Time-resolved noninvasive optical parameters determination in three-dimensional biological tissue using finite difference time domain analysis with nonuniform grids for diffusion equations

The finite difference time domain (FDTD) analysis based on nonuniform grids for solving the diffusion equations in biological tissue has been proposed. It has been confirmed that the analysis greatly reduce the time for calculating time-resolved reflectance in three-dimensional scattering medium preserving the numerical accuracies. Based on the analysis, noninvasive optical parameters determination in three-dimensional inhomogeneous medium divided into 192 homogeneous cubic cell 10 (mm) in edge lengths has been studied. Chi-square fitting for theoretical and experimental time-resolved reflectance was optimized by the downhill simplex method. As a result, it has been become clear that absorption coefficients in all cubic cells in scattering medium are estimated within the accuracy of 10% by utilizing only time-resolved reflectance.

Time-resolved reflectance of an optical pulse from an adult head model utilizing the finite difference time domain (FDTD) analysis

Time-resolved reflectance of an optical pulse from a four-layered adult head model including cerebrospinal fluid (CSF) has been analyzed based on the finite difference time domain analysis (FDTD) which has been formulated by authors. It has been confirmed that the intensity of the reflected light and the mean time of flight dependences on the source-detector separations estimated from the time-resolved reflectance agree quite well with the previously reported experiments and calculations. The sensitivities for detecting optical property changes of gray and white matter beyond CSF in time-resolved reflectance have been evaluated and it has been become clear that they are enhanced compared with the sensitivities in the intensity of the reflected light and the mean time of flight. The derivatives of reflectance with respect to the absorption coefficients of gray and white matter have shown that the presence of CSF improves the capabilities of time-resolved reflectance for detecting optical property changes in the brain in actual noise limited photon detection systems.