Unveiling the heart dynamics using ultrafast ultrasound : from wave physics to clinical prospects

Abstract

Cardiovascular diseases (CVDs) remain a persistent and serious global health challenge, leading to a significant number of preventable premature deaths. The effective identification of risk factors for CVD prevention is thus of utmost importance. Echocardiography, a cost-effective, real-time, and portable imaging modality, is routinely used to assess cardiac function. Quantitative echocardiographic parameters, such as ejection fraction and global longitudinal strain, are clinically adopted for the evaluation of both systolic and diastolic heart function. However, these parameters do not directly measure myocardial contractility. In recent decades, shear wave imaging (SWI) based on acoustic radiation force has emerged as a noninvasive technique for measuring tissue stiffness, which offers valuable insights into internal forces within the myocardium, enabling a more direct assessment of cardiac systolic and diastolic functions. However, its application to the human heart faces challenges primarily due to the frame rate limited by the large imaging depth and acoustic force attenuation. Thus, the discovery of naturally occurring elastic waves has sparked interest, particularly the aortic valve closure (AVC) and mitral valve closure (MVC) waves. These waves have been explored as potential clinical tools for assessing myocardial stiffness. Nonetheless, AVC and MVC waves occur at the beginning of the isovolumetric period, during which myocardial stiffness undergoes significant changes. As a result, the velocities of AVC and MVC waves do not strictly represent end-systolic or end-diastolic myocardial mechanical properties. This thesis stems from our recent discovery of two vibrational waves during the late diastasis phase. This phase offers a better opportunity to evaluate myocardial passive stiffness compared to the AVC and MVC waves. Our motivation is to visualize, understand the origin and mechanism, and explore the clinical potential of these waves. This thesis first demonstrates the enhancement of heart wall vibration visualization by utilizing a randomized singular value decomposition (rSVD) filter to remove artifacts in vivo. The simulation platform developed in the thesis assesses the feasibility and reliability of various filters appropriate for echocardiography. Subsequently, we hypothesize that fluid-structure (i.e., blood-myocardium) interaction may contribute to the generation of natural waves in the heart. This leads to the extraction of blood signals using rSVD filter. For the first time in echocardiography, the robustness of rSVD filter is demonstrated. Results from 127 human subjects demonstrate high reproducibility of the observed vibrational waves among individuals. Furthermore, significant differences in wave velocities between healthy volunteers and hypertension patients suggest the potential of these vibrational waves in effectively assessing myocardial stiffness. This thesis further challenges the prevailing assumption that natural waves originate exclusively from specific sources by employing ω-k space analysis, revealing the multi-directionality of natural vibrational waves. Our findings provide unprecedented evidence that vibrational waves could potentially be used to detect and map myocardial structural anisotropy (i.e., fiber orientation) within the heart wall. The thesis has implications for detecting the onset of myocardial ischemia and advancing the study of fluid-structure interaction in the heart. It explores the physics of intrinsic events and their clinical prospects, offering new insights into unanswered research questions.