PhD Oral Exam - Wissam Abdallah, Mechanical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Aortic regurgitation (AR) increases the hemodynamic load on the left ventricle (LV), altering blood transport, energy balance, and intracardiac forces. Conventional clinical assessment relies primarily on regurgitant fraction (RF), which provides a global severity estimate but fails to capture the complex flow and mechanical alterations underlying disease progression. Four-dimensional flow magnetic resonance imaging (4D-flow MRI) has emerged as a powerful tool to non-invasively assess cardiovascular hemodynamics more than standard volumetric measures. This thesis integrates flow visualization, energetic quantification, and force analysis techniques to comprehensively characterize LV hemodynamics in patients affected by AR. First, quantitative analyses of kinetic energy (KE), vorticity (VRT), and viscous energy dissipation (VED) revealed significant elevations in AR patients compared to healthy controls, with strong correlations to RF, highlighting their potential as complementary markers of disease severity. Second, hemodynamic force (HDF) analysis identified increased heterogeneity and directional imbalance in AR patients, with enhanced transverse forces and reduced longitudinal dominance during both systole and diastole. Finally, three-dimensional Lagrangian coherent structures (LCS) were extracted for the first time in patients with AR, demonstrating that healthy LV flow is organized into coherent mitral jet structures that promote efficient systolic ejection, whereas AR patients exhibit complex jet interactions that disrupt blood transport. Together, these studies provide new insights into the impact of AR on LV flow dynamics, energetics, and force distribution. By combining advanced imaging biomarkers with mechanistic analyses, this work underscores the potential of flow-based techniques combined with 4D-flow MRI to improve the non-invasive evaluation and stratification of patients with AR.