A direct comparison of natural and acoustic-radiation-force-induced cardiac mechanical waves

Abstract

Natural and active shear wave elastography (SWE) are potential ultrasound-based techniques to non-invasively assess myocardial stiffness, which could improve current diagnosis of heart failure. This study aims to bridge the knowledge gap between both techniques and discuss their respective impacts on cardiac stiffness evaluation. We recorded the mechanical waves occurring after aortic and mitral valve closure (AVC, MVC) and those induced by acoustic radiation force throughout the cardiac cycle in four pigs after sternotomy. Natural SWE showed a higher feasibility than active SWE, which is an advantage for clinical application. Median propagation speeds of 2.5–4.0 m/s and 1.6–4.0 m/s were obtained after AVC and MVC, whereas ARF-based median speeds of 0.9–1.2 m/s and 2.1–3.8 m/s were reported for diastole and systole, respectively. The different wave characteristics in both methods, such as the frequency content, complicate the direct comparison of waves. Nevertheless, a good match was found in propagation speeds between natural and active SWE at the moment of valve closure, and the natural waves showed higher propagation speeds than in diastole. Furthermore, the results demonstrated that the natural waves occur in between diastole and systole identified with active SWE, and thus represent a myocardial stiffness in between relaxation and contraction.

Assessing cardiac stiffness using ultrasound shear wave elastography

“…Besides passive material properties, cardiac contraction affects shear wave speed measurements (see It should, nonetheless, be noted that the timing of the mechanical waves after valve closure within the isovolumic contraction/relaxation phase is unclear. Recent work combining ARF-based and natural SWE (Keijzer et al, 2020a) was able to detect both wave types in the same acquisition as visualized in Figure 9a and b. This permitted to investigate the timing of natural wave measurements with respect to ARF-assessed cardiac dynamics: waves after valve closure occurred when the myocardium is in between relaxation and contraction (Figure 9c).…”

“…For ARF-based SWE, two shear waves are created in the center of the field of view (in the IVS); whereas for natural SWE one shear wave is propagating from base to apex. Adapted with permission from Keijzer and Caenen et al (Keijzer et al, 2020a).…”

“…This result shows that many factors, such as fluid loading, contraction speed, contraction synchronicity, or even stenosis of outflow track or mitral valve, can affect the exact moment of valve closure, and hence the instantaneous speed of the induced wave. (Keijzer et al, 2020a).…”

“…A negligible change in diastolic wave speed was reported for ARF-based SWE (Δ0.04 m/s for ΔEDP of 14.7 mmHg (Pernot et al, 2011) and Δ0.5 kPa for ΔEDP of 5 mmHg (Pislaru et al, 2014b)), whereas the wave speed after MVC increased significantly with increased EDP (Δ4.9 m/s for ΔEDP of 21.7 mmHg: R=0.83 and p<0.001 (Bezy et al, 2020b)). Differences in shear wave excitation properties, experimental set-up and investigated pressure range partly explain the reported difference in the sensitivity of shear wave speed to pressure, but more importantly, the end-diastolic moment of the shear wave recording for ARF-based SWE might still be in the quasi-linear part of the stress-strain relationship whereas this might be different for the shear wave after MVC for which, again, the exact timing within the isovolumic contraction phase is unclear (Keijzer et al, 2020a;Pernot and Villemain, 2019).…”

“…Additionally, the authors in (Villemain et al, 2018) found no statistical difference between measurements at baseline and after three months (n=15). Even though the feasibility of ARF-based SWE throughout the cardiac cycle is not yet studied in the clinics, a previous preclinical study (Keijzer et al, 2020a) reported a success rate of 32 % for 65 ARF-based SWE acquisitions (n=4). This demonstrates the SNR challenges for in vivo ARFbased SWE.…”