CEE Seminar - Professor Bojan Guzina, University of Minnesota

The ¿Sixth Sense¿ of Ultrasound: Probing Nonlinear Elasticity With Acoustic Radiation Force This study deals with a sustained body force ¿ the so-called acoustic radiation force ¿ in tissue-like solids, generated by a focused ultrasound field subject to low-frequency modulation. This intermediate-asymptotics problem, which bears relevance to a number of emerging biomedical applications, is characterized by a number of small (but non-vanishing) parameters including the Mach number, the ratio between the modulation and ultrasound frequency, and the ratio of the shear to bulk modulus. On approximating the response of soft tissues as that of a nonlinear viscoelastic solid with heat conduction, the featured second-order problem is tackled via a scaling paradigm wherein the transverse coordinates are scaled by the width of the focal region, while the axial and temporal coordinate are each split into a ¿¿fast¿¿ and ¿¿slow¿¿ component. In this way the acoustic radiation force (ARF), giving rise to the mean tissue motion, is exacted by computing the ¿¿fast¿¿ time average of the germane field equations. A comparison with the existing theory reveals a number of features that are brought to light by the new formulation, including the contribution to the ARF of ultrasound modulation, as well as the fact that the ARF in (isotropic) tissue-like solids depends linearly on a particular third-order