| The determination of the optical properties of turbid biological media is of primary importance in several areas of medicine and biotechnology. In particular, the quantitative determination and spatial localization of the optical scattering and absorbing properties of biological tissue would allow for the non-invasive and non-ionizing imaging of tissue structure and the monitoring of physiology. Until recently, such characterization of thick, highly scattering biological tissues using visible and near infrared light has been thwarted because of the inability to determine the absolute optical properties of thick tissues.; This thesis presents the development of the concept, physical model, and experimental study of diffuse photon density waves in thick turbid media. The goal of this work is to determine the applicability of photon density waves to the optical tomography and spectroscopy of thick, multiply scattering media. Toward this end, analytic expressions based on the diffusion approximation to the Boltzmann transport equation are derived for the case of an isotropically emitting, sinusoidally intensity-modulated point source of light immersed in an infinite, macroscopically uniform, multiply scattering medium. These frequency-domain expressions are given in terms of the optical properties of the medium, and they predict that the photon density propagates outward from the light source as a spherical wave of constant phase velocity. Experiments are performed which support the validity of these frequency-domain expressions, and provide a basis for the understanding of photon transport in turbid media containing absorbing and/or reflecting objects. Further experiments demonstrate the feasibility of using frequency-domain data in conjunction with a frequency-domain diffusion model to determine the absolute optical parameters of thick, multiply scattering media. |
