Given that the diffusive motion of RBCs dominates the decay of , Eq. (15) thus stands as the quantitative relationship between the blood flow index measured by DCS and the true absolute blood flow. Recall that in order to get , we first need to know the average optical properties of the tissue and , which would generally be obtained from an independent NIRS measurement31,32 and possibly also estimated with multidistance DCS measurements.30 To then convert the to , we need to know the proportionality between shear flow and the RBC diffusion coefficient, the reduced scattering coefficient of the blood , and the radius of the blood vessels. In this paper, we used a value of obtained from Goldsmith and Marlow.15 We note that this is a value obtained for a hematocrit of 40% to 47% and that was found to linearly increase with hematocrit from 0% to 45% and then to plateau and reverse.33 The reduced scattering coefficient of blood is linearly proportional to hematocrit from 0% to 40%, the highest value measured by Meinke et al.21 In principle, if one has an independent measure of hematocrit, from a blood draw for instance, one can then determine the appropriate value to use for and . The remaining factor needed to estimate is the vessel radius . Although we performed simulations using vessels with a common radius, in reality DCS will measure vessels with a distribution of radii. We anticipate that Eq. (15) will still be valid when measuring a distribution of vessel radii, but that the effective vessel radius will represent a complex nonlinear dependence on the distribution of vessel radii and the corresponding RBC speed distribution.