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Infrared light scattering methods have been developed and employed tonon-invasively monitor human cerebral blood flow (CBF). However, the number ofreflected photons that interact with the brain is low when detecting blood flowin deep tissue. To tackle this photon-starved problem, we present anddemonstrate the idea of interferometric speckle visibility spectroscopy (ISVS).In ISVS, an interferometric detection scheme is used to boost the weak signallight. The blood flow dynamics are inferred from the speckle statistics of asingle frame speckle pattern. We experimentally demonstrated the improvement ofmeasurement fidelity by introducing interferometric detection when the signalphoton number is insufficient. We apply the ISVS system to monitor the humanCBF in situations where the light intensity is $\sim$100-fold less than that incommon diffuse correlation spectroscopy (DCS) implementations. Due to the largenumber of pixels ($\sim 2\times 10^5$) used to capture light in the ISVSsystem, we are able to collect a similar number of photons within one exposuretime as in normal DCS implementations. Our system operates at a sampling rateof 100 Hz. At the exposure time of 2 ms, the average signal photon electronnumber is $\sim$0.95 count/pixel, yielding a single pixel interferometricmeasurement signal-to-noise ratio (SNR) of $\sim$0.97. The total $\sim 2\times10^5$ pixels provide an expected overall SNR of 436. We successfullydemonstrate that the ISVS system is able to monitor the human brain pulsatileblood flow, as well as the blood flow change when a human subject is doing abreath holding task.

Interferometric speckle visibility spectroscopy (ISVS) for human cerebral blood flow monitoring