Imaging of shear waves induced by Lorentz force in soft solids

One of the main challenge in shear wave elastography is the shear wave source. Most of nowadays techniques are using external vibrator or acoustic radiation force. Otherwise, by applying an electrical current in a conductor submitted to a magnetic field, a displacement is induced by Lorentz force. This displacement is propagating as a combination of compressional and shear waves. In this study, we observed for the first time shear waves induced by Lorentz force in a tissue-mimicking phantom. In our experiment, a 10 ms long, 80 V peak-to-peak electrical signal was applied with two electrodes in a gelatin phantom placed in a 300 mT magnetic field. The phantom has an electrical conductivity of 1 S/m, close to the human soft tissue conductivity. An ultrasonic probe was acquiring an image in ultrafast mode at 1000 frames per second with a Verasonics ultrasound scanner. Displacements in the phantom were measured with a speckle-tracking technique based on correlations between each image. Results showed that a displacement of a few micrometers is induced by Lorentz force which results in a shear wave propagation at a speed of 1.4 ± 0.1 m/s. These results could lead to a shear wave elastography technique based on the combination of electrical current and magnetic field.