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

Abstract

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.

Non-invasive imaging of Young’s modulus and Poisson’s ratio in cancers in vivo

“…Most of retrievable methods assume specific boundary conditions such as total uniformity of the background, stress-free lateral boundaries etc., which are rarely true in complex tissue environments. Most of these methods perform well for tumors of specific shapes such as disk (2D)/sphere (3D) [10], [36], [39], [40] but have poorer performance in tumors of other shapes such as ellipse. In case of soft tumor or tumor of high YM contrast, the strain inside the tumor changes small for a large change in the tumor YM.…”

“…As the available methods rely only on the change of axial strain neglecting the change in the lateral strain, they cannot perform efficiently when the YM contrast between the tumor and normal tissue is larger than or when the tumor is softer than the background [15], [36], [40]. In many cases, the tumor is assumed to be very small so that certain ratios such as ratios of sample-radius-to-tumor-radius, compressor-radius-totumor-radius and distance between applied force and tumorto-tumor-radius are greater than a predefined value [36], [39]. Determination of heterogeneous distribution of YM inside the tumor and normal tissue is another challenge [41].…”

Elastography: history, principles, and technique comparison

“…Recently work has begun at the FDA on modeling some of the factors that may degrade the quality of strain ratio estimates. Work using simple phantoms and phantom models has shown that the location of the ''normal'' tissue ROI can greatly affect the strain ratio value [26]. This is due to distortion of the strain values in surrounding tissue caused by the presence of a lesion.…”