Optical Blood Flow Monitoring in Humans Using SNSPDs and High-Density SPAD Cameras.

Continuous and non-invasive monitoring of cerebral blood flow (CBF) is essential for managing acute brain injuries. Time-domain diffuse correlation spectroscopy (TD-DCS) enables depth-sensitive microvascular blood flow assessment using time- of-flight information in diffuse media like living tissue. This paper presents recent developments in TD-DCS using superconducting nanowire single-photon detectors (SNSPDs) and high-density single-photon avalanche diode (SPAD) arrays. We demonstrate improved signal-to-noise (SNR) and depth sensitivity at 1064 nm wavelengths using photon time-gating, and optimized instrument response functions. Experimental results from head-of-bed (HOB) and pressure modulation protocols validated the system's capability in isolating deeper cerebral signals. The additional assessment of SPAD-based detectors with preliminary hand grip and HOB protocols showed results complementing our CW-DCS system. This places the SPAD-based approach in a more advantageous position than traditional CW-DCS systems, especially in terms of SNR and scalability. With future enhancements such as fast time- gating and improved quantum efficiency, SPAD arrays can bridge the gap for implementing time-domain systems in clinical settings due to their sensitivity, compactness and cost-effective features. Similarly, SNSPD arrays can become a viable alternative or complement to SPAD arrays in TD-DCS, especially for deep-tissue imaging where performance near 1064 nm is desirable.