Assessing the coronary circulation with contrast-enhanced echocardiography has high clinical relevance. However, it is not being routinely performed in clinical practice because the current clinical tools generally cannot provide adequate image quality. The contrast agent's visibility in the myocardium is generally poor, impaired by motion and nonlinear propagation artifacts. The established multipulse contrast schemes (MPCSs) and the more experimental singular value decomposition (SVD) filter also fall short to solve these issues. Here, we propose a scheme to process amplitude modulation/amplitude-modulated pulse inversion (AM/AMPI) echoes with higher order SVD (HOSVD) instead of conventionally summing the complementary pulses. The echoes from the complementary pulses form a separate dimension in the HOSVD algorithm. Then, removing the ranks in that dimension with dominant coherent signals coming from tissue scattering would provide the contrast detection. We performed both in vitro and in vivo experiments to assess the performance of our proposed method in comparison with the current standard methods. A flow phantom study shows that HOSVD on AM pulsing exceeds the contrast-to-background ratio (CBR) of conventional AM and an SVD filter by 10 and 14 dB, respectively. In vivo porcine heart results also demonstrate that, compared to AM, HOSVD improves CBR in open-chest acquisition (up to 19 dB) and contrast ratio (CR) in closed-chest acquisition (3 dB).

High-frame-rate (HFR) echo-particle image velocimetry (echoPIV) is a promising tool for measuring intracardiac blood flow dynamics. In this study, we investigate the optimal ultrasound contrast agent (UCA: SonoVue) infusion rate and acoustic output to use for HFR echoPIV (PRF = 4900 Hz) in the left ventricle (LV) of patients. Three infusion rates (0.3, 0.6, and 1.2 ml/min) and five acoustic output amplitudes (by varying transmit voltage: 5, 10, 15, 20, and 30 V - corresponding to mechanical indices of 0.01, 0.02, 0.03, 0.04, and 0.06 at 60-mm depth) were tested in 20 patients admitted for symptoms of heart failure. We assess the accuracy of HFR echoPIV against pulsed-wave Doppler acquisitions obtained for mitral inflow and aortic outflow. In terms of image quality, the 1.2-ml/min infusion rate provided the highest contrast-to-background ratio (CBR) (3-dB improvement over 0.3 ml/min). The highest acoustic output tested resulted in the lowest CBR. Increased acoustic output also resulted in increased microbubble disruption. For the echoPIV results, the 1.2-ml/min infusion rate provided the best vector quality and accuracy; mid-range acoustic outputs (corresponding to 15-20-V transmit voltages) provided the best agreement with the pulsed-wave Doppler. Overall, the highest infusion rate (1.2 ml/min) and mid-range acoustic output amplitudes provided the best image quality and echoPIV results.

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.

In cardiac high-frame-rate color tissue Doppler imaging (TDI), a wave-like pattern travels over the interventricular septum (IVS) after atrial contraction. The propagation velocity of this myocardial stretch post-atrial contraction (MSPa) was proposed as a measure of left ventricular stiffness. The aim of our study was to investigate the MSPa in patients with hypertrophic cardiomyopathy (HCM) compared with healthy volunteers. Forty-two healthy volunteers and 33 HCM patients underwent high-frame-rate (>500 Hz) TDI apical echocardiography. MSPa was visible in TDI, M-mode and speckle tracking. When assuming a wave propagating with constant velocity, MSPa in healthy volunteers (1.6 ± 0.3 m/s) did not differ from that in HCM patients (1.8 ± 0.8 m/s, p = 0.14). Yet, in 42% of patients with HCM, the MSPa had a non-constant velocity over the wall: in the basal IVS, the velocity was lower (1.4 ± 0.5 m/s), and in the mid-IVS, much higher (6.1 ± 3.4 m/s, p < 0.0001), and this effect was related to the septal thickness. The reason is hypothesized to be the reaching of maximal longitudinal myocardial distension in HCM patients.

We apply a high frame rate (over 500 Hz) tissue Doppler method to measure the propagation velocity of naturally occurring shear waves (SW) generated by aortic and mitral valves closure. The aim of this work is to demonstrate clinical relevance. We included 45 healthy volunteers and 43 patients with hypertrophic cardiomyopathy (HCM). The mitral SW (4.68 ± 0.66 m/s) was consistently faster than the aortic (3.51 ± 0.38 m/s) in all volunteers (p < 0.0001). In HCM patients, SW velocity correlated with E/e’ ratio (r = 0.346, p = 0.04 for aortic SW and r = 0.667, p = 0.04 for mitral SW). A subgroup of 20 volunteers were matched for age and gender to 20 HCM patients. In HCM, the mean velocity of 5.1 ± 0.7 m/s for the aortic SW (3.61 ± 0.46 m/s in matched volunteers, p < 0.0001) and 6.88 ± 1.12 m/s for the mitral SW(4.65 ± 0.77 m/s in matched volunteers, p < 0.0001). A threshold of 4 m/s for the aortic SW correctly classified pathologic myocardium with a sensitivity of 95% and specificity of 90%. Naturally occurring SW can be used to assess differences between normal and pathologic myocardium.

