Acoustic radiation force and streaming induced by focused nonlinear ultrasound in a dissipative medium

Based on asymptotic methods recently developed in nonlinear acoustics, analytical solutions of the equations for the radiation force induced by nonlinear focused ultrasound in a dissipative medium are considered. Equations describing spatial structure of the radiation force field in the paraxial region of the ultrasound beam and the spatial–temporal behavior of the induced nonlinear streaming are derived. The equations enable analytical investigation of dependencies of radiation pressure and resulting streaming on the acoustic field and medium parameters. Estimates have shown that nonlinearity of medium can significantly enhance radiation force in the focal region at the intensities lower than those used in ultrasound devices for medical imaging. The initiation of acoustical streaming by radiation force is considered, and the spatial and temporal characteristics of induced flow are discussed. Both acoustic and hydrodynamic nonlinearities are taken into account. The paper concludes by discussing possible medical and industrial applications of ultrasound radiation force and induced acoustical streaming.