Abstract:
Background: Abnormal intracardiac blood flow patterns may exacerbate ventricular remodeling leading to adverse cardiac outcomes. Four-dimensional flow cardiac magnetic resonance imaging (4D flow) enables study of a spectrum of flow features, including vorticity and its impact on cardiac architecture. Right ventricular remodeling in repaired tetralogy of Fallot adults (rToF) occurs in response to volume overload created by chronic pulmonary regurgitation (PR). This group is regularly followed up by MRI to determine optimal timing of interventions. Although rToF patients are routinely evaluated using 2D slice-based imaging, 4D flow MRI has been suggested to be an accurate, non-invasive, and faster evaluation with the potential to replace the current multi-slice imaging protocols. Moreover, it introduces novel parameters such as vorticity. Cardiac atlases can quantify heart shapes as morphometric scores, to identify relationships between shape and clinical factors. Exploring the relationship between vorticity and shape variations remains uncharted.
Aim: This thesis aimed to examine the 4D flow MRI acquisition and its applicability in the clinical MRI service at Auckland Hospital. Our proposition was to enhance the existing clinical scan protocol with 4D flow, perform external and internal validation of 4D flow MRI against the current workflow and explore right ventricular vorticity across patient groups. We also aimed to correlate and quantify relationships between vorticity and shape derived from atlas-based analysis of biventricular shape.
Methods: Controls (n=15), rToF (n=15) and left ventricular hypertrophy (LVH, n=15) groups were scanned between January 2019 and February 2020. A systematic review of studies that examined rToF groups with 4D Flow MRI was performed. The method validation included examination of corrections and comparison with current volumetric analysis data. Internal validation was also carried out, and qualitative analysis and visualisation were performed with Siemens 4D flow software. The impact on clinical decision-making based on pulmonary artery system categorisation was examined. Finally, intraventricular vorticity was examined with comparison across groups and correlation with cardiac function parameters and atlas-based principal component analysis scores on a biventricular shape atlas.
Results: 4D flow MRI showed high correlation with 2D flow, and higher internal consistency than current 2D flow. Decisions based on pulmonary regurgitation volumes showed excellent agreement with 2D evaluation. 4D flow also showed faster workflow, and faster acquisition time (an average of 8 minutes with quiet breathing). Right ventricular outflow tract vorticity demonstrated high correlation with pulmonary regurgitation and flow rates, with distinctive visual qualities across groups of different pathologies. Vorticity shape correlation also gave an insight on relationships between flow and the architecture of the right ventricle in rToF.
Conclusion: 4D flow MRI is ready for routine integration in the clinical workflow, enabling replacement of multi-slice 2D acquisitions to expedite workflow, and enables evaluation of the relationships between flow and remodeling.