Characterising the large strain mechanical properties of soft tissues is challenging due to their complex nonlinear behaviour and various experimental limitations. Oscillatory shear experiments are commonly used to quantify stiffness in soft tissues in vitro but a major limitation is that they should not be used to quantify the shear moduli of material beyond the linear viscoelastic regime (typically in the range of 0.1- 1% in soft tissues). Magnetic Resonance Elastography (MRE) is a novel imaging method that can measure the stiffness of tissues non-invasively. The setup of the experiment only allows small amplitude strain properties to be measured and as such, MRE has never been used to measure large deformation properties in vivo.

Rheology experiments on liver and skeletal muscle tissue in vitro were conducted and Fourier transform (FT) rheology was used to calculate the decomposed higher order harmonic contributions of the shear moduli. Eccentric rheology was used to measure the anisotropy of muscle. Histology was used to analyse the effect of large strain on tissue microstructure. Two imaging modalities, MRE and SPAMM, were used to measure large deformation properties in the calf muscles of human subjects in vivo by inducing plantarflexion.

Results showed distinct microstructural changes that correlated with the nonlinear response in liver (rearrangement of hepatic cords) and skeletal muscle (destruction of extracellular matrix and intersecting fibres). Increasing preload (axial strain) was shown to incur irreversible mechanical damage to liver that was accompanied by an increase in stiffness. Finite element methods showed a more homogenous strain field in the eccentric configuration compared to the traditional concentric configuration but a similar effective strain applied across the whole sample. In vivo experiments using MRE and SPAMM showed that the shortening of calf muscles (caused by plantarflexion) resulted in a decrease in stiffness. These experiments showed that in vivo large deformation properties could be measured using MRE by physiologically inducing strain.

Overall, these studies have shown that these novel methods can be used to characterise the large deformation mechanical properties of soft tissue in vitro and in vivo in a meaningful and reliable way