Development of a technique to measure local scattering in turbid media using backscattered light at the surface for noninvasive turbidity evaluation of blood in subcutaneous blood vessels

Abstract

For noninvasive skin-surface evaluation of turbidity in subcutaneous blood vessels we have developed a technique to estimate the reduced scattering coefficient from spatially resolved backscattered light. The solution of the diffusion approximation was used to derive an analytical solution for the effective attenuation coefficient as a function of the spatially resolved reflectance with respect to the source-detector distance. The reduced scattering coefficient can be calculated from the effective attenuation coefficient. This represents the blood turbidity or serum triglyceride concentration. An exact solution for the reduced scattering coefficient was newly obtained as a function of the effective attenuation coefficient using the special diffusion coefficient, which expands the applicability of the diffusion approximation to the case of human blood. To eliminate the effects of strong scattering in the surrounding tissue we introduced a differential principle using spatially resolved reflectance measured at positions on and off the blood vessel. The results of Monte Carlo simulation demonstrate the validity of the proposed technique even in the case of blood, which does not necessarily satisfy the conditions of the diffusion approximation. The small dependence on absorption variation in the practical range and robustness against the measurement error were verified. With the differential principle we can estimate blood turbidity by suppressing the effect of the surrounding tissue. With this technique, one can expect more than 50 times higher sensitivity for blood turbidity than that obtained without using this principle. The validity of the simulation and the applicability of the proposed technique were verified with measurements using a model phantom of subcutaneous blood vessels in a tissue-simulating turbid medium.