Magnetic resonance elastography (MRE) is a novel experimental technique for probing the dynamic shear modulus of soft biological tissue non-invasively and in vivo. MRE utilizes a standard MRI scanner to acquire images of propagating shear waves through a specimen that is subject to external harmonic mechanical actuation; commonly at frequencies in excess of 200Hz. At steady state, the wavelength of the propagating shear wave can be used to estimate the shear modulus of the tissue. Dynamic shear testing (DST) is also used to characterize soft biomaterials. Thin samples of the material are subject to oscillatory shear strains. Shear force is measured, and converted to shear stress — analysis of this data of a range of frequencies gives a complex shear modulus. The data analysis method assumes that the shear displacement is linear and shear strain is constant through the thickness of the sample. In soft tissues, very thin samples are typically used to avoid inertial effects at higher frequencies. As the thickness of the sample decreases, it is more difficult to cut samples of uniform thickness and to maintain structural integrity of the sample. Thus in practice, measurements of brain tissue properties using DST without inertial correction are limited to low frequencies. In this work, we bridge the frequency regimes of DST and MRE by testing thick samples using DST over a range of frequencies that generates a shear wave in the sample, with a corresponding peak in the measured shear force. The frequency and magnitude of this peak give additional information about the complex shear modulus of the material being tested, and these DST results are interpreted using a finite element (FE) model of the sample. Using this method, we can obtain an estimate of shear modulus in an intermediate frequency regime between that of standard DST and MRE.
Viscoelastic properties of soft gels: comparison of magnetic resonance elastography and dynamic shear testing in the shear wave regime