Dynamic Elastography using Multi-directional Excitation on Prestressed Isotropic Phantoms

Dynamic elastography is an imaging technique that maps shear wave motion through material and has been applied in multiple modalities including optics, ultrasound, and magnetic resonance. Elastography offers a quantitative and non-invasive method of diagnosing and staging diseases where changes in viscoelastic muscle properties occurs(dystrophy, fibrosis, tumors). The mechanical properties of soft tissue are complex because they depend on both material and structure. It is important to consider the structure of the striated soft muscle tissue as anisotropic, incompressible, and heterogeneous. Current studies have shown that skeletal muscle exhibits a distinct mechanical response to active versus passive loading and neglect the effect of waveguides coupled with this loading. To better understand the effect of prestress and waveguides on mechanical wave motion, a novel experimental setup was made to introduce torsionally polarized waves into an isotropic phantom. Using two in-phase piezo actuators, a silicone phantom is rotated on one end and torsional waves propagate through the phantom. Weight is also added to the phantom to simulate a strain of 2.5, 5, 10, and 20%. Surface level wave motion was captured using Scanning Laser Doppler Vibrometry(SLDV). SLDV data is used to obtain the complex shear modulus as a function of prestress and excitation frequency, with transformation acousto-elastogrpahy(TAE) used to differentiate true shear modulus values from prestress effects. Finite element models were constructed to simulate the static and dynamic deformations being tested and compared to experimental results. Last, torsional excitation delivery and prestress decoupling were also tested using MRE. Experimental complex shear modulus values were acquired and corrected using TAE with promising results. Analysis of fractional Voigt model parameters identified a range of frequencies of interest for the torsional wave setup, and provides a good basis for exploration of higher order terms and complexities in material models.