Understanding the Contrast Mechanism in Rotation Elastogram: A Parametric Study

Ultrasound elastography has been found to be useful in different clinical applications. For example, in breast imaging, axial strain elastography provides information related to tissue stiffness, which is used to characterize breast lesions as either benign or malignant. In addition, these lesions also differ in their bonding properties. Benign breast lesions are loosely bonded and malignant breast lesions are firmly bonded to the surrounding tissues. Therefore, only benign breast lesions will rotate/slip on the application of deformation. This rotation of lesions can be visualized with rotation elastography, which utilizes axial and lateral shear strain components. The contrast obtained in rotation elastography depends on various mechanical as well as ultrasound elastography parameters. However, there is no reported work that provides an understanding of the influence of these parameters on the visualized rotation contrast. In this work, the authors studied the rotation contrast by varying the mechanical parameters such as the inclusion b/a ratio, relative inclusion-background Young's modulus, amount of applied deformation and orientation of the inclusion. First, the authors performed finite-element analysis to understand the fundamental rotation contrast of the inclusion. Next, rotation elastograms obtained from ultrasound simulations in Field II and experiments on tissue-mimicking phantoms were investigated. Mean contrast was used as a metric to evaluate the quality of rotation elastograms in finite-element analysis, and contrast-to-noise ratio was used in Field II simulations and phantom experiments. The results indicate that rotation contrast was observed only in the case of loosely bonded inclusions. Further, the rotation contrast was found to depend on the inclusion asymmetry and its orientation with respect to the axis of deformation. Interestingly, it was found that a loosely bonded inclusion contrasts with surrounding tissue in rotation elastography, even in the absence of any inclusion-background modulus contrast.