Pushing The Physical Limits Of Wave Propagation In Soft Tissues: An Add-On To Shear Wave Elastography

Shear wave elastography (SWE) relies on the generation and tracking of coherent shear waves to image the tissue's Young’s modulus (E). To this end, SWE locally estimates the shear wave speed which is directly linked to E under the hypothesis that the tissue behaves as a near incompressible, locally homogeneous, isotropic, and linearly elastic solid. Recent technological developments have allowed SWE to be implemented in commercial ultrasound and magnetic resonance imaging systems, quickly becoming a new imaging modality in medicine and biology. It has been successfully applied to breast, prostate, and liver. However, the abovementioned hypothesis sets a limit to the applicability of SWE. Tissue is by nature visco-elastic and nonlinear. Neglecting this behavior results in loss of information that can potentially be used in the development of new biomarkers to assess tissue’s health. Further understanding of the wave physics involved in SWE will allow imaging new properties like viscosity, anisotropy or nonlinear elasticity that may improve the diagnostic capability of SWE and extend its applications to other tissues like skeletal muscle, tendon, bladder, or the crystalline lens to name a few.

Several research groups are working on pushing the physical limits of shear wave elastography (SWE), either through the development of more complete wave propagation models or by gaining a deeper understanding on the wave physics to identify new physical properties to be incorporated into medical images. Therefore, the goal of this research topic is to present the latest experimental and theoretical developments on wave physics in soft tissue that can contribute to extending the applicability of SWE or the development of new biomarkers.
Soft tissues are composed of hierarchically organized complex structures with different geometries and subject to different boundary conditions. Analytical physical models for wave propagation tend to simplify this structure. To make progress, it is necessary to make the models more complex either by taking more variables into account or by using numerical simulations of the differential equations. It is even possible to estimate the parameters by dispensing with a detailed physical model of the medium and using data-driven models. In addition, interrogating the tissue at different spatial scales (i.e., wavelength) can also reveal its complex structure and yield new information.

This research topic is focused on pushing the limits of standard elastography methods to add relevant information for its applications in medicine or biology in the form of Original Research Papers, Data Report Papers, or Review papers. Examples of themes to be addressed here are:
- Physical modelling of wave propagation in soft tissues
- Viscoleasticity imaging
- Nonlinear imaging
- Anisotropy
- Guided wave propagation in tissues
- Improving shear wave imaging of small lesions.
- Improving imaging of deep tissues
- Development of phantom materials that mimc different elastic properties of tissues
- Novel instrumentation to assess tissue elastic properties

Keywords: Shear anisotropy, Nonlinear elasticity, Guided waves, Surface waves, signal processing, Elastography, Soft tissues, Viscoelasticity

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