Theoretical and experimental investigation of the effect of sulci on light propagation in brain tissue

The effect of the layered surface tissues of the head on the measurement of brain oxygenation by near infrared spectroscopy (NIRS) has been investigated by both time-of-flight measurement and Monte Carlo simulation on models consisting of three or four separate layered homogeneous media. The clear cerebrospinal fluid (CSF) layer surrounding the brain has previously been shown to significantly affect light distribution, and in the head the brain surface is deeply folded with many CSF filled sulci. Therefore the most sophisticated model has four layers including a clear 'CSF' layer containing slots imitating sulci on the brain. Simpler models are also used and the light distributions in each are compared to examine the effect of the CSF layer. The mean optical pathlength of each model calculated from the temporal point spread function (TPSF) of the time-of-flight measurement agrees well with the Monte Carlo prediction. The fractional pathlength in each of the layers and the spatial sensitivity profile, which indicates the volume of tissue interrogated, are calculated by the Monte Carlo method. Results show that the light distribution in the head is highly affected by the existence of the clear CSF layer, and the optical pathlength and spatial sensitivity profile in the models with a clear layer are quite different from those without. However the presence of the sulci hardly affect the light distribution, the results from the sophisticated brain model with sulci being almost the same as those of the model with a simple CSF layer.