of the USC Neurorestoration Center; Danny J Wang, professor at the USC Institute for Neuroimaging and Informatics; and Charles Liu, professor of clinical neurological surgery at the Keck School and director of the USC Neurorestoration Center.
After severe head trauma, some patients undergo decompressive hemicraniectomy, a life-saving procedure in which a large part of the skull is removed to control the pressure from swelling in the brain. Liu and Russin work with many such patients at the Rancho Los Amigos National Rehabilitation Center in Downey, Calif., Where Liu is chief of innovation and research. After recovering from an acute injury, but before skull reconstruction surgery, selected patients participated in this study to determine how imaging technology works.
“One obstacle that we still have to overcome is the skull,” Wang said. “It’s an acoustic lens, but it’s bad, so it also distorts our signal with attenuation. It’s like looking out through a wavy window,” he says. “But they have a population of patients who have had a hemicraniectomy. They’re missing part of their skull, so we can picture them.”
“Neuroimaging is central to the development of new treatment paradigms, and this demonstration is a very important step towards the development of a powerful new tool to complement current approaches such as MRI-based techniques,” said Russin .
Liu agrees, adding that “many of the most exciting therapeutic approaches for functional restoration involve neuromodulation strategies that cannot be studied in the MRI environment, and we look forward to using this new technology to better. understand and refine our treatments. this study may ultimately require new treatments, so it’s a great way to help develop a tool that will ultimately benefit them. “
To image a patient, the research team shaves his head (a step Wang says they’re trying to eliminate) so the laser light can shine on his scalp. The patient then lies down on a table with the head partially resting in a bowl containing the laser source, ultrasonic sensors and water. The water acts as a “mediator,” acoustically coupling the sensors to the scalp surface and allowing them to effectively pick up the signals, Wang explains. It is analogous to the gel that is placed on the skin when a patient has an ultrasound.
Going forward, Wang says research will need to focus on solving problems caused by hair and scalp. He said it might be possible to avoid shaving a patient’s head if optical fibers can be used to deliver the pulses of laser light between hair follicles on the scalp. And he also hopes to eventually use the technology on patients with intact skulls.
“We need a way to counter the distortion caused by the skull,” he says, adding that such a corrective “lens” will most likely be a more powerful data processing algorithm that can compensate for the distortion when it comes down to it. assemble a picture.
An article describing the technology, titled “Massively parallel functional photoacoustic CT scan of the human brain, appears in the May 31 issue of the journal Nature Biomedical Engineering. Caltech’s co-authors are Associate Postdoctoral Fellows Shuai Na and Li Lin (PhD ’20); Peng Hu, graduate student in medical engineering; Konstantin Maslov, member of the scientific staff of the Andrew and Peggy Cherng medical engineering department; Junhui Shi, former postdoctoral researcher now at Zhijiang Laboratories; and Xiaoyun Yuan, former visiting scholar, now at Tsinghua University in Beijing. USC co-authors are Jonathan Russin, Charles Y. Liu, Kay Jann, Lirong Yan, and Danny Wang.
Funding for the research was provided by the National Institutes of Health.