There are two main approaches for modeling the blood medium and its ultrasound
scattering characteristics. One approach models the blood as a large collection of
particle objects [53, 54]. The main advantage of this approach is that the principle of
superposition can be applied to sum the backscattered wavelets from each individual
RBC. Another approach models the blood as a random continuum, where the insonified
scattering volume is assumed to consist of many scattering RBCs, which together form
a continuum whose density ρ and compressibility κ change due to fluctuations in blood
cell concentration, causing the scattering of incoming ultrasound pressure waves [52,
55]. The two models can explain different properties known to exist for the scattering of
blood, but neither are consistent with measurements of the backscattering coefficient in
presence of phenomena such as turbulence, shear rate, and varying hematocrit [56, 57].
A unified approach where a hybrid of the two models have also been proposed to
provide a higher level of accuracy [58]. A more thorough review of the different models
proposed is also given here. There is a general agreement in both models, that the
scattering of ultrasound from blood can be described as a zero-mean Gaussian process
due to the large number of scattering red blood cells within an ultrasound resolution
cell. Considering the complex demodulated signal, a corresponding complex Gaussian
process is given.