Experimental assessment of the CSF contribution to light propagation in the adult head

General equation for the differential pathlength factor of the frontal human head depending on wavelength and age

“…[33][34][35] For example, Okada et al 34 concluded that for a source-detector distance of 5 cm, the mean light propagation distance in the medium, i.e., d DPF, is composed of 65% contribution from the scalp and skull, 35% from the CSF, and 5% from the GM. However, later works demonstrated that the light-piping effect of the CSF is reduced due to the presence of scattering structures, i.e., arachnoid trabeculae, within the CSF, [36][37][38] reducing the influence of the CSF. To account for the mean pathlength in each tissue layer, the concept of the "partial differential pathlength" (also termed "partial optical path length"), i.e., the mean pathlength of the light in a specific layer, the "partial differential pathlength factor," and the "partial pathlength factor," were introduced.…”