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We present a novel, cost-efficient methodology to simulate aortic haemodynamics in a patient-specific, compliant aorta using an MRI data fusion process. Based on a previously-developed Moving Boundary Method, this technique circumvents the high computational cost and numerous structural modelling assumptions required by traditional Fluid-Structure Interaction techniques. Without the need for Computed Tomography (CT) data, the MRI images required to construct the simulation can be obtained during a single imaging session. Black Blood MR Angiography and 2D Cine-MRI data were used to reconstruct the luminal geometry and calibrate wall movement specifically to each region of the aorta. 4D-Flow MRI and non-invasive pressure measurements informed patient-specific inlet and outlet boundary conditions. Simulated wall movement closely matched 2D Cine-MRI measurements throughout the aorta, and physiological pressure and flow distributions in CFD were achieved within 3.3% of patient-specific targets. Excellent agreement with 4D-Flow MRI velocity data was observed. Conversely, a rigid-wall simulation under-predicted peak flow rate and systolic maximum velocities whilst predicting a mean Time-Averaged Wall Shear Stress (TAWSS) 13% higher than the compliant simulation. The excellent agreement observed between compliant simulation results and MRI is testament to the accuracy and efficiency of this MRI-based technique.

Type-B aortic dissection (TBAD) is a disease in which a tear develops in the intimal layer of the descending aorta forming a true lumen and false lumen (FL). Because disease outcomes are thought to be influenced by haemodynamic quantities such as pressure and wall shear stress (WSS), their analysis via numerical simulations may provide valuable clinical insights. Major aortic branches are routinely included in simulations but minor branches are virtually always neglected, despite being implicated in TBAD progression and the development of complications. As minor branches are estimated to carry about 7–21% of cardiac output, neglecting them may affect simulation accuracy. We present the first simulation of TBAD with all pairs of intercostal, subcostal and lumbar arteries, using 4D-flow MRI (4DMR) to inform patient-specific boundary conditions. Compared to an equivalent case without minor branches, their inclusion improved agreement with 4DMR velocities, reduced time-averaged WSS (TAWSS) and transmural pressure and elevated oscillatory shear in regions where FL dilatation and calcification were observed in vivo. Minor branch inclusion resulted in differences of 60-75% in these metrics of potential clinical relevance, indicating a need to account for minor branch flow loss if simulation accuracy is sought.

Type-B Aortic Dissection (TBAD) is a disease in which a tear develops in the intimal layer of the descending aorta forming a true lumen (TL) and false lumen (FL). Because disease outcomes are thought to be influenced by haemodynamic quantities such as pressure and Wall Shear Stress (WSS), their analysis via numerical simulations may provide valuable clinical insights. Major aortic branches are routinely included in simulations but minor branches are virtually always neglected, despite being implicated in TBAD progression and the development of complications. As minor branches are estimated to carry about 7-21% of cardiac output, neglecting them may affect simulation accuracy. We present the first simulation of TBAD with all pairs of intercostal, subcostal and lumbar arteries, using 4D-Flow MRI (4DMR) to inform patient-specific boundary conditions. Compared to an identical case without these branches, their inclusion improved agreement with 4DMR velocities, reduced Time-Averaged WSS (TAWSS) by up to 58\% and elevated oscillatory shear in regions where FL dilatation and calcification are observed in-vivo. Transmural pressure (TMP) was up to 61\% lower when minor branches were included. These effects indicate a need to account for minor branch flow loss if accuracy in clinically-relevant metrics is sought.

Type-B Aortic Dissection (TBAD) is cardiovascular disease in which a tear develops in the intimal layer of the descending aorta, allowing pressurised blood to delaminate the intimal and medial layers to form a true and false lumen. In medically managed patients, long-term aneurysmal dilatation of the false lumen is considered virtually inevitable and is associated with poorer disease outcomes. The pathophysiological mechanisms driving false lumen dilatation are not yet understood, though haemodynamic factors are believed to play a key role. Recent analysis via Computational Fluid Dynamics (CFD) and 4D-Flow MRI have demonstrated links between flow helicity, oscillatory wall shear stress and aneurysmal dilatation of the false lumen. In this study, we compare simulations using the gold-standard three-dimensional, three-component inlet velocity profile (3D IVP) extracted from 4DMR data with flow-matched flat and through-plane profiles that remain widely used in the absence of 4DMR. Furthermore, we assess the impact of 4DMR imaging errors on the flow solution by scaling the 3D IVP components to the degree of imaging error observed in previous studies. We demonstrate that secondary inlet flows affect the distribution of oscillatory shear and helicity throughout the FL, and that even the 3D IVP exhibited notable differences in helicity and oscillatory shear when modulated to account for imaging errors. These results illustrate that the quality of inlet velocity conditions in simulations of TBAD may greatly affect their clinical value, and efforts to further enhance their patient-specific accuracy are warranted.

IntroductionCompliance mismatch between the aortic wall and Dacron Grafts is a clinical problem concerning aortic haemodynamics and morphological degeneration. The aortic stiffness introduced by grafts can lead to an increased left ventricular (LV) afterload. This study quantifies the impact of compliance mismatch by virtually testing different Type-B aortic dissection (TBAD) surgical grafting strategies in patient-specific, compliant computational fluid dynamics (CFD) simulations.Materials and MethodsA post-operative case of TBAD was segmented from computed tomography angiography data. Three virtual surgeries were generated using different grafts; two additional cases with compliant grafts were assessed. Compliant CFD simulations were performed using a patient-specific inlet flow rate and three-element Windkessel outlet boundary conditions informed by 2D-Flow Magnetic Resonance Imaging (2DMRI) data. The wall compliance was calibrated using Cine-MRI images. Pressure, wall shear stress (WSS) indices and energy loss (EL) were computed.ResultsIncreased aortic stiffness and longer grafts increased aortic pressure and EL. Implementing a compliant graft matching the aortic compliance of the patient reduced the pulse pressure by 11% and EL by 4%. The endothelial cell activation potential (ECAP) differed the most within the aneurysm, where the maximum percentage difference between the reference case and the mid (MDA) and complete (CDA) descending aorta replacements increased by 16% and 20%, respectively.ConclusionThis study highlights the negative impact of increased graft length on LV condition after surgical aortic replacement in TBAD. To mitigate the associated risks to the patient, graft manufacturers should allocate more resources toward developing compliant biomimetic grafts.