Absorption and scattering of laser light in biological media--Mathematical modeling and methods for determining the optical properties.

摘要

Laser light interaction with biological media is studied. First, a mathematical model for light scattering in inhomogeneous media is developed. Second, methods for determining the optical properties of tissue such as absorption and scattering coefficients and the phase function are introduced. The proposed seven flux model approximates light scattering using seven directional fluxes. An analytic solution for a one dimensional geometry and numerical solutions for higher dimensional geometries are presented. The behavior of the seven flux model is examined by correlating it with more accurate solutions of radiative transfer theory. Predicted reflection and transmission are accurate. However, the fluence rates at the front surface are higher, but become consistent with the solutions of radiative transfer theory beyond about one optical depth. The proposed model provides improved results compared with the currently-used models such as Kubelka-Munk theory and the diffusion approximation, and as well as allowing geometrical flexibilities which are difficult to achieve with more accurate solutions. Next, new methods are developed to determine optical properties. Absorption and scattering coefficients are determined based on transmission measurements for isotropic scattering. Additional measurements of the phase function and collimated attenuation are required for anisotropic scattering. Measurements with the gel samples and the human aortic walls at the wavelength of 633 nm show highly forward scattering behaviors. A conventional isotropic scattering assumption should be carefully applied. Since the proposed methods require simple procedures compared with the methods based on both reflection and transmission, experimental errors can be reduced. Furthermore, the problem of the internal reflections is decoupled from optical property evaluation.