Toward a velocity-resolved microvascular blood flow measure by decomposition of the laser Doppler spectrum.

Tissue microcirculation, as measured by laser Doppler flowmetry (LDF), comprises capillary, arterial, and venous blood flow. With the classical LDF approach, it has been impossible to differentiate between different vascular compartments. We suggest an alternative LDF algorithm that estimates at least three concentration measures of flowing red blood cells (RBCs), each associated with a predefined, physiologically relevant, absolute velocity in millimeters per second. As the RBC flow velocity depends on the dimension of the blood vessel, this approach might enable a microcirculatory flow differentiation. The LDF concentration estimates are derived by fitting predefined Monte Carlo simulated, single-velocity spectra to a measured, multiple-velocity LDF spectrum. Validation measurements, using both single- and double-tube flow phantoms perfused with a microsphere solution, show that it is possible to estimate velocity and concentration changes, and to differentiate between flows with different velocities. Our theory is also applied to RBC flow measurements. A Gegenbauer kernel phase function (alpha(gk)=1.05; g(gk)=0.93), with an anisotropy factor of 0.987 at 786 nm, is found suitable for modeling Doppler scattering by RBCs diluted in physiological saline. The method is developed for low concentrations of RBCs, but can in theory be extended to cover multiple Doppler scattering.