Viscoelastic characterization of soft tissues by Dynamic Micro-Elastography (DME) in the frequency range of 300–1500 Hz

Mechanical characterization of living tissues and organs are of interest because information on their viscoelastic properties in the presence of diseases can affect therapy planning. This article proposes the Dynamic Micro-Elastography (DME) method to characterize elasticity and viscosity parameters from the acoustical properties (velocity and attenuation) of monochromatic and transient shear waves in a large frequency range (300 – 1500 Hz). To overcome spatial and temporal limitations of conventional systems, we used a high frequency transducer (25 MHz) and a shear wave gated strategy to reconstruct ultra fast RF frame sequences (16000 images per second). Viscoelasticity of agar-gelatin materials, porcine blood clots and porcine liver samples were investigated. As previously observed in the literature at lower frequencies, the liver tissue appeared highly viscous between 300 – 1500 Hz, whereas the gel and blood clot presented constant elasticity values over that range of frequency. A second experiment undertaken on a small animal organ (rat liver) proved that such high frequency waves, tracked with our high resolution system, permit to study its mechanical properties. To conclude, this characterization tool is adequate to investigate soft tissue rheological behavior evolution as a function of frequency. Moreover, because wavelengths of propagating shear waves are very small (≪ 2 mm at 1500 Hz) and the motion tracking system very accurate, DME could also be applied to characterize millimetric organs.