Determination of the optical properties of anisotropic biological media using isotropic and anisotropic diffusion models

Abstract

The anisotropic light propagation in biological tissue is investigated in the steady-state and time domains. Monte Carlo simulations performed for tissue that has anisotropic optical properties show that the steady-state and time-resolved reflectance depends strongly on the measurement direction. We examined the determination of the optical properties using an isotropic diffusion model and found that in the time domain, in contrast to steady-state spatially-resolved reflectance measurements, the obtained absorption coefficient does not depend on the measurement direction and is close to the correct value. We performed measurements of the steady-state and time-resolved reflectance from porcine and bovine tendon which confirmed the theoretical findings. In addition, we compared the results obtained from Monte Carlo simulations with the solutions of the anisotropic diffusion theory for reflectance from semi-infinite media and for transmittance from slabs. In contrast to the literature, we found that the anisotropic diffusion equation is, in general, not a valid approximation to the anisotropic light propagation even in the diffusive regime.