Monte Carlo simulations of laser Doppler blood flow measurements in tissue

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Abstract
Light propagation in a model for blood perfusion in tissue was simulated with Monte Carlo calculations to investigate the dependence of the output of laser Doppler perfusion meters on the configuration of the optical probe and on the multiple scattering of photons by moving particles in the tissue. Laser Doppler perfusion meters registrating the first moment 〈ν〉 and the first weighted moment 〈ν〉s of the spectral power density S(ν) of intensity fluctuations on a detector viewing tissue illuminated by a laser are considered. The model was scaled up about a factor of 10 compared with real tissue, to make experimental tests possible. From the simulations of the Doppler scattering, it will be shown that the location of the effective probe volume of the perfusion meter can be extended to deeper layers in tissue by increasing the distance between the illuminating light beam and the detector. This opens the possibility to measure perfusion in skin layers as a function of the distance to the surface. Other calculations show how the degree of multiple scattering of individual photons by moving cells determines which flow parameter is measured with the perfusion meter. If the degree is low, the output of the meter depends linearly on the mean velocity of cells. For high degrees, a dependence on the root mean square value of this distribution is found. At a high moving particle concentration, multiple scattering by moving particles also results in deviations from the linear dependence of 〈ν〉 on the concentration of moving particles and in deviations from the concentration independence of 〈ν〉s. Intensity distributions of light inside the tissue model were obtained from the simulations.