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

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 analog 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 required to outpace decorrelation while preserving TOF contrast. 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 .