The effect of overlying tissue on NIR light propagation in neonatal brain

The effect of the layered surface tissue of the neonatal head on near infrared spectroscopy (NIRS) measurement of brain oxygenation has been investigated by Monte Carlo simulation. Four different cylindrical models have been adopted in order to investigate the effect of the clear cerebrospinal fluid (CSF) layer surrounding the brain on the light propagation. The simplest is a four layered model without a clear CSF layer whilst the most sophisticated model consists of five layers including the clear layer containing slots imitating the sulci in the brain. The fractional optical pathlength in each layer and spatial sensitivity profiles are predicted to examine the contribution of each layer to the NIRS signal. The clear CSF layer was found to affect the light propagation in the neonatal head, causing the light to follow the CSF boundary. This phenomena reduces the contribution of absorption changes in the white matter to the NIRS signal. However the contribution of the brain exceeds 75 % at an optode spacing of 50 mm in the neonatal head, a much greater contribution than observed for the adult brain. The spatial sensitivity around the sulci increases slightly, but the sulcus structure does not significantly affect the optical pathlength and general trend of the spatial sensitivity profile.