Abstract
.Elastography measures tissue strain, which can be interpreted under certain simplifying assumptions to be representative of the underlying stiffness distribution. This is useful in cancer diagnosis where tumors tend to have a different stiffness to healthy tissue and has also shown potential to provide indication of the degree of bonding at tumor–tissue boundaries, which is clinically useful because of its dependence on tumor pathology. We consider the changes in axial strain for the case of a symmetrical model undergoing uniaxial compression, studied by characterizing changes in tumor contrast transfer efficiency (CTE), inclusion to background strain contrast and strain contrast generated by slip motion, as a function of Young’s modulus contrast and applied strain. We present results from a finite element simulation and an evaluation of these results using tissue-mimicking phantoms. The simulation results show that a discontinuity in displacement data at the tumor boundary, caused by the surrounding tissue slipping past the tumor, creates a halo of “pseudostrain” across the tumor boundary. Mobile tumors also appear stiffer on elastograms than adhered tumors, to the extent that tumors that have the same Young’s modulus as the background may in fact be visible as low-strain regions, or those that are softer than the background may appear to be stiffer than the background. Tumor mobility also causes characteristic strain heterogeneity within the tumor, which exhibits low strain close to the slippery boundary and increasing strain toward the center of the tumor. These results were reproduced in phantom experiments. In addition, phantom experiments demonstrated that when fluid lubrication is present at the boundary, these effects become applied strain-dependent as well as modulus-dependent, in a systematic and characteristic manner. The knowledge generated by this study is expected to aid interpretation of clinical strain elastograms by helping to avoid misinterpretation as well as provide additional diagnostic criteria stated in the paper and stimulate further research into the application of elastography to tumor mobility assessment.
Highlights
Quasi-static ultrasound elastography generates images of tissue strain, referred to as elastograms, which aim to reflect the underlying relative tissue stiffness.[1]
The discontinuity observed in the axial displacement field causes a halo of high axial strain to surround the mobile inclusion in the gradient-based strain image [Fig. 2(f)], which is not seen in the element-based strain image [Fig. 2(d)]
We suggest that the internal variation in the mobile inclusion axial strain distribution is due to the heterogeneous transmission of stress into the inclusion due to the freedom of the background tissue to slip across the inclusion surface
Summary
Quasi-static ultrasound elastography generates images of tissue strain, referred to as elastograms, which aim to reflect the underlying relative tissue stiffness.[1]. Strain is a useful quantity to measure due to its dependence upon tissue stiffness, which is clinically relevant because pathological changes are often accompanied by altered tissue stiffness.[6,7] The term elastography has since been used to describe a wide range of biomechanical imaging methods including deep loading of the tissue using acoustic radiation force[8,9] and those that display: shear elastic modulus by measuring the speed of shear waves,[10,11,12] Young’s modulus (YM). E., not bonded to) the surrounding tissue, has been used to help differentiate between benign and malignant tumors;[36] local infiltration of malignant tumors, for example, is believed to increase mechanical integration between tumor and surrounding tissue.
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