A new technique for measuring blood flow in the human brain is based on conventional digital camera technology. The method could be significantly cheaper and more robust than prior approaches and is envisioned for use in patients with stroke or traumatic brain injury.
Shining a light into a cloudy solution causes light particles, or photons, to scatter in different directions. The diffuse correlation spectroscopy (DCS) technique uses essentially this approach to look inside someone’s skull. Laser light is shined on the head, and as photons from the laser pass through the skull and brain, they are scattered by blood and tissue. A detector placed elsewhere on the head, where the photons make their way out again, picks up the light fluctuations due to blood motion. These fluctuations provide information about blood flow.
The light signal is very weak, and becomes even weaker the further it passes through the skull and brain tissue. DCS therefore requires a number of very sensitive, expensive single photon counting detectors. Boosting the light going in also risks burning the patient’s skin.
Biomedical engineers from University of California, Davis sought to improve the process by exploiting the fact that overlapping light waves will reinforce or cancel each other out, like overlapping ripples on a pond. Their method involves splitting the light beam into a sample path which goes into the patient’s head and another, stronger, reference beam that is routed to reconnect with the sample beam before going to the detector. This boosts the signal, meaning that instead of needing about 20 photon-counting detectors that cost a few thousand dollars each, the researchers could use a single CMOS-based digital camera chip for a fraction of the price.
The researchers explain that the strong reference light enhances the weaker signal from the sample, and call the method interferometric diffusing wave spectroscopy (iDWS). An added advantage is that they do not need to turn off the room lights while making measurements with iDWS,
The research is published in the journal Optica.