Measured Hyperelastic Properties of Cervical Tissue with Shear-Wave Elastography.

The nonlinear mechanical behaviour of cervical tissue causes unpredictable changes in measured elastograms when pressure is applied. These uncontrolled variables prevent the reliable measurement of tissue elasticity in a clinical setting. Measuring the nonlinear properties of tissue is difficult due to the need for both shear modulus and strain to be taken simultaneously. A simulation-based method is proposed in this paper to resolve this. This study describes the nonlinear behaviour of cervical tissue using the hyperelastic material models of Demiray-Fung and Veronda-Westmann. Elastograms from 33 low-risk patients between 18 and 22 weeks gestation were obtained. The average measured properties of the hyperelastic material models are: Demiray-Fung-A1α = 2.07 (1.65-2.58) kPa, α = 6.74 (4.07-19.55); Veronda-Westmann-C1C2 = 4.12 (3.24-5.04) kPa, C2 = 4.86 (2.86-14.28). The Demiray-Fung and Veronda-Westmann models performed similarly in fitting to the elastograms with an average root mean square deviation of 0.41 and 0.47 ms-1, respectively. The use of hyperelastic material models to calibrate shear-wave speed measurements improved the consistency of measurements. This method could be applied in a large-scale clinical setting but requires updated models and higher data resolution.