The effect of shear waves in an elastic sphere on the radiation force from a quasi-Gaussian beam

Ultrasound beams are capable of exerting radiation force on scattering or absorbing obstacles. Previously, our team developed a technology to reposition kidney stones using this approach. It is convenient to study the influence of different parameters using a theoretical model based on a spherical shape stone and a quasi-Gaussian acoustic beam. In such an approach, only two geometrical parameters are involved, namely the beam width and the sphere diameter. The radiation force depends on their ratio, as well as on the elastic properties of the liquid and the stone. In this work, numerical modeling was performed to calculate the force acting on an elastic sphere using previously developed theory. Our numerical modeling indicates that the force on a stone is strongest when the beam is slightly wider than the stone. Also, the force created by a narrow beam appears to be the strongest when the beam is targeted to the side of the sphere. These peculiarities of the radiation force are explained by more effective generation of shear waves inside the stone resulting from their effective coupling with the acoustic waves in liquid at the stone edge.