Investigating Shear Wave Physics in a Generic Pediatric Left Ventricular Model via In Vitro Experiments and Finite Element Simulations

Simulation of stress in a blood vessel due to plaque sediments in coronary artery disease

Atherosclerosis is a cardiovascular disease mainly caused by plaque deposition in blood vessels. Plaque comprises components such as thrombosis, fibrin, collagen, and lipid core. It plays an essential role in inducing rupture in a blood vessel. Generally, Plaque could be described as three kinds of elastic models: cellular Plaque, hypocellular Plaque, and calcified Plaque. The present study aimed to investigate the behavior of atherosclerotic plaque rupture according to different lipid cores using Fluid-Structure Interaction (FSI). The blood vessel was also varied with different thicknesses (0.05, 0.25, and 0.5 mm). In this study, FSI simulation with a cellular plaque model with various thicknesses was investigated to obtain information on plaque rupture. Results revealed that the blood vessel with Plaque having a lipid core represents higher stresses than those without a lipid core. Blood vessels' thin thickness, like a thin cap, results in more considerable than Von Mises stress. The result also suggests that even at low fracture stress, the risk of rupture due to platelet decomposition at the gap was more significant for cellular plaques.

Ultrasound Shear Wave Elastography in Cardiology

The advent of high-frame rate imaging in ultrasound allowed the development of shear wave elastography as a noninvasive alternative for myocardial stiffness assessment. It measures mechanical waves propagating along the cardiac wall with speeds that are related to stiffness. The use of cardiac shear wave elastography in clinical studies is increasing, but a proper understanding of the different factors that affect wave propagation is required to correctly interpret results because of the heart's thin-walled geometry and intricate material properties. The aims of this review are to give an overview of the general concepts in cardiac shear wave elastography and to discuss in depth the effects of age, hemodynamic loading, cardiac morphology, fiber architecture, contractility, viscoelasticity, and system-dependent factors on the measurements, with a focus on clinical application. It also describes how these factors should be considered during acquisition, analysis, and reporting to ensure an accurate, robust, and reproducible measurement of the shear wave.

Advanced Imaging Technologies for Assessing Tetralogy of Fallot: Insights Into Flow Dynamics

Tetralogy of Fallot is the most common cyanotic congenital heart defect requiring surgical repair. Although surgical interventions have significantly reduced mortality, postrepair complications, such as pulmonary valve regurgitation and stenosis, may lead to adverse outcomes, including right ventricular dysfunction and increased risks of morbidity and mortality. This review explores the potential of advanced imaging technologies, including 4-dimensional-flow magnetic resonance imaging and high-frame-rate echocardiography, in providing valuable insights into blood flow dynamics and energy parameters. Quantitative measures, such as energy loss and vorticity, along with qualitative flow analysis, can provide additional insights into adverse haemodynamics at a potentially earlier and more reversible stage. Furthermore, personalized patient-specific information from these imaging modalities aids in guiding treatment decisions and monitoring postoperative interventions effectively. By characterizing flow patterns, these advanced imaging techniques hold great promise in improving the assessment and management of tetralogy of Fallot, providing tailored insights. However, further research and longitudinal studies are required to fully establish their clinical utility and potential impact on patient care.

Impact of Ventricular Geometric Characteristics on Myocardial Stiffness Assessment Using Shear-Wave Velocity in Healthy Children and Young Adults

BACKGROUND

Diastolic myocardial stiffness (MS) can serve as a key diagnostic parameter for congenital or acquired heart diseases. Using shear modulus and shear-wave velocity (SWV), shear-wave elastography (SWE) is an emerging ultrasound-based technique that can allow noninvasive assessment of MS. However, MS extrinsic parameters such as left ventricular geometric characteristics could affect shear-wave propagation. The aims of this study were to determine a range of normal values of MS using SWE in age groups of healthy children and young adults and to explore the impact of left ventricular geometric characteristics on SWE.

METHODS

Sixty healthy volunteers were recruited in the study and divided into 2 groups: neonates (0-1 months old, n = 15) and >1 month old (1 month to 45 years of age, n = 45). SWE was performed using the Verasonics Vantage systems with a phased-array ultrasound probe. The anteroseptal basal segment was assessed in two views. SWE was electrocardiographically triggered during the end-diastolic phase. Conventional echocardiography was performed to assess ventricular function and anatomy. Results are presented as stiffness values along with mean velocity measurements and SDs. Simple and multivariate linear regression analyses were performed.

