Abstract: Radiation force from ultrasound has been used to excite monochromatic waves in tissues. The shear wave propagation speed is related to the shear elasticity and shear viscosity of the tissue. In previous researches

the thinking that shear wave propagation speed is a constant in isotropic media was widely used to estimate the viscoelastic modulus. Nevertheless

our research shows that shear wave propagation speed in isotropic media follows the fact that it’s not a constant but regularly vary with vibration phase of measuring point in shear direction

which appears sinusoidal change approximately with phase . Our work indicates that the variance of shear wave propagation speed is mainly caused by the change of local density with time. The modified formulas of shear wave propagation speed and density are presented. Finally

shear elasticity modulus is deduced respectively with constant speed and modified

which demonstrates the essential of modification and the error of shear elasticity modulus fall by more than 10%.