Wearable frequency-domain near-infrared spectroscopy (FD-NIRS) system

Frequency-domain near-infrared spectroscopy (FD-NIRS) is a noninvasive in vivo sensing and imaging technique that is used to quantify tissue composition and oxygen metabolism in the brain, muscle, and other tissues. However, the size and complexity of FD-NIRS instrumentation have largely limited its use to laboratory and clinical research settings. To expand the use of FD-NIRS into continuous monitoring applications and in naturalistic environments, we report a novel real-time multi-frequency (50-350 MHz) wearable FD-NIRS system based on a custom application-specific integrated circuit (ASIC). The wearable device includes 685 nm and 850 nm laser diodes and a silicon photomultiplier (SiPM) detector. The system has an optical property accuracy of 0.0007 mm^-1 for absorption and 0.08 mm^-1 for reduced scattering at a 14.7 Hz measurement rate, evaluated using tissue-simulating phantoms, and is capable of capturing single-frequency measurements up to 1 kHz with both wavelengths. Without a battery, it weighs 37 grams and measures less than 7 x 3 x 3 cm in size. For proof-of-concept, we demonstrate measurement of an arteriovenous occlusion of the human forearm. Overall, this work demonstrates the feasibility of quantitative FD-NIRS tissue optical spectroscopy for wearable health monitoring.