RESULTS

For neonates, mean MS was 1.87 ± 0.79 kPa (range, 0.59-2.91 kPa; mean SWV, 1.37 ± 0.57 m/sec), with high variability and no correlation with age (P = .239). For this age group, no statistically significant correlation was found between MS and any demographic or echocardiographic parameters (P > .05). For the >1 month old group, a mean MS value of 1.67 ± 0.53 kPa was observed (range, 0.6-3 kPa; mean SWV, 1.29 ± 0.49 m/sec) for healthy volunteers. When paired for age, no sex-related difference was observed (P = .55). In univariate linear regression analysis, age (r = 0.83, P < .01), diastolic interventricular septal thickness (r = 0.72, P < .01), and left ventricular end-diastolic diameter (r = 0.67, P < .01) were the parameters with the highest correlation coefficients with MS. In a multiple linear regression analysis incorporating these three parameters as cofounding factors, age was the only statistically significant parameters (r = 0.81, P = .02).

CONCLUSION

Diastolic MS increases linearly in children and young adults. Diastolic MS correlates more robustly with age than with myocardial and left ventricular geometric characteristics. However, the geometry affects SWV, implying the need to determine well-established boundaries in future studies for the clinical application of SWE.

Comparing Myocardial Shear Wave Propagation Velocity Estimation Methods Based on Tissue Displacement, Velocity and Acceleration Data

Shear wave elastography (SWE) is a promising technique used to assess cardiac function through the evaluation of cardiac stiffness non-invasively. However, in the literature, SWE varies in terms of tissue motion data (displacement, velocity or acceleration); method used to characterize mechanical wave propagation (time domain [TD] vs. frequency domain [FD]); and the metric reported (wave speed [WS], shear or Young's modulus). This variety of reported methodologies complicates comparison of reported findings and sheds doubt on which methodology better approximates the true myocardial properties. We therefore conducted a simulation study to investigate the accuracy of various SWE data analysis approaches while varying cardiac geometry and stiffness. Lower WS values were obtained by the TD method compared with the FD method. Acceleration-based WS estimates in the TD were systematically larger than those based on velocity (∼10% difference). These observations were confirmed by TD analysis of 32 in vivo SWE mechanical wave measurements. In vivo data quality is typically too low for accurate FD analysis. Therefore, our study suggests using acceleration-based TD analysis for in vivo SWE to minimize underestimation of the true WS and, thus, to maximize the sensitivity of SWE to detect stiffness changes resulting from pathology.

Error analysis and reliability of zero-order Lamb mode inversion for waveguide characterization

In recent years, several fitting techniques have been presented to reconstruct the parameters of a plate from its Lamb wave dispersion curves. Published studies show that these techniques can yield high accuracy results and have the potential of reconstructing several parameters at once. The precision with which parameters can be reconstructed by inverting Lamb wave dispersion curves, however, remains an open question of fundamental importance to many applications. In this work, we introduce a method of analyzing dispersion curves that yields quantitative information on the precision with which the parameters can be extracted. In our method, rather than employing error minimization algorithms, we compare a target dispersion curve to a database of theoretical ones that covers a given parameter space. By calculating a measure of dissimilarity (error) for every point in the parameter space, we reconstruct the distribution of the error in that space, beside the location of its minimum. We then introduce dimensionless quantities that describe the distribution of this error, thus yielding information about the spread of similar curves in the parameter space. We demonstrate our approach by considering both idealized and realistic scenarios, analyzing the dispersion curves obtained numerically for a plate and experimentally for a pipe. Our results show that the precision with which each parameter is reconstructed depends on the mode used, as well as the frequency range in which it is considered.

Assessing cardiac stiffness using ultrasound shear wave elastography

Shear wave elastography offers a new dimension to echocardiography: it measures myocardial stiffness. Therefore, it could provide additional insights into the pathophysiology of cardiac diseases affecting myocardial stiffness and potentially improve diagnosis or guide patient treatment. The technique detects fast mechanical waves on the heart wall with high frame rate echography, and converts their propagation velocity into a stiffness value. A proper interpretation of shear wave data is required as the shear wave interacts with the intrinsic, yet dynamically changing geometrical and material characteristics of the heart under pressure. This dramatically alters the wave physics of the propagating wave, demanding adapted processing methods compared to other shear wave elastography applications as breast tumor and liver stiffness staging. Furthermore, several advanced analysis methods have been proposed to extract supplementary material features such as viscosity and anisotropy, potentially offering additional diagnostic value. This review explains the general mechanical concepts underlying cardiac shear wave elastography and provides an overview of the preclinical and clinical studies within the field. We also identify the mechanical and technical challenges ahead to make shear wave elastography a valuable tool for clinical practice.

