Experimental evidence for anisotropic diffusion of light in white matter tissue

We report on the observation of anisotropic propagation of light through ex vivo white matter from a human brain sample. White matter comprises bundles of axons which exhibit preferential alignment directions in different regions. This is associated with an anisotropic response which, however, has not been characterized yet in the field of biomedical optics. We test this hypothesis experimentally by probing transverse propagation of light across two perpendicular directions in a post-mortem human brain cortex sample. Using recently derived solutions to the anisotropic diffusive equation, we characterize the scattering properties along both directions at a probe wavelength of 820 nm, well within the near-infrared window of biological tissue, with the aim to improve the diagnostic value of near-infrared studies of light transport through brain tissue, and other structurally anisotropic tissues in general.