Attenuation and Scattering of Light in the Brain

Within brain tissue, near-infrared photons encounter absorption by chromophores (hemoglobin, cytochrome oxidase) and elastic scattering by cellular membranes, organelles, and myelinated fibers. The combined effects are characterized by the absorption coefficient (μ_a) and reduced scattering coefficient (μ_s'), which depend on wavelength, tissue composition, and physiological state, critically affecting signal attenuation and spatial resolution.

Advanced fNIRS techniques employ time-domain and frequency-domain measurements to separately estimate μ_a and μ_s', allowing quantitative determination of hemoglobin concentration changes. Accurate modeling of light–tissue interactions underpins image reconstruction and enhances the reliability of hemodynamic mapping in functional brain studies.