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Speckle Visibility Spectroscopy

Diffusing wave spectroscopy is a well-known set of methods to measure the temporal dynamics of samples. In DWS, dynamic samples scatter the incident coherent light, and the information of the temporal dynamics is encoded in the scattered light. To record and analyze the light signal, there exist two types of methods—temporal sampling methods and speckle ensemble methods. Temporal sampling methods, including diffuse correlation spectroscopy, use one or multiple large bandwidth detectors to sample well and analyze the temporal light signal to infer the sample temporal dynamics. Speckle ensemble methods, including speckle visibility spectroscopy, use a high-pixel-count camera sensor to capture a speckle pattern and use the speckle contrast to infer sample temporal dynamics. We theoretically and experimentally investigate the decorrelation time (τ) measurement accuracy or signal-to-noise ratio (SNR) of the two types of methods has a unified and similar fundamental expression based on the number of independent observables (NIO) and the photon flux.

Speckle visibility spectroscopy temporal (with a single detector) and spatial (camera) methods
Credit: https://doi.org/10.1063/5.0021988 - https://doi.org/10.1063/5.0034576

Given a time measurement duration, the NIO in temporal sampling methods is constrained by the measurement duration, while speckle ensemble methods can outperform by using simultaneous sampling channels to scale up the NIO significantly. In the case of optical brain monitoring, the interplay of these factors favors speckle ensemble methods. We illustrate that this important engineering consideration is consistent with the previous research on blood pulsatile flow measurements, where a speckle ensemble method operating at 100-fold lower photon flux than a conventional temporal sampling system can achieve a comparable SNR.

Speckle visibility spectroscopy temporal (photodetector) and spatial (camera) results
Credit: https://doi.org/10.1063/5.0034576

Diverse light scattering methods have been developed and employed to monitor human cerebral blood flow (CBF) in a non-invasive, harmful way. However, in most methods, if not all, the number of collected photons interacting with the brain is low when detecting blood flow in deep tissue. To tackle this photon starved problem, we investigate the idea of interferometric speckle visibility spectroscopy (ISVS). In ISVS, an interferometric detection scheme is used to boost the weak signal light. The blood flow dynamics are inferred from the speckle statistics of a single frame speckle pattern.

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