Quantifying Tissue-Stiffening Behaviour of the cervix in Shear-wave Elastography

Shear-wave elastography is a useful tool in imaging mechanical properties of soft-tissue. It has great potential to improve the predictive accuracy of preterm birth. This imaging modality, however, has high inter- and intra-observer variability. This is due its sensitivity to changes in pressure and the natural variation in elasticity across the cervix. The aim of this study is to use the artefacts caused by the pressure-dependent increase in shear-wave speed to derive fourth order hyperelastic elasticity tensor of the tissue. The method proposed is as follows: the strain-map of the tissue is extracted by tracking the deformation of the cervix in two raw B-mode images of the ultrasound. Each B-mode image is correlated to a shear-wave map acquired at the same pressure. The relation of the shear-wave speed to the strain is used to derive the parameters of the hyperelastic material model of cervical tissue. A continuous measure of the cervical elasticity parameters is taken. The maps of the parameters are then used to predict the delivery date. In this paper, this method is demonstrated through simulation in FEBio in preparation for a large-scale clinical study at the Royal Prince Alfred Hospital. In the simulation, a tissue of Mooney Rivilin hyperelastic material is indented from 0 mm to 6 mm using a model of the ultrasound probe. The maximum factor increase in shear modulus was 2.63 times which translates to 62 % increase in measured shear-wave speed. The function of the change in shear-wave speed is unique to the parameters of the model.