Finite element investigation of a void region on light propagation in scattering media using the radiative transfer equation

Abstract

Applications of the diffuse optical tomography to a thyroid cancer detection have been investigated by using the optical contrast. For the application, one needs to investigate effects of two factors (the diffusive reflection and refractive-index mismatch) at the tissue-trachea interface on light propagation in the human neck. In a previous work, the human neck was modeled as a two-dimensional circular heterogeneous medium consisting of a background medium and a circular void region, and the effects of the two factors at the medium-void interface in the simple heterogeneous medium were investigated using the time-dependent radiative transfer equation (RTE) in the time domain (TD). Nevertheless, the effects of the two factors in the continuous wave (CW) is still unclear because a crossover from the ballistic light to diffusive light in the CW differs from that in the TD. In this paper, we investigated the effects of the two factors using the steady-state RTE in the CW for the simple heterogeneous medium. The numerical results showed that the diffusive reflection dominated over the refractive-index mismatch at the medium-void interface even when the void region was located near the light source, where light is scattered anisotropically. The results in the CW were quite different from those in the TD, where the strong effects of the refractive-index mismatch were observed. The difference in the effects between the TD and CW is probably ascribed to a fact the refractive index influences in a shorter time scale compared to the time period for the average in the TD.Applications of the diffuse optical tomography to a thyroid cancer detection have been investigated by using the optical contrast. For the application, one needs to investigate effects of two factors (the diffusive reflection and refractive-index mismatch) at the tissue-trachea interface on light propagation in the human neck. In a previous work, the human neck was modeled as a two-dimensional circular heterogeneous medium consisting of a background medium and a circular void region, and the effects of the two factors at the medium-void interface in the simple heterogeneous medium were investigated using the time-dependent radiative transfer equation (RTE) in the time domain (TD). Nevertheless, the effects of the two factors in the continuous wave (CW) is still unclear because a crossover from the ballistic light to diffusive light in the CW differs from that in the TD. In this paper, we investigated the effects of the two factors using the steady-state RTE in the CW for the simple heterogeneous medium. T...