Effects of Different Optical Properties of Head Tissues on Near-Infrared Spectroscopy Using Monte Carlo Simulations

In near-infrared spectroscopy (NIRS), it is crucial to have an accurate and realistic model of photon transport in the adult head for obtaining accurate brain oxygenation values. There are several studies on the influence of thickness, the morphology of extracerebral layers, and source-detector distance on the sensitivity of NIRS to the brain. However, the optical properties of the different layers vary between different publications. How is the performance of NIRS affected when the real optical properties differ from the assumed ones?We aim to investigate the influence of variation in scattering and absorption in a five-layered head model (scalp, skull, CSF, grey and white matter). We performed Monte Carlo simulations focusing on a five-layered slab mesh. The range of optical properties is based on a review of the published literature. We assessed the effect on light propagation by measuring the difference in the mean partial path lengths, attenuation, and the number of the detected photons between the different optical properties performing Monte Carlo simulations. For changes in the reduced scattering, we found that the upper layers tend to have a negative impact. In contrast, changes in lower layers tend to impact the brain's influence metrics positively. Furthermore, for small source-detector distances, the relative percentage difference between lower and higher values is greater than larger distances. Conclusions: We conclude that the assumption of different optical properties has a substantial effect on the sensitivity to the brain. This means that it is important to determine the correct optical properties for NIRS measurements in vitro and in vivo.