Imaging of Shear Waves Induced by Lorentz Force in Soft Tissues

Abstract

This study presents the first observation of elastic shear waves generated in soft solids using a dynamic electromagnetic field. The first and second experiments of this study showed that Lorentz force can induce a displacement in a soft phantom and that this displacement was detectable by an ultrasound scanner using speckle-tracking algorithms. For a 100 mT magnetic field and a 10 ms, 100 mA peak-to-peak electrical burst, the displacement reached a magnitude of 1 µm. In the third experiment, we showed that Lorentz force can induce shear waves in a phantom. A physical model using electromagnetic and elasticity equations was proposed. Computer simulations were in good agreement with experimental results. The shear waves induced by Lorentz force were used in the last experiment to estimate the elasticity of a swine liver sample. The displacement of a conductor in a magnetic field induces eddy currents. Conversely, the application of an electrical current in a conductor placed in a magnetic field induces a displacement due to Lorentz force [1]. These two phenomena are currently investigated to produce medical images [2]. In the technique called Lorentz Force Electrical Impedance Tomography [3], also known as Magneto-Acoustical Electrical Tomography [4], an ultrasound beam is focused in a tissue placed in a magnetic field. The displacement of the tissue due to ultrasound in a magnetic field induces an electrical current. The current is measured using electrodes and has been used to produce tissue electrical conductivity interface images. In a "reverse" mode, injecting an electrical current in a tissue placed in a magnetic field induces a displacement due to Lorentz force. As in the megahertz range, shear waves decay over a few micrometers, the displacement propagates only through compression waves. These waves can be detected using ultrasound transducers to produce electrical conductivity images. One implementation of this method is called Magneto-Acoustic Tomography with Magnetic Induction [5].We hypothesized in this study that applying a low frequency (10-1000 Hz) electrical current through a tissue placed in a magnetic field would produce a shear wave within the medium. This could notably have applications in shear wave elastography [6], [7], [8], a medical imaging technique used to map the mechanical properties of biological tissues. The mechanical properties of biological tissues are known to be viscoelastic (hence frequency-dependent) [9], [10], [11], often anisotropic, e.g. along muscle fibers [12], and nonlinear (changing with pre-stress). However, in the field of medical imaging, most applications rely on a simple model, assuming an elastic isotropic linear solid. The viscoelasticity effect has been shown to have only second effect orders [13] and the synthetic phantoms as used in this study can reasonably be considered as fully isotropic and linear [14], [15]. Under these assumptions, tissue elasticity can be described by two parameters only, e.g. the shear modulus µ and Poisson's ratio. The shear modulus ...

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“…Yet, due to the much larger attenuation of shear wave than acoustic longitudinal wave in normal soft tissue (several orders of magnitude difference), sensitivity is an issue when measuring the shear wave directly. In a recent study (Grasland-Mongrain et al , 2014), Lorentz force induced tissue displacement on the order of 1 micrometer was successfully detected by ultrasound speckle tracking. This provides another possible path for imaging both the electrical and elastic properties of tissue using shear wave, which may be a potential research direction for MAT-MI.…”