Functional imaging of brain activity at small source-detector distances using fast-gated Single-Photon Avalanche Diodes

Functional near infrared spectroscopy (fNIRS) is an emerging tool to non-invasively monitor task-related hemodynamic changes in the human brain. We have proposed a novel approach to fNIRS exploiting time-resolved reflectance at a source-detector distance of few millimeters, which is predicted to have better performances than measurements realized at few centimeters. The implementation of this approach is not trivial due to the huge number of early photons that overwhelms the fewer late photons carrying information from the brain. In our instrument we exploited a fast-gating (<400ps) electronics to enable a silicon Single-Photon Avalanche Diode at a given delay after the laser pulse, and reconstructing the photon time distribution by using time-correlated single-photon counting. Measurements on phantoms demonstrated the capabilities of the technique to detect a local optical perturbation buried within an homogeneous medium with good contrast, good spatial resolution and attaining photon-timing resolutions of 100ps (FWHM) and photon-counting dynamic ranges over 8 decades. Furthermore, preliminary in-vivo results show the possibility to detect a cerebral activation in the motor cortex