Numerical wave speed sensitivity study for assessment of myocardial elasticity in a simplified linear elastic and isotropic left ventricle model

Since tissue elasticity can change with pathology, noninvasive assessment of elasticity has received increasing attention. Emerging methods for assessing cardiac elasticity utilize either an external source to induce propagating shear waves or intrinsic longitudinal waves created by natural cardiac events such as left ventricle stretching that occurs due to atrial kick during late diastole. However, the effect of morphological variations that occur in diseased hearts on this longitudinal stretch wave and the corresponding estimate of elasticity is not well understood and is an active area of research. This study investigated the sensitivity of longitudinal wave speed to material properties and chamber geometry parameters through numerical simulations using a finite element model of a bullet-shaped chamber with homogeneous isotropic linear elastic material properties. A longitudinal impulse displacement was applied to the base edge of the model to investigate wave propagation from this boundary. Parametric studies were performed for variables of interest related to geometry and material properties. The wave speeds estimated from simulation results were used to determine wave speed sensitivity to each variable. Wave speed was found to be a strong function of material elasticity and a weak function of chamber geometry and viscous damping. Simulated wave speed as a function of elasticity was in good agreement with wave speeds determined from an analytical expression for longitudinal wave speed in elastic thin plates. These promising preliminary results increase our understanding of how these parameters affect intrinsic longitudinal wave speed and warrant future studies addressing the impact of patient-specific model geometry, material anisotropy and hyperelasticity, and boundary conditions on wave speed.

Tapering of the interventricular septum can affect ultrasound shear wave elastography: An in vitro and in silico study

Shear wave elastography (SWE) has the potential to determine cardiac tissue stiffness from non-invasive shear wave speed measurements, important, e.g., for predicting heart failure. Previous studies showed that waves traveling in the interventricular septum (IVS) may display Lamb-like dispersive behaviour, introducing a thickness-frequency dependency in the wave speed. However, the IVS tapers across its length, which complicates wave speed estimation by introducing an additional variable to account for. The goal of this work is to assess the impact of tapering thickness on SWE. The investigation is performed by combining in vitro experiments with acoustic radiation force (ARF) and 2D finite element simulations, to isolate the effect of the tapering curve on ARF-induced and natural waves in the heart. The experiments show a 11% deceleration during propagation from the thick to the thin end of an IVS-mimicking tapered phantom plate. The numerical analysis shows that neglecting the thickness variation in the wavenumber-frequency domain can introduce errors of more than 30% in the estimation of the shear modulus, and that the exact tapering curve, rather than the overall thickness reduction, determines the dispersive behaviour of the wave. These results suggest that septal geometry should be accounted for when deriving cardiac stiffness with SWE.

Analysis of multiple shear wave modes in a nonlinear soft solid: Experiments and finite element simulations with a tilted acoustic radiation force

Tissue nonlinearity is conventionally measured in shear wave elastography by studying the change in wave speed caused by the tissue deformation, generally known as the acoustoelastic effect. However, these measurements have mainly focused on the excitation and detection of one specific shear mode, while it is theoretically known that the analysis of multiple wave modes offers more information about tissue material properties that can potentially be used to refine disease diagnosis. This work demonstrated proof of concept using experiments and finite element simulations in a uniaxially stretched phantom by tilting the acoustic radiation force excitation axis with respect to the material's symmetry axis. Using this unique set-up, we were able to visualize two propagating shear wave modes across the stretch direction for stretches larger than 140%. Complementary simulations were performed using material parameters determined from mechanical testing, which enabled us to convert the observed shear wave behavior into a correct representative constitutive law for the phantom material, i.e. the Isihara model. This demonstrates the potential of measuring shear wave propagation in combination with shear wave modeling in complex materials as a non-invasive alternative for mechanical testing.

An in silico framework to analyze the anisotropic shear wave mechanics in cardiac shear wave elastography

Shear wave elastography (SWE) is a potential tool to non-invasively assess cardiac muscle stiffness. This study focused on the effect of the orthotropic material properties and mechanical loading on the performance of cardiac SWE, as it is known that these factors contribute to complex 3D anisotropic shear wave propagation. To investigate the specific impact of these complexities, we constructed a finite element model with an orthotropic material law subjected to different uniaxial stretches to simulate SWE in the stressed cardiac wall. Group and phase speed were analyzed in function of tissue thickness and virtual probe rotation angle. Tissue stretching increased the group and phase speed of the simulated shear wave, especially in the direction of the muscle fiber. As the model provided access to the true fiber orientation and material properties, we assessed the accuracy of two fiber orientation extraction methods based on SWE. We found a higher accuracy (but lower robustness) when extracting fiber orientations based on the location of maximal shear wave speed instead of the angle of the major axis of the ellipsoidal group speed surface. Both methods had a comparable performance for the center region of the cardiac wall, and performed less well towards the edges. Lastly, we also assessed the (theoretical) impact of pathology on shear wave physics and characterization in the model. It was found that SWE was able to detect changes in fiber orientation and material characteristics, potentially associated with cardiac pathologies such as myocardial fibrosis. Furthermore, the model showed clearly altered shear wave patterns for the fibrotic myocardium compared to the healthy myocardium, which forms an initial but promising outcome of this modeling study.