Different shear wave elastography methods have been proposed to measure cardiac material properties. This study compared shear waves naturally generated by aortic and mitral valve closure to those externally induced with an acoustic radiation force throughout the cardiac cycle. The shear wave timing and propagation speeds were measured in four pigs with open-chest recordings. Despite spatial and temporal differences in excitation source, the propagation speeds of the natural shear waves were found to be in the same range as the propagation speeds of the active shear waves. The results also suggested a large inter-beat variability for the natural shear waves.

For the quantification of myocardial function, myocardial stiffness can potentially be measured non-invasively using shear wave elastography. Clinical diagnosis requires high precision. In 10 healthy volunteers, we studied the reproducibility of the measurement of propagation speeds of shear waves induced by aortic and mitral valve closure (AVC, MVC). Inter-scan was slightly higher but in similar ranges as intra-scan variability (AVC: 0.67 m/s (interquartile range [IQR]: 0.40–0.86 m/s) versus 0.38 m/s (IQR: 0.26–0.68 m/s), MVC: 0.61 m/s (IQR: 0.26–0.94 m/s) versus 0.26 m/s (IQR: 0.15–0.46 m/s)). For AVC, the propagation speeds obtained on different day were not statistically different (p = 0.13). We observed different propagation speeds between 2 systems (AVC: 3.23–4.25 m/s [Zonare ZS3] versus 1.82–4.76 m/s [Philips iE33]), p = 0.04). No statistical difference was observed between observers (AVC: p = 0.35). Our results suggest that measurement inaccuracies dominate the variabilities measured among healthy volunteers. Therefore, measurement precision can be improved by averaging over multiple heartbeats.

The propagation velocity of shear waves relates to tissue stiffness. We prove that a regular clinical cardiac ultrasound system can determine shear wave velocity with a conventional unmodified tissue Doppler imaging (TDI) application. The investigation was performed on five tissue phantoms with different stiffness using a research platform capable of inducing and tracking shear waves and a clinical cardiac system (Philips iE33, achieving frame rates of 400–700 Hz in TDI by tuning the normal system settings). We also tested the technique in vivo on a normal individual and on typical pathologies modifying the consistency of the left ventricular wall. The research platform scanner was used as reference. Shear wave velocities measured with TDI on the clinical cardiac system were very close to those measured by the research platform scanner. The mean difference between the clinical and research systems was 0.18 ± 0.22 m/s, and the limits of agreement, from −0.27 to +0.63 m/s. In vivo, the velocity of the wave induced by aortic valve closure in the interventricular septum increased in patients with expected increased wall stiffness.

Background: The closure of the valves generates shear waves in the heart walls. The propagation velocity of shear waves relates to stiffness. This could potentially be used to estimate the stiffness of the myocardium, with huge potential implications in pathologies characterized by a deterioration of the diastolic properties of the left ventricle. In an earlier phantom study we already validated shear wave tracking with a clinical ultrasound system in cardiac mode.

Purpose: In this study we aimed to measure the shear waves velocity in normal individuals.

Methods: 12 healthy volunteers, mean age=37±10, 33% females, were investigated using a clinical scanner (Philips iE33), equipped with a S5-1 probe, using a clinical tissue Doppler (TDI) application. ECG and phonocardiogram (PCG) were synchronously recorded. We achieved a TDI frame rate of >500Hz by carefully tuning normal system settings. Data were processed offline in Philips Qlab 8 to extract tissue velocity along a virtual M-mode line in the basal third of the interventricular septum, in parasternal long axis view. This tissue velocity showed a propagating wave pattern after closure of the valves. The slope of the wave front velocity in a space-time panel was measured to obtain the shear wave propagation velocity. The velocity of the shear waves induced by the closure of the mitral valve (1st heart sound) and aortic valve (2nd heart sound) was averaged over 4 heartbeats for every subject.

Results: Shear waves were visible after each closure of the heart valves, synchronous to the heart sounds. The figure shows one heart cycle of a subject, with the mean velocity along a virtual M-mode line in the upper panel, synchronous to the ECG signal (green line) and phonocardiogram (yellow line) in the lower panel. The slope of the shear waves is marked with dotted lines and the onset of the heart sounds with white lines. In our healthy volunteer group the mean velocity of the shear wave induced by mitral valve closure was 4.8±0.7m/s, standard error of 0.14 m/s. The mean velocity after aortic valve closure was 3.4±0.5m/s, standard error of 0.09 m/s. We consistently found that for any subject the velocity after mitral valve closure was higher than after aortic valve closure.

Conclusion: The velocity of the shear waves generated by the closure of the heart valves can be measured in normal individuals using a clinical TDI application. The shear wave induced after mitral valve closure was consistently faster than after aortic valve closure. Abstract P1138 Figure.

Abstract P1138 Figure.

This paper describes an in-vivo pilot study in which the systolic myocardial stiffness of four pigs was measured with various acoustic techniques. We compare the propagation velocity of shear waves in open chest and closed chest experiments, in which the open-chest shear wave was either physiologically induced by aortic valve closure (N=3), or externally induced by acoustic-radiation force (N=1). The results of closed-chest versus open-chest recordings are consistent within 1 unit of standard error, whereas the acoustic-radiation force measurement showed lowest standard deviation (7% versus 13%).