Ultrashort Laser Radiation Transfer in heterogeneous biological tissues

Abstract

Recent work has suggested that time-dependent reflectance and transmittance of light scattering can provide a valuable, non-invasive means to quantitatively probe tissue morphology. Radiation absorption has also been applied to applications in biomedical treatment such as laser surgery and photodynamic therapy. Fundamental to these laser applications is the determination of time-dependent light distribution in scattering-absorbing biological tissues. Modeling of time-dependent photon migration has traditionally been performed using either Monte Carlo (MC) method or deterministic methods based on diffusion approximation. The statistical MC method is time consuming and its results are subject to statistical errors. The diffusion theory presumes that the scattering predominate and the medium be optically diffuse, so that the angle-dependent radiant intensity is replaced by an angle-independent photon flux and the Boltzmann radiative transport equation is simplified as a diffusion equation. However, the diffuse approximation is hardly applicable to heterogeneous biological tissues with non-scattering or low-scattering regions, not to mention that experiments have shown that it fails to match experimental data when the tissue sample is not optically diffuse. The development of accurate simulation of photon migration in heterogeneous media is demanded and in press.In this work, time-dependent short-pulsed laser radiation transport in heterogeneous biological tissues is simulated using discrete ordinates method (DOM). Formulations for solving the transient radiation transfer equation are deduced for three-dimensional geometries. The sensitivity and accuracy of the DOM are examined. The false radiation propagation and numerical diffusion associated with the differencing schemes are discussed. Parametric studies are performed in order to check the effects of boundary conditions, inhomogeneity, multidimensionality, scattering albedo, etc. The characteristics of short-pulsed laser transport and its applications in biomedical diagnostics and treatment are demonstrated.