Concepts and applications of ultrafast cardiac ultrasound imaging

Evolution of Natural Myocardial Shear Wave Behavior in Young Hearts: Determinant Factors and Reproducibility Analysis

BACKGROUND

Myocardial diastolic function assessment in children by conventional echocardiography is challenging. High-frame rate echocardiography facilitates the assessment of myocardial stiffness, a key factor in diastolic function, by measuring the propagation velocities of myocardial shear waves (SWs). However, normal values of natural SWs in children are currently lacking. The aim of this study was to explore the behavior of natural SWs among children and adolescents, their reproducibility, and the factors affecting SW velocities from childhood into adulthood.

METHODS

One hundred six healthy children (2-18 years of age) and 62 adults (19-80 years of age) were recruited. High-frame rate images were acquired using a modified commercial scanner. An anatomic M-mode line was drawn along the ventricular septum, and propagation velocities of natural SWs after mitral valve closure were measured in the tissue acceleration-coded M-mode display.

RESULTS

Throughout life, SW velocities after mitral valve closure exhibited pronounced age dependency (r = 0.73; P < .001). Among the pediatric population, SW velocities correlated significantly with measures of cardiac geometry (septal thickness and left ventricular end-diastolic dimension), local hemodynamics (systolic blood pressure), and echocardiographic parameters of systolic and diastolic function (global longitudinal strain, mitral E/e' ratio, isovolumic relaxation time, and mitral deceleration time) (P < .001). In a multivariate analysis including all these factors, the predictors of SW velocities were age, mitral E/e', and global longitudinal strain (r = 0.81).

CONCLUSIONS

Natural myocardial SW velocities in children can be detected and measured. SW velocities showed significant dependence on age and diastolic function. Natural SWs could be a promising additive tool for the assessment of diastolic function among children.

Evaluation of left ventricular stiffness with echocardiography

Half of patients with heart failure are presented with preserved ejection fraction (HFpEF). The pathophysiology of these patients is complex, but increased left ventricular (LV) stiffness has been proven to play a key role. However, the application of this parameter is limited due to the requirement for invasive catheterization for its measurement. With advances in ultrasound technology, significant progress has been made in the noninvasive assessment of LV chamber or myocardial stiffness using echocardiography. Therefore, this review aims to summarize the pathophysiological mechanisms, correlations with invasive LV stiffness constants, applications in different populations, as well as the limitations of echocardiography-derived indices for the assessment of both LV chamber and myocardial stiffness. Indices of LV chamber stiffness, such as the ratio of E/e' divided by left ventricular end-diastolic volume (E/e'/LVEDV), the ratio of E/SRe (early diastolic strain rates)/LVEDV, and diastolic pressure-volume quotient (DPVQ), are derived from the relationship between echocardiographic parameters of LV filling pressure (LVFP) and LV size. However, these methods are surrogate and lumped measurements, relying on E/e' or E/SRe for evaluating LVFP. The limitations of E/e' or E/SRe in the assessment of LVFP may contribute to the moderate correlation between E/e'/LVEDV or E/SRe/LVEDV and LV stiffness constants. Even the most validated measurement (DPVQ) is considered unreliable in individual patients. In comparison to E/e'/LVEDV and E/SRe/LVEDV, indices like time-velocity integral (TVI) measurements of pulmonary venous and transmitral flows may demonstrate better performance in assessing LV chamber stiffness, as evidenced by their higher correlation with LV stiffness constants. However, only one study has been conducted on the exploration and application of TVI in the literature, and the accuracy of assessing LV chamber stiffness remains to be confirmed. Regarding echocardiographic indices for LV myocardial stiffness evaluation, parameters such as epicardial movement index (EMI)/ diastolic wall strain (DWS), intrinsic velocity propagation of myocardial stretch (iVP), and shear wave imaging (SWI) have been proposed. While the alteration of DWS and its predictive value for adverse outcomes in various populations have been widely validated, it has been found that DWS may be better considered as an overall marker of cardiac function performance rather than pure myocardial stiffness. Although the effectiveness of iVP and SWI in assessing left ventricular myocardial stiffness has been demonstrated in animal models and clinical studies, both indices have their limitations. Overall, it seems that currently no echocardiography-derived indices can reliably and accurately assess LV stiffness, despite the development of several parameters. Therefore, a comprehensive evaluation of LV stiffness using all available parameters may be more accurate and enable earlier detection of alterations in LV stiffness.

