NIRS signals measured on the adult head contain contributions from the brain and from overlying tissue. It was shown recently that measured distributions of times of flight (DTOF) of photons allow to deduce absorption changes occurring in different layers of the head. This method relies on time-dependent mean partial pathlengths calculated by Monte Carlo simulations for assumed background optical properties of the various tissues. Deconvolution of the measured DTOF is required using the instrumental response function. We propose an alternative method to estimate absorption changes in various tissue layers by analyzing changes of moments of DTOFs (integral, mean time of flight and variance) recorded at various source-detector separations. The sensitivity factors corresponding to integral, mean time of flight and variance were obtained by Monte Carlo simulations for a layered model of the head. From experimentally derived mean time of flight and variance the contributions of the instrumental response function were subtracted. The proposed method was applied to multi-distance time-domain measurements during functional stimulation of the brain of healthy volunteers.
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