  Elastography provides significant information on staging of fibrosis in
patients with liver disease and may be of some value in assessing steatosis.
However, there remain questions as to the role of steatosis and fibrosis as
cofactors influencing the viscoelastic measurements of the liver tissues,
particularly shear wave speed (SWS) and shear wave attenuation (SWA). In this
study, by employing the theory of composite elastic media as well as two
independent experimental measurements on fat-in-gelatin phantoms and also
finite element simulations, it is consistently shown that fat and fibrosis
jointly influence the SWS and SWA measurements. At a constant level of fat,
fibrosis stages can influence the SWA by factors of 2-4. Moreover, the rate of
increase in SWA with increasing fat is strongly influenced by the stages of
fibrosis; softer background cases (low fibrosis stages) have higher rate of SWA
increase with fat than those with stiffer moduli (higher fibrosis stages).
Meanwhile, SWS results are influenced by the presence of fat, however the
degree of variability is more subtle. The results indicate the importance of
jointly considering fat and fibrosis as contributors to SWS and SWA
measurements in complex liver tissues and in the design and interpretation of
clinical trials.
