Functional diffuse reflectance spectroscopy at small source-detector distances based on fast-gated single-photon avalanche diodes

In this review we present the instrumental and theoretical developments for functional diffuse reflectance spectroscopy at small source-detector distances. We proposed the possibility to perform photon migration measurements at null or small inter-fiber distances demonstrating the improvement of this novel approach in terms of achievable contrast, spatial resolution and number of detected photons. We developed a novel system to perform time-resolved diffuse reflectance measurement at small source detector separation based on a single photon avalanche photodiode (SPAD) operated in fast time gated mode and a broadband fiber laser. By means of time gating it is possible to detect longer lived photons neglecting initial ones. We show results both on homogeneous and inhomogeneous tissue phantoms demonstrating a dynamic range of 7 orders of magnitude and a temporal range of 6 nanoseconds. Furthermore, this approach proved valuable to detect brain activity.