Generation of personalized synthetic 3-dimensional inlet velocity profiles for computational fluid dynamics simulations of type B aortic dissection.

Computational fluid dynamics (CFD) simulations have shown promise in assessing type B aortic dissection (TBAD) to predict disease progression, and inlet velocity profiles (IVPs) are essential for such simulations. To truly capture patient-specific hemodynamic features, 3D IVPs extracted from 4D-flow magnetic resonance imaging (4D MRI) should be used, but 4D MRI is not commonly available. A new workflow was devised to generate personalized synthetic 3D IVPs that can replace 4D MRI-derived IVPs in CFD simulations. Based on 3D IVPs extracted from 4D MRI of 33 TBAD patients, statistical shape modelling and principal component analysis were performed to generate 270 synthetic 3D IVPs accounting for specific flow features. The synthetic 3D IVPs were then scaled and fine-tuned to match patient-specific stroke volume and systole-to-diastole ratio. The performance of personalized synthetic IVPs in CFD simulations was evaluated against patient-specific IVPs and compared with parabolic and flat IVPs. Our results showed that the synthetic 3D IVP was sufficient for faithful reproduction of hemodynamics throughout the aorta. In the ascending aorta (AAo), where non-patient-specific IVPs failed to replicate in vivo flow features in previous studies, the personalized synthetic IVP was able to match not only the flow pattern but also time-averaged wall shear stress (TAWSS), with a mean TAWSS difference of 5.9 %, which was up to 36.5 % by idealized IVPs. Additionally, the predicted retrograde flow index in both the AAo (8.36 %) and descending aorta (8.17 %) matched closely the results obtained with the 4D MRI-derived IVP (7.36 % and 6.55 %). The maximum false lumen pressure difference was reduced to 11.6 % from 68.8 % by the parabolic IVP and 72.6 % by the flat IVP. This study demonstrates the superiority of personalized synthetic 3D IVPs over commonly adopted parabolic or flat IVPs and offers a viable alternative to 4D MRI-derived IVP for CFD simulations of TBAD.

Wang K ，Armour CH ，Hanna L ，Gibbs R ，Xu XY ... -  《-》

A combined 4D flow MR imaging and fluid-structure interaction analysis of ascending thoracic aortic aneurysms.

This study aimed to characterize the altered hemodynamics and wall mechanics in ascending thoracic aortic aneurysms (ATAA) by employing fully coupled two-way fluid-structure interaction (FSI) analyses. Our FSI models incorporated hyperelastic wall mechanical properties, prestress, and patient-specific inlet velocity profiles (IVP) extracted from 4D flow magnetic resonance imaging (MRI). By performing FSI analyses on 7 patient-specific ATAA models and 6 healthy aortas, the primary objective of the study was to compare hemodynamic and biomechanical features in ATAA versus healthy controls. A secondary objective was to examine the need for 4D flow MRI-derived IVP in FSI simulations by comparing results with those using two commonly adopted idealized IVPs: Flat-IVP and Para-IVP for selected cases. Our results show that, compared to the healthy aortas, the ATAA models exhibited highly disturbed blood flow in the ascending aorta. Consequently, maximum turbulent kinetic energy (TKE) at peak systole (155.0 ± 188.4 Pa) and maximum time-averaged wall shear stress (TAWSS) (8.6 ± 6.5 Pa) were significantly higher in the ATAA cohort, compared to 0.6 ± 0.5 Pa and 2.8 ± 0.7 Pa in the healthy aortas. Peak wall stress was also nearly doubled in the ATAA group (414 ± 108 kPa vs. 215 ± 31 kPa). Additionally, comparisons of simulation results across models with different IVPs underscore the importance of prescribing 3D-IVP at the inlet, especially for ATAA cases. Using idealized IVPs in two selected ATAA models (P1 and P7) substantially reduced the maximum TKE from 571 Pa to 0.01 Pa (Flat-IVP) and 0.02 Pa (Para-IVP) in P1 and from 73 Pa to 0.01 Pa (Flat-IVP) and 0.08 Pa (Para-IVP) in P7, while the maximum TAWSS in the ascending aorta decreased from 9.6 Pa to 0.7 Pa (Flat-IVP) and 0.9 Pa (Para-IVP) in P1, and from 3.6 Pa to 1.2 Pa and 0.9 Pa, respectively, in P7. Moreover, idealized IVPs also caused the peak wall stress to reduce by up to 11.5% in P1 with severe aortic valve stenosis, and by up to 2% in P7 with mild aortic regurgitation. These results highlight the importance of FSI simulations combined with 4D flow MRI in capturing realistic hemodynamic and biomechanical changes in aneurysmal aortas.

Zhu Y ，Armour C ，Li B ，Pirola S ，Salmasi Y ，Athanasiou T ，O'Regan DP ，Xu XY ... -  《-》

Interval changes in four-dimensional flow-derived in vivo hemodynamics stratify aortic growth in type B aortic dissection patients.

