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

Abstract

Shear wave elastography (SWE) is a potentially valuable tool to noninvasively assess ventricular function in children with cardiac disorders, which could help in the early detection of abnormalities in muscle characteristics. Initial experiments demonstrated the potential of this technique in measuring ventricular stiffness; however, its performance remains to be validated as complicated shear wave (SW) propagation characteristics are expected to arise due to the complex non-homogenous structure of the myocardium. In this work, we investigated the (i) accuracy of different shear modulus estimation techniques (time-of-flight (TOF) method and phase velocity analysis) across myocardial thickness and (ii) effect of the ventricular geometry, surroundings, acoustic loading, and material viscoelasticity on SW physics. A generic pediatric (10-15-year old) left ventricular model was studied numerically and experimentally. For the SWE experiments, a polyvinylalcohol replicate of the cardiac geometry was fabricated and SW acquisitions were performed on different ventricular areas using varying probe orientations. Additionally, the phantom's stiffness was obtained via mechanical tests. The results of the SWE experiments revealed the following trends for stiffness estimation across the phantom's thickness: a slight stiffness overestimation for phase speed analysis and a clear stiffness underestimation for the TOF method for all acquisitions. The computational model provided valuable 3-D insights in the physical factors influencing SW patterns, especially the surroundings (water), interface force, and viscoelasticity. In conclusion, this paper presents a validation study of two commonly used shear modulus estimators for different ventricular locations and the essential role of SW modeling in understanding SW physics in the pediatric myocardium.

Effect of Ultrafast Imaging on Shear Wave Visualization and Characterization: An Experimental and Computational Study in a Pediatric Ventricular Model

“…Further details on this phantom can be found in a recent publication from our group [23]. Shear waves were generated and imaged by a SL15-4 linear transducer with 256 elements, a pitch of 200 µm and an elevation focus of~30 mm, connected to the Aixplorer system (SuperSonic Imagine, Aix-en-Provence, France).…”

“…The modeling methodology for the pushing and imaging sequence is concisely described below. The reader is referred to [23] for further details about the pushing sequence, comprising the first two modeling parts. …”

“…Volume force b and interface pressure π are calculated based on the time-averaged acoustic intensity I, of which its spatial distribution is derived by simulating acoustic probe pressures mimicking the push sequence (see Table 1) with Field II and its magnitude is scaled to 1500 W/cm 2 [26], as follows [21,27]: Table 2. The PVA's Young's modulus and viscoelastic behavior were mechanically determined on a uniaxial tensile testing machine (Instron 5944, Norwood, MA, USA), whereas its density and speed of sound were measured using the principle of Archimedes [28] and an oscilloscope respectively (for more details on all measurements, we refer to [23]). The resulting spatial distribution of the volume force in the axial-lateral plane is shown in the bottom-left panel of Figure 2.…”

“…The multiphysics model was employed in combination with SWE experiments, for validation purposes. This combined approach was applied on an idealized left ventricular phantom model with pediatric dimensions, as this has been demonstrated to evoke dispersive guided wave propagation patterns due to left ventricular geometry [23]. The proposed multiphysics model in this work thus adds an extra modeling layer to the previously presented SWE biomechanics model in [23], expanding our scope from studying the effect of biomechanical factors on shear wave physics to investigating the effect of imaging factors on shear wave physics.…”

“…This combined approach was applied on an idealized left ventricular phantom model with pediatric dimensions, as this has been demonstrated to evoke dispersive guided wave propagation patterns due to left ventricular geometry [23]. The proposed multiphysics model in this work thus adds an extra modeling layer to the previously presented SWE biomechanics model in [23], expanding our scope from studying the effect of biomechanical factors on shear wave physics to investigating the effect of imaging factors on shear wave physics. Our objective can be translated into two main study questions: (i) study the effect of compounding through comparison of single and compounded plane wave acquisitions from SWE experiments, for which more in-depth insights are realized by modeling both acquisitions using the multiphysics methodology, and (ii) study the effect of ultrafast imaging by analyzing the mechanical versus imaged shear wave acquisitions in the simulations.…”