Hybrid interferometric near infrared spectroscopy (hybrid iNIRS) enables time-of-flight–resolved non-invasive blood flow monitoring in humans in vivo

Abstract

Abstract Interferometric near-infrared spectroscopy (iNIRS) uniquely offers time-of-flight (TOF) resolution for depth-resolved optical property and blood-flow assessment in tissue, but single-mode collection constrains photon throughput and single-channel implementations impose stringent analogue bandwidth and digitization rates to resolve both TOF and speckle dynamics. Conversely, continuous-wave parallel interferometric NIRS (CW-πNIRS) boosts photon detection via spatial multiplexing on camera sensors, yet sacrifices TOF information and remains limited by camera readout. Here we reconcile these trade-offs with a hybrid swept-source, hybrid iNIRS platform that combines TOF encoding with multi-speckle, heterodyne detection. Liquid-phantom experiments map the trade-space among sweep speed and speckle decorrelation, indicating that sweep rates ≥ 10 kHz are sufficient to outpace decorrelation while preserving TOF contrast under controlled conditions. In contrast, in vivo measurements require higher sweep rates (typically 50–100 kHz) to accommodate faster physiological dynamics. In vivo forearm and forehead measurements demonstrate depth-resolved blood flow measurements during cuff occlusion. By simultaneously overcoming photon-starvation and electronic-bandwidth ceilings, this approach establishes a new operating regime for diffuse optical monitoring and provides a scalable hardware foundation for haemodynamic sensing in vivo.