[Echocardiography with high frame rates in the clinical practice : Principles, applications and perspectives]

Continuous developments in cardiovascular imaging, software and hardware have led to technological advancements that open new ways for assessing myocardial mechanics, hemodynamics, and function. Through new scan modalities, echocardiographic scanners can nowadays achieve very high frame rates up to 5000 frames s-1 which enables a wide variety of new applications, including shear wave elastography, ultrafast speckle tracking, the visualization of intracardiac blood flow and myocardial perfusion imaging. This review provides an overview of these advances and demonstrates possible applications and their potential added value in the clinical practice.

A simplified and accelerated implementation of SVD for filtering ultrafast power Doppler images

BACKGROUND AND OBJECTIVE

Ultrafast Power Doppler (UPD) is a growing ultrasound modality for imaging and diagnosing microvasculature disease. A key element of UPD is using singular value decomposition (SVD) as a highly selective filter for tissue and electronic noise. However, two significant drawbacks of SVD are its computational burden and the complexity of its algorithms. These limitations hinder the development of fast and specific SVD algorithms for UPD imaging. This study introduces power SVD (pSVD), a simplified and accelerated algorithm for filtering tissue and noise in UPD images.

METHODS

pSVD exploits several mathematical properties of SVD specific to UPD images. In particular, pSVD allows the direct computation of blood-related SVD components from the temporal singular vectors. This feature simplifies the expression of SVD while significantly accelerating its computation. After detailing the theory behind pSVD, we evaluate its performances in several in vitro and in vivo experiments and compare it to SVD and randomized SVD (rSVD).

RESULTS

pSVD strongly decreases the running time of SVD (between 5 and 12 times in vivo) without impacting the quality of UPD images. Compared to rSVD, pSVD can be significantly faster (up to 3 times) or slightly slower but gives access to more estimators to isolate tissue subspaces.

CONCLUSION

pSVD is highly valuable for implementing UPD imaging in clinical ultrasound and provides a better understanding of SVD for ultrasound imaging in general.

Impact of Loading and Myocardial Mechanical Properties on Natural Shear Waves: Comparison to Pressure-Volume Loops

BACKGROUND

Shear wave elastography (SWE) has been proposed as a novel noninvasive method for the assessment of myocardial stiffness, a relevant determinant of diastolic function. It is based on tracking the propagation of shear waves, induced, for instance, by mitral valve closure (MVC), in the myocardium. The speed of propagation is directly related to myocardial stiffness, which is defined by the local slope of the nonlinear stress-strain relation. Therefore, the operating myocardial stiffness can be altered by both changes in loading and myocardial mechanical properties.

OBJECTIVES

This study sought to evaluate the capability of SWE to quantify myocardial stiffness changes in vivo by varying loading and myocardial tissue properties and to compare SWE against pressure-volume loop analysis, a gold standard reference method.

METHODS

In 15 pigs, conventional and high-frame rate echocardiographic data sets were acquired simultaneously with pressure-volume loop data after acutely changing preload and afterload and after inducting an ischemia/reperfusion (I/R) injury.

RESULTS

Shear wave speed after MVC significantly increased by augmenting preload and afterload (3.2 ± 0.8 m/s vs 4.6 ± 1.2 m/s and 4.6 ± 1.0 m/s, respectively; P = 0.001). Preload reduction had no significant effect on shear wave speed compared to baseline (P = 0.118). I/R injury resulted in significantly higher shear wave speed after MVC (6.1 ± 1.2 m/s; P < 0.001). Shear wave speed after MVC had a strong correlation with the chamber stiffness constant β (r = 0.63; P < 0.001) and operating chamber stiffness dP/dV before induction of an I/R injury (r = 0.78; P < 0.001) and after (r = 0.83; P < 0.001).

CONCLUSIONS

Shear wave speed after MVC was influenced by both acute changes in loading and myocardial mechanical properties, reflecting changes in operating myocardial stiffness, and was strongly related to chamber stiffness, invasively derived by pressure-volume loop analysis. SWE provides a novel noninvasive method for the assessment of left ventricular myocardial properties.