Brain functional imaging at small source-detector distances based on fast-gated Single-Photon Avalanche Diodes

In this work we focused on time-resolved measurements in diffusive media performed at small (few millimeters) source-detector distances in reflectance geometry. This configuration has been predicted to have better contrast, better spatial resolution, and lower noise than the typical measurements performed at few centimeters. In our instrumental set-up we exploited a fast-gating (rise-time < 400 ps) front-end electronics enabling a silicon Single-Photon Avalanche Diode (SPAD) for time-correlated single-photon counting. By means of this detector, we can acquire "late" photons of the diffused light collected 2 mm apart from the injection point. This is possible because the fast gated SPAD rejects the huge amount of "early" photons which otherwise would saturate the detection electronic chain. The time resolution of the set-up is 100 ps. The instrument has been validated on both homogeneous and inhomogeneous (high absorbing inclusion at different depths inside) tissue phantoms with different optical properties. We obtained diffused time-resolved curves with dynamic ranges of about 107. Moreover, we demonstrated good agreement between the measured time-resolved contrasts and those calculated by Monte Carlo numerical simulations. © 2009 SPIE.