Image reconstruction of local shear and viscosity moduli for a finite-amplitude acoustic modulated radiation force

Localized narrowband low-frequency shear waves can be non-invasively generated within tissue, by a modulated finite-amplitude radiation force, resulting from the interference of two focused quasi-CW ultrasound beams of slightly different frequencies. Assuming a Voigt viscoelastic model, this paper describes the use of a finite-element-method (FEM) model, to simulate two-dimensional shear wave propagation in viscoelastic media, containing circular inclusions (lesions). Using this model an inverse approach is used to extract maps of the local shear modulus and viscosity. The performance is evaluated based on two modified quality metrics commonly used in elastography: the lesion contrast and the contrast-transfer-efficiency (CTE). It is shown that accurate reconstruction can be achieved not only with the fundamental, but also with the higher harmonics, as well as, with a low-frequency component that occurs for high viscosity and high modulation frequencies.