Estimation of shear modulus ratio between inclusion and background using strain ratios in 2-D ultrasound elastography

The goal of this study is to assess the effects of region of interest (ROI) selection and lesion size on estimates of shear modulus ratio from strain ratios to quantify relative stiffness of breast tumors. A theoretical model and finite element method (FEM) simulations of lesions with various shear modulus ratios are created for a 2-D plane strain deformation. Both the lesion and the surrounding tissue are assumed to be linearly elastic, isotropic, homogenous, and incompressible. The results from the model and simulations are in agreement that the lesion-to-surrounding shear modulus ratio is linearly proportional to the axial normal strain ratio for small lesions when the ROI in the surrounding tissue is at least four lesion diameters away from the lesion. For larger lesions, FEM simulations show that the estimated strain ratio using the same ROI location increases with the lesion size and would overestimate the shear modulus ratio. Therefore, a correction factor is necessary for large breast lesions when strain ratios are used to estimate the shear modulus ratio. We also demonstrate that strain elastograms calculated using a speckle tracking method on simulated RF data are accurate enough to observe the same effect on strain ratio estimation. This result is confirmed using experimental data acquired from two tissue-mimicking phantoms. Our findings will help clinicians to estimate strain ratios and shear modulus ratios more accurately for more reliable comparison of one clinical examination to another.