Step aside, huge magnets and radioactive tracers—soon some brain activity will be revealed by simply training dozens of red lights on the scalp. A new study in Nature Photonics finds this optical technique can replicate functional MRI experiments, and it is more comfortable, more portable and less expensive.
The method is an enhancement of diffuse optical tomography (DOT), in which a device shines tiny points of red light at a subject's scalp and analyzes the light that bounces back. The red light reflects off red hemoglobin in the blood but does not interact as much with tissues of other colors, which allows researchers to recover an fMRI-like image of changing blood flow in the brain at work. For years researchers attempting to use DOT have been limited by the difficulty of packing many heavy light sources and detectors into the small area around the head. They also needed better techniques for analyzing the flood of data that the detectors collected.
Now researchers at Washington University in St. Louis and the University of Birmingham in England report they have solved those problems and made the first high-density DOT (HD-DOT) brain scans. The team first engineered a “double halo” structure to support the weight of 96 lights and 92 detectors, more than double the number in earlier arrays. The investigators also dealt with the computing challenges associated with that many lights—for example, they figured out how to filter out interference from blood flow in the scalp and other tissues. The team then used HD-DOT to successfully replicate fMRI studies of vision and language processing—a task impossible for other fMRI alternatives, such as functional near-infrared spectroscopy or electroencephalography, which do not cover a large enough swath of the brain or have sufficient resolution to pinpoint active brain areas. Finally, the team scanned the brains of people who have implanted electrodes for Parkinson's disease—something fMRI can never do because the machine generates electromagnetic waves that can destroy electronic devices such as pacemakers.
Although HD-DOT can penetrate only about two centimeters, which means the method will never totally replace fMRI, commercial versions will cost about a tenth as much and be more portable, says lead author Adam Eggebrecht, a physicist at the Washington University School of Medicine. And that two-centimeter depth is enough to investigate many of the brain's higher-order cognitive functions, which largely take place in the cerebral cortex, the outer folds of the brain. Researchers are already using HD-DOT to study brain function in Parkinson's patients and children with autism, Eggebrecht says.