Aortic diameter growth in type B aortic dissection (TBAD) is associated with progressive aortic dilation, resulting in increased mortality in patients with both de novo TBAD (dnTBAD) and residual dissection after type A dissection repair (rTAAD). Preemptive thoracic endovascular aortic repair may improve mortality in patients with TBAD, although it is unclear which patients may benefit most from early intervention. In vivo hemodynamic assessment using four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) has been used to characterize TBAD patients with growing aortas. In this longitudinal study, we investigated whether changes over time in 4D flow-derived true and false lumen (TL and FL) hemodynamic parameters correlate with aortic growth rate, which is a marker of increased risk. We retrospectively identified TBAD patients with baseline and follow-up 4D flow CMR at least 120 days apart. Patients with TBAD intervention before baseline or between scans were excluded. 4D flow CMR data analysis included segmentation of the TL and FL, followed by voxel-wise calculation of TL and FL total kinetic energy (KE), maximum velocity (MV), mean forward flow (FF), and mean reverse flow (RF). Changes over time (Δ) were calculated for all hemodynamic parameters. Maximal diameter in the descending aorta was measured from magnetic resonance angiogram images acquired at the time of 4D flow. Aortic growth rate was defined as the change in diameter divided by baseline diameter and standardized to scan interval. Thirty-two patients met inclusion criteria (age: 56.9 ± 14.1 years, female: 13, n = 19 rTAAD, n = 13 dnTBAD). Mean follow-up time was 538 days (range: 135-1689). Baseline aortic diameter did not correlate with growth rate. In the entire cohort, Δ FL MV (Spearman's rho [rho] = 0.37, p = 0.04) and Δ FL RF (rho = 0.45, p = 0.01) correlated with growth rate. In rTAAD only, Δ FL MV (rho = 0.48, p = 0.04) and Δ FL RF (rho = 0.51, p = 0.03) correlated with growth rate, while in dnTBAD only, Δ TL KE (rho = 0.63, p = 0.02) and Δ TL MV (rho = 0.69, p = 0.01) correlated with growth rate. 4D flow-derived longitudinal hemodynamic changes correlate with aortic growth rate in TBAD and may provide additional prognostic value for risk stratification. 4D flow MRI could be integrated into existing imaging protocols to allow for the identification of TBAD patients who would benefit from preemptive surgical or endovascular intervention.

Engel J ，Kilinc O ，Weiss E ，Baraboo J ，Mehta C ，Hoel A ，Malaisrie SC ，Markl M ，Allen BD ... -  《-》

Patient-specific compliant simulation framework informed by 4DMRI-extracted pulse wave Velocity: Application post-TEVAR.

We introduce a new computational framework that utilises Pulse Wave Velocity (PWV) extracted directly from 4D flow MRI (4DMRI) to inform patient-specific compliant computational fluid dynamics (CFD) simulations of a Type-B aortic dissection (TBAD), post-thoracic endovascular aortic repair (TEVAR). The thoracic aortic geometry, a 3D inlet velocity profile (IVP) and dynamic outlet boundary conditions are derived from 4DMRI and brachial pressure patient data. A moving boundary method (MBM) is applied to simulate aortic wall displacement. The aortic wall stiffness is estimated through two methods: one relying on area-based distensibility and the other utilising regional pulse wave velocity (RPWV) distensibility, further fine-tuned to align with in vivo values. Predicted pressures and outlet flow rates were within 2.3 % of target values. RPWV-based simulations were more accurate in replicating in vivo hemodynamics than the area-based ones. RPWVs were closely predicted in most regions, except the endograft. Systolic flow reversal ratios (SFRR) were accurately captured, while differences above 60 % in in-plane rotational flow (IRF) between the simulations were observed. Significant disparities in predicted wall shear stress (WSS)-based indices were observed between the two approaches, especially the endothelial cell activation potential (ECAP). At the isthmus, the RPWV-driven simulation indicated a mean ECAP>1.4 Pa-1 (critical threshold), indicating areas potentially prone to thrombosis, not captured by the area-based simulation. RPWV-driven simulation results agree well with 4DMRI measurements, validating the proposed pipeline and facilitating a comprehensive assessment of surgical decision-making scenarios and potential complications, such as thrombosis and aortic growth.

Girardin L ，Lind N ，von Tengg-Kobligk H ，Balabani S ，Díaz-Zuccarini V ... -  《-》

Multiyear Interval Changes in Aortic Wall Shear Stress in Patients with Bicuspid Aortic Valve Assessed by 4D Flow MRI.

In patients with bicuspid aortic valve (BAV), 4D flow MRI can quantify regions exposed to abnormal aortic hemodynamics, including high wall shear stress (WSS), a known stimulus for arterial wall dysfunction. However, the long-term multiscan reproducibility of 4D flow MRI-derived hemodynamic parameters is unknown. To investigate the long-term stability of 4D flow MRI-derived peak velocity, WSS, and WSS-derived heatmaps in patients with BAV undergoing multiyear surveillance imaging. Retrospective. 20 BAV patients (mean age 48.4 ± 13.9 years; 14 males) with five 4D flow MRI scans, with intervals of at least 6 months between scans, and 125 controls (mean age: 50.7 ± 15.8 years; 67 males). 1.5 and 3.0T, prospectively ECG and respiratory navigator-gated aortic 4D flow MRI. Automated AI-based 4D flow analysis pipelines were used for data preprocessing, aorta 3D segmentation, and quantification of ascending aorta (AAo) peak velocity, peak systolic WSS, and heatmap-derived relative area of elevated WSS compared to WSS ranges in age and sex-matched normative control populations. Growth rate was derived from the maximum AAo diameters measured on the first and fifth MRI scans. One-way repeated measures analysis of variance. P < 0.05 indicated significance. One hundred 4D flow MRI exams (five per patient) were analyzed. The mean total follow-up duration was 5.5 ± 1.1 years, and the average growth rate was 0.3 ± 0.2 mm/year. Peak velocity, peak systolic WSS, and relative area of elevated WSS did not change significantly over the follow-up period (P = 0.64, P = 0.69, and P = 0.35, respectively). The patterns and areas of elevated WSS demonstrated good reproducibility on semiquantitative assessment. 4D flow MRI-derived peak velocity, WSS, and WSS-derived heatmaps showed good multiyear and multiscan stability in BAV patients with low aortic growth rates. These findings underscore the reliability of these metrics in monitoring BAV patients for potential risk of dilation. 3 TECHNICAