A time-domain NIR brain imager applied in functional stimulation experiments

Abstract

We developed a time-domain brain imager that is based on picosecond diode lasers, a multimode fiber switch and multi-channel time-correlated single photon counting. It allows to record time-resolved diffuse reflectance for 16 source-detector pairs within typically 1 s. Data analysis was based on the evaluation of moments of measured distributions of times of flight of photons. To show the relevance of these moments for achieving depth selectivity, three-dimensional sensitivities of integral, mean time of flight and variance to absorption changes were calculated using a perturbation approach based on the diffusion equation for photon density for a homogeneous semi-infinite medium. It turned out that variance is almost exclusively sensitive to deep layers, whereas the integral reflects changes in deep as well as in superficial layers. The lateral resolution of the imager was demonstrated by a phantom experiment. Results of a motor stimulation experiment on a healthy volunteer strongly suggest that variance reveals mainly the cerebral activation whereas the integral may additionally contain significant systemic contributions.