Shear wave elastography can assess the in-vivo nonlinear mechanical behavior of heel-pad

Abstract

This study combines non-invasive mechanical testing with finite element (FE) modelling to assess for the first time the reliability of shear wave (SW) elastography for the quantitative assessment of the in-vivo nonlinear mechanical behavior of heel-pad. The heel-pads of five volunteers were compressed using a custom-made ultrasound indentation device. Tissue deformation was assessed from B-mode ultrasound and force was measured using a load cell to calculate the force - deformation graph of the indentation test. These results were used to design subject specific FE models and to inverse engineer the tissue's hyperelastic material coefficients and its stress - strain behavior. SW speed was measured for different levels of compression (from 0% to 50% compression). SW speed for 0% compression was used to assess the initial stiffness of heel-pad (i.e. initial shear modulus, initial Young's modulus). Changes in SW speed with increasing compressive loading were used to quantify the tissue's nonlinear mechanical behavior based on the theory of acoustoelasticity. Statistical analysis of results showed significant correlation between SW-based and FE-based estimations of initial stiffness, but SW underestimated initial shear modulus by 64%(±16). A linear relationship was found between the SW-based and FE-based estimations of nonlinear behavior. The results of this study indicate that SW elastography is capable of reliably assessing differences in stiffness, but the absolute values of stiffness should be used with caution. Measuring changes in SW speed for different magnitudes of compression enables the quantification of the tissue's nonlinear behavior which can significantly enhance the diagnostic value of SW elastography.

Two-Dimensional Shear Wave Elastography of Normal Soft Tissue Organs in Adult Beagle Dogs; Interobserver Agreement and Sources of Variability

“…In a study about spatial dependent mechanical properties of the heel pad using SWE, Young's modulus was not used considering the heterogeneity of the tissue (Lin et al, 2017). Thus, SWE should be used to assess the changes in stiffness of the tissue but not to quantify stiffness itself (Chatzistergos et al, 2018).…”

An In-Vivo Model For Overloading-Induced Soft Tissue Injury

“…Localised stiffening was assessed by measuring changes in the distribution of SW propagation speed in an axial imaging plane at the centre of the heel (Figure 1a). Previous research from the authors of this study had shown that increased or decreased SW speed in the plantar soft tissue is a reliable indicator for tissue stiffening or softening respectively 4 . To precondition the plantar soft tissue and eliminate the effect of loading history, the participants were asked to walk barefoot the length of the lab twice before imaging (≈80 steps).…”

“…In this study, changes in tissue biomechanics were assessed in 5mm wide circular areas close to the skin surface (Figure 1c-e). The effect of overloading in deeper tissues was not assessed because SW elastography cannot produce reliable measurements close to bony surfaces 4,5 . The use of alternative elastography techniques could enable the investigation of the effect of overloading on different plantar soft tissue layers 19 .…”

“…To ensure no compression was applied to the tissue during imaging, special care was taken to always maintain a visible layer of ultrasound gel between the probe and skin. This was necessary to avoid any misleading increase in SW speed due to externally applied compression 4 . The probe was kept in the same position for at least 10 seconds or until a stable SW map was achieved before capturing a frame for analysis.…”

Measured Hyperelastic Properties of Cervical Tissue with Shear-Wave Elastography

“…The nonlinear behaviour of tissue causes errors in the measurements made since, in most clinical settings, the pressure from ultrasound probes is unregulated. This phenomenon, known as “tissue stiffening” or “strain stiffening”, is present in most soft tissues, for example, breast tissue [ 11 ], heel pad [ 12 , 13 ], calf muscle [ 14 ] and thyroid gland [ 15 ]. The occurrence of tissue stiffening is especially detrimental for cervical elastography as the probe is in direct contact with the tissue, and the shape of the probe causes non-uniform deformation [ 16 ].…”