Effect of local and global absorption perturbation in layered media on NIR signal

Near infrared light propagation in the adult head has been shown to be considerably affected by the inhomogeneity of tissue. This indicates that the contribution to the NIR signal by absorption perturbations in each layer of the head is different. In this study, the change in NIR signal caused by a local or global absorption perturbation in a particular layer of the head has been analysed by Monte Carlo prediction. A slab model of the adult human head consisting of five layers is used and the absorption coefficient in the whole region of a layer is changed for global absorption perturbations and in a small region of 10 mm by 10 mm by 5 mm rectangular solid for the local absorption perturbation. The differential pathlengths, which is partial derivative of the attenuation, were calculated from the differences in detected light intensity caused by the perturbations. Since the position of the local absorption perturbation with respect to the optodes affects the NIR signal, the absorption coefficient is changed in a local region either just below an optode or below the middle point of the optodes and the resulting attenuation in NIR signals are calculated.