Influence of extracerebral layers on estimates of optical properties with continuous wave near infrared spectroscopy: analysis based on multi-layered brain tissue architecture and Monte Carlo simulation.

Continuous wave near-infrared spectroscopy (CW-NIRS) can be used to measure cerebral activity because it is noninvasive, simple and portable. However, the performance of the CW-NIRS is distorted by the presence of extracerebral layer. Change of optical parameters in gray matter layer will then be inappropriately converted into the brain activity response. In the current study, a five-layer structure model constitute of scalp, skull, cerebrospinal fluid, gray matter and white matter is adopted and the mixture of the Intralipid, India ink and agar is applied to fabricate human brain tissue. To simulate optical properties in deep layer due to the brain activity, the absorption coefficients of gray matter are increased by 5%, 10%, 15%, 20%, and 25% relative to the baseline. The NIRS measurement system was designed to detect the changes in the absorption coefficients of the gray matter and quantitatively analyse the influence of the extracerebral layers. Monte Carlo technique is performed to compensate partial volume effect (PVE) introduced by the extracerebral layers. The results of the in-vitro experiments show that the measured absorption coefficients are about 9% of the standard value and the relative error is about 91% due to the extracerebral layers. The influence of the extracerebral layers is suppressed by correcting PVE with Monte Carlo simulations and the average relative error is improved to only about 6% for the whole data set. Therefore, the measurement and analysis of the brain activity could be further strengthened if the anatomic structure of the head could be predicted with Monte Carlo method or other technologies.