New aspects in assessment of changes in width of subarachnoid space with near-infrared transillumination/backscattering sounding, part 1: Monte Carlo numerical modeling.

A modified Monte Carlo method was used for numerical modeling of the propagation of near-infrared radiation (NIR) within the anatomical layers of the human head. The distribution of NIR transmission between particular anatomical layers in the measurement region (frontal tubers) of the head was obtained. The study demonstrates the effect of the cardiac pump function-dependent changes in the width of the subarachnoid space (SAS) on the intensity of the backscattered radiation. It was proved that the influence of this factor increases with increasing distance between the observation point and the location of the NIR source placed on the surface of the head. Moreover, with sufficiently small NIR detector-source distance, the contribution of the optic radiation propagated within the SAS to the total signal received is negligibly low, which gives a basis for estimation of the modulatory influence of blood circulation within the superficial skin layer on the total intensity of the backscattered radiation. The dimensions of anatomical layers used in the study are real values measured in a female patient, in whom--due to unique circumstances--it was possible to make measurements followed by recordings in clinical conditions, a situation essential for verification of the results of numerical modeling.