Effect of incident light intensity and source-detector separation on photon migration depth in turbid media

Abstract

Diffuse optical tomography (DOT) and near-infrared spectroscopy (NIRS) aretechniques that suffer from an uncertainty of photon migration pathlength which isimportant for information of the tissue depth that is probed. We have investigated the hypothesis that probing depth is a function of the incident light power and source-detectordistance. The hypothesis is tested both numerically by Photon Migration Imaging (PMI)Toolbox (finite-element model diffusion approximation to the radiative transfer equation)and experimentally using continuous wave coherent light. Increasing the light power simultaneously increases the measured photon fluence, more importantly its effects ondistribution of photon density can be seen by forming perturbations in the media.Simulations and experiments showed that it is possible to detect the presence of a layerwith a higher absorption coefficient than the upper layer and its depth using the fact that different source - detector pairs̕ diffuse photons have different depths of most probablepath of migration. We also showed that a change in the photon distribution with increasingintensity of incident light has no virtual contribution to this derivation.|Keywords: Light propagation in tissue, diffuse optical tomography, photon migration and probe geometry.