Ultrasonic shear wave propagation in soft tissues for microelastography

Microelastography is an imaging technique capable of mapping the biomechanical properties of soft tissues on a microscopic scale. Its higher resolution, compared to traditional elastography, allows researchers and physicians to observe disease-related mechanical changes with far more precision.

To better understand the underlying physics of microelastography, Laloy-Borgna et al. explored the effects of high-frequency shear waves on gel phantoms that mimicked single cells. They demonstrated for the first time that ultrasonic shear waves can propagate in soft solids, despite the assumption that shear waves in tissue fade out before the ultrasonic regime due to viscoelastic dissipation.

The experimental setup consisted of an ultrafast camera, an optical microscope, and a piezoelectric transducer that induced shear waves in the sample. The researchers recorded movies of the phantoms vibrating, ran them through a phase-tracking algorithm, and plotted a dispersion curve of phase velocity as a function of wave frequency.

They identified three different areas in the dispersion curve, depending on the frequency range. At lower frequencies, the velocity increases quickly with frequency, and Lamb waves are guided by the geometry of the sample. As the frequency increases, bulk shear waves that obey the Voigt model take over. At a certain point, an evanescent wave region appears, where phase velocity can no longer be measured.

The transition between shear waves and evanescent waves represents a cutoff frequency above where shear waves no longer propagate, which can be predicted using only the elasticity and viscosity of the medium. Being able to now calculate this value allows researchers in microelastography to choose an ideal frequency to work with for a particular biological tissue application.

Source: “Micro-elastography: Toward ultrasonic shear waves in soft solids,” by G. Laloy-Borgna, A. Zorgani, and S. Catheline, Applied Physics Letters (2021). The article can be accessed at http://doi.org/10.1063/5.0039816 .