Simultaneous estimation of absolute concentrations of chromophores and the differential pathlength factor in forearm muscle using spectral derivatives

A method for estimating the absolute concentrations of chromophores in highly scattering tissues using near-infrared spectra is presented, which also yields estimates of the differential pathlength factor (DPF) and the power-dependency of scattering on wavelength. It involves measuring the attenuation spectral gradient and comparing it with an expression derived from diffusion theory. The validity of the approach is first explored using a diffusion model of light propagation in a homogenous slab, which is used to simulate measurements of diffuse reflectance in the adult forearm muscle during a vascular occlusion. Thereafter, the method is applied to experimental measurements performed on the forearms of five volunteers. The simulation results suggest that accuracy is significantly enhanced if some derived parameters are constrained to ranges which are physiologically realistic, and that the absolute concentrations of oxy- and deoxy-hemoglobin are estimated to within 40% and 20% of the true values, respectively. Furthermore, the wavelength-averaged DPF can be estimated to within around 10%. The measurements on volunteers revealed broadly consistent concentrations of the hemoglobins in the range 2-105 µM, and differential pathlength factors in the range 2.2-5.1.