Time-resolved near-infrared spectroscopy and imaging of the adult human brain.

Near-infrared spectroscopy (NIRS) of the human brain is aiming at the non-invasive determination of concentration changes of oxy- and deoxyhemoglobin in the cortex. However, it usually relies on the assumption of spatially homogeneous absorption changes. To overcome this limitation we performed instrumental and methodological developments of time-resolved NIRS with the aim to achieve depth resolution. We present our recently developed time-domain near-infrared brain imager based on picosecond diode lasers and time-correlated single photon counting (TCSPC) which can be used at the bedside. To achieve depth localization of absorption changes we analysed statistical moments (integral, mean time of flight and variance) of measured time-of-flight distributions of diffusely reflected photons. In particular, variance has a selective sensitivity to deep absorptions changes and provides a suitable representation of cerebral signals. The separation of cerebral and extracerebral changes of hemoglobin concentrations is demonstrated for a motor stimulation experiment.