Investigation of the effect of discrete scatterers in CSF layer on optical path length in the brain

Adequate modelling of light propagation in the complex and heterogeneous tissue of the human head is very important for quantitative near infrared spectroscopy and optical imaging. The presence of a clear and non-scattering CSF layer around the brain has been previously shown to strongly affect light propagation in the head. However the CSF layer is not totally filled with a non-scattering fluid and quite a few fine arachnoid trabeculae are actually present in the layer. In this study light propagation in an adult head model with discrete scatterers distributed within the CSF layer has been predicted by Monte Carlo simulation in order to investigate the effect of scattering caused by the arachnoid trabeculae in the CSF layer. Results show that the presence of the arachnoid trabeculae affect the total optical path length, a parameter which can be directly measured by time-resolved measurement. However, the partial optical path length in the brain tissue, which relates the sensitivity of the near infrared spectroscopy signal to absorption changes in the brain is strongly affected by the CSF layer even in the presence of the arachnoid trabeculae. The increased partial optical path length results from an increased lateral spreading of the NIR light within the grey matter of the cortex.