Assessment of Myocardial Stiffness With New Echocardiographic Methods

Figures

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  Figure 1. Examples of different types of elastic moduli: Young’s modulus relates perpendicular stress to perpendicular strain (A), shear modulus relates tangential stress to shear strain (B). In a 3-dimensional object, deformation may be described by up to 81 different deformation tensors.8) A = area; ΔL = difference in length; E = Young’s modulus; ε = strain; F = Force; G = shear modulus; γ = shear strain; L = length; σ = stress; τ = shear stress; tan θ = resulting shear of the material.

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  Figure 2. Myocardial stiffness and chamber stiffness. Myocardial stiffness (A) describes the relative elongation of the myocardial (strain, ε) wall in response to the wall stress (σ). This wall stress is caused by the filling pressure of the cavity but also modulated by the wall thickness and chamber size and shape. The end-diastolic stress-strain relation (black curves) is curvilinear and can be characterized as a whole by an exponential function with the stiffness constant k as dominant variable. The operational stiffness (red dot) represents the instantaneous stiffness (Δσ over Δε) at a certain preload (wall stress). An increase in preload causes a shift (blue arrow) towards a higher operating myocardial stiffness without changes of intrinsic myocardial properties (along the same stress-strain relation, same k). Changes in intrinsic myocardial properties because of pathologic processes (e.g., fibrosis or ischemic scar) change the entire stress strain relation towards a steeper curve (increased stiffness constant k). The operating myocardial stiffness increases, even at the same preload (green arrow). Chamber stiffness (B) is a result of myocardial stiffness, modulated by wall thickness…

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  Figure 3. Example of a SW and a longitudinal wave. Schematic representation of a longitudinal wave of which the particle displacement is in line with the wave propagation (A) and a shear or transverse wave of which the particle displacement is perpendicular to the wave propagation (B).12) SW = shear wave.

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  Figure 4. Overview of ultrasound-based techniques to determine myocardial stiffness. (A) SWE. The 2 most used techniques within SWE are shown: natural SWE, where SWs that arise from closure of the valves are tracked and ARF induced SWE, where SW are induced by sending a focused ultrasound push towards the septum. (B) Cardiac time-harmonic elastography. SWs are induced in the whole imaging window by sending low-frequency vibrations with an external vibrator. (C) Acoustic radiation force impulse imaging: This techniques looks directly at tissue displacement induced by an acoustic radiation force impulse to determine tissue stiffness. It is based on the principle that the magnitude of the ARF-displacement is inversely related to tissue stiffness. ARF = acoustic radiation force; LV = left ventricle; SW = shear wave; SWE = shear wave elastography.

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  Figure 5. Schematic overview of different approaches to increase the temporal resolution of ultrasound imaging, such as multiline acquisition and diverging wave imaging with and without compounding.12)

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  Figure 6. Technical steps to perform SWE. Three steps needed to perform SWE are 1) acquire a high frame rate image of around 1,000 frames per second. 2) post-processing of the images and an M-mode is drawn across the mid-septum from base to apex to generate a tissue velocity or acceleration map. 3) SWs are represented as tilted green bands in the velocity/acceleration maps. The slope of these bands are measured manually or semi-automatically to determine SW speed. SW = shear wave; SWE = shear wave elastography.

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  Figure 7. Measuring shear wave slope semi-automatically. The wave of interest is indicated with a mouse click and then segmented. The algorithm searches for the locations of the peak accelerations within the area and the borders of the segmented area. It then corrects the locations of the peak accelerations based on the edges of the segmented area. Finally, these corrected locations are used for a linear regression to determine the propagation speed of the wave.

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  Figure 8. Example of cardiac time-harmonic elastography. (A) Anatomical overview of the measurements performed in the parasternal long-axis view. (B) Representative B-mode images and SWS maps of a healthy control subject and a hypertrophic cardiomyopathy patient. Regions of interest are demarcated by white dotted lines. Regions of interest of the septum and posterior wall were manually selected. A third region of interest (white dashed line in the SWS maps) was automatically identified to select a larger region of major portions of the myocardium visible in the image without any user interaction (adapted from Meyer et al.5)). SWS = shear wave speed; wTTR = wild-type transthyretin amyloidosis.

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  Figure 9. Influencing factors and applications. (A) shows factors that need to be considered when interpreting SW speed. Factors intrinsic to the heart are shown in blue, while factors related to demographics are shown in green. (B) shows the clinical applications of SWE: applications directly related to cardiac function are given in blue, while technical applications are shown in green. LV = left ventricle; RV = right ventricle; SWE = shear wave elastography; 3D = three-dimensional.

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  Figure 10. Correlations between myocardial stiffness or SW speed and age. (A) shows a linear correlation between myocardial stiffness (derived from acoustic radiation force-induced SW speed) in diastole and age in HVs (adapted from Malik et al.28)). (B) shows the logarithmic relationship between natural SW speed after MVC and age in healthy volunteers. HV = healthy volunteer; MVC = mitral valve closure; SW = shear wave.

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  Figure 11. SW speed is influenced by loading conditions of the heart and changes in myocardial tissue properties.32) I/R = ischemia-reperfusion; MVC = mitral valve closure; SW = shear wave.

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  Figure 12. SW speed correlates with LVEDP and can accurately predict elevated filling pressures. (A) shows a correlation between SW speed and LVEDP. (B) shows the receiver operating characteristic analysis for SW speed to detect elevated LVEDP (adapted from Werner et al.34)). AUC = area under the curve; CI = confidence interval; LVEDP = left ventricular end-diastolic pressure; MVC = mitral valve closure; SW = shear wave.