Noninvasive Method for Measuring Brain Blood Flow Developed

A noninvasive method for measuring brain blood flow with light has been developed by researchers from the University of California, Davis, and used to detect brain activation. The new method, functional interferometric diffusing wave spectroscopy, or fiDWS, could be used for assessing brain injuries, or in neuroscience research.

"Now we can assess how well the brain regulates blood flow, and even detect brain activation noninvasively in adult humans, using principles similar to functional magnetic resonance imaging (fMRI), but at a fraction of the cost," said Vivek Srinivasan, senior author on the study published yesterday in Science Advances.

Measuring cerebral blood flow is important for diagnosing strokes, and for predicting secondary damage in subarachnoid hemorrhages or traumatic brain injuries. Existing technology is expensive and cannot be applied continuously or at the bedside. For example, current techniques to image cerebral blood flow require expensive MRI or computed tomography scanners. There are light-based technologies, such as near-infrared spectroscopy, but these also have drawbacks in accuracy.

The new method takes advantage of the fact that near-infrared light can penetrate through body tissues. If you shine a near-infrared laser on someone's forehead, the light will be scattered many times by tissue, including blood cells. By picking up the fluctuation signal of the light that finds its way back out of the skull and scalp, you can get information about blood flow inside the brain. Naturally, that signal is extremely weak. The team overcame that problem by making use of interferometry through which a strong light wave can boost a weak light wave by increasing its detected energy.

They first split the laser beam into "sample" and "reference" paths. The sample beam goes into the patient's head and the reference beam is routed so that it reconnects with the sample beam before going to the detector. Through interferometry, the stronger reference beam boosts the weak sample signal. This allowed the team to measure the output with the type of light-detecting chip found in digital cameras, instead of expensive photon counting detectors. They then use software to calculate a blood flow index for different locations in the brain.