Transmural Wave Speed Gradient May Distinguish Intrinsic Myocardial Stiffening From Preload-Induced Changes in Operational Stiffness in Shear Wave Elastography

Background: Shear wave elastography (SWE) is a promising technique to non-invasively assess myocardial stiffness based on the propagation speed of mechanical waves. However, a high wave propagation speed can either be attributed to an elevated intrinsic myocardial stiffness or to a preload-induced increase in operational stiffness. Objective: Our objective was to find a way to discriminate intrinsic myocardial stiffening from stiffening caused by an increased pressure in SWE. Methods: We used the finite element method to study the shear wave propagation patterns when stiffness and/or pressure is elevated, compared to normal stiffness and pressure. Numerical findings were verified in a few human subjects. Results: The transmural wave speed gradient was able to distinguish changes in intrinsic stiffness from those induced by differing hemodynamic load (a speed of ±3.2 m/s in parasternal short-axis (PSAX) view was associated with a wave speed gradient of −0.17 ± 0.15 m/s/mm when pressure was elevated compared to 0.04 ± 0.05 m/s/mm when stiffness was elevated). The gradient however decreased when stiffness increased (decrease with a factor 3 in PSAX when stiffness doubled at 20 mmHg). The human data analysis confirmed the presence of a wave speed gradient in a patient with elevated ventricular pressure. Conclusion: Cardiac SWE modeling is a useful tool to gain additional insights into the complex wave physics and to guide post-processing. The transmural differences in wave speed may help to distinguish loading-induced stiffening from intrinsic stiffness changes. Significance: The transmural wave speed gradient has potential as a new diagnostic parameter for future clinical studies.