Interferometry Enables Measurement of Brain Blood Flow, Activity

A spectroscopy technique developed at the University of California, Davis (UC Davis) is able to noninvasively measure brain blood flow and activity. The method, called functional interferometric diffusing wave spectroscopy (fiDWS), has the potential for use in assessing brain injuries and in neuroscience research. It also promises to be cheaper than existing technology, which cannot be applied continuously or at the bedside.

The method harnesses near-infrared light’s ability to penetrate through body tissue; if one shines a near-infrared laser on a person’s forehead, the light will be scattered many times by the tissue, including blood cells. By picking up the fluctuation signal of the light that finds its way back out of the skull and scalp, it is possible to ascertain information about blood flow within the brain.

A volunteer tests the functional interferometric diffusing wave spectroscopy method, which is used to detect blood flow and activity within the brain. Courtesy of Wenjun Zhou, UC Davis.

Such a signal is weak, however, and the team of Vivek Srinivasan, adjunct professor of biomedical engineering at UC Davis and senior author of a study describing the method, and postdoctoral researcher Wenjun Zhou, used interferometry to overcome the limitation. Interferometry, the ability of lightwaves to superimpose, reinforce, or cancel one another out, enables a strong lightwave to boost a weak lightwave by bolstering its detected energy.

The researchers’ method first splits a laser beam into “sample” and “reference” paths. The sample beam is directed into the patient’s head, and the reference beam is routed in such a way that it reconnects with the sample beam before going to a detector. Through interferometry, the stronger reference beam boosts the weak sample signal. This allowed the team to measure the output with a CMOS sensor rather than an expensive photon detector. The team then used software to calculate a blood flow index for different locations within the brain.

Tests found that blood flow could be measured more rapidly and deeper below the surface than with current light-based technology for the same purpose. Imaging cerebral blood flow is typically done with MRI or computed tomography, both of which are expensive and neither of which is portable. Light-based technologies exist, such as near-infrared spectroscopy, but they lack accuracy. The researchers were able to measure pulsating cerebral blood flow and could detect changes when volunteers were given a mild increase in carbon dioxide. When the volunteers were given a simple math problem, the researchers could measure activation of the prefrontal cortex through the forehead.

The research was published in Science Advances (www.doi.org/10.1126/sciadv.abe0150).