Monte Carlo simulation of near-infrared light propagation in a realistic, three-dimensional human head and brain model

A software, based on the Monte Carlo technique, able to simulate near infrared photon propagation within a three dimensional human head and brain model obtained by MRI images, is described.
Near-InfraRed Spectroscopy (NIRS) can be employed to monitor continuously and non-invasively local changes in haemodynamics and oxygenation of human tissues. In particular this technique can be useful to measure brain activity from haemodynamic changes consequent to local oxygen consumption.
In order to identify the spatial distribution of the tissues interrogated by the NIRS analysis and the sensitivities of the detected NIRS signals to the brain activation, it is important to determine the photon path inside the tissue. The described software is able to convert a whole stack of MRI images of a human head in a three-dimensional map of optical properties of the various tissues. Then, with the Monte Carlo technique, it simulates propagation and detection of near infrared light in the whole volume. Each photon is followed in its trajectory within the tissue, taking into account scattering and absorption processes occurring along its path. External boundaries are considered too: the software is able to identify outlines of the head image, and when photons exit such outlines, to account for reflection and refraction from the surface.
Results can be displayed by a custom interface, or they can be converted in a format compatible with the most common MRI image viewers.