Shear waves generated with magnetomotive force on an embedded sphere

The characterization of the material properties of tissue and tissue mimicking materials has been is an important area of study to develop methods for elasticity imaging. Many methods utilize acoustic radiation force to produce shear waves and to extract elastic or viscoelastic properties of the medium by measuring the propagation of the shear waves. However, the push beam geometry used to generate the shear waves can induce motion that may cause bias for conventional processing methods. Towards the aim of characterizing a medium with a simpler excitation distribution, we propose the use of magnetomotive force on an embedded metallic sphere to generate shear waves. The shear wave motion can be compared with an analytic Green's function for a spherical source. A gelatin phantom with an embedded steel sphere of 2 mm in diameter was placed above an electromagnet with diameter of 101.6 mm. A direct current (DC) voltage was applied for 20 ms to drag the sphere and the motion after the cessation of the excitation pulse was measured. A linear array transducer configured to perform compound plane wave imaging at a frame rate of 3.8 kHz was utilized to measure the shear wave motion. A Green's function simulation was used to compare with the motion induced in the experimental setting. The Green's function and the experimental results are in good agreement. Magnetomotive-generated shear waves could provide a unique tool to characterize material properties without the complications of an acoustic radiation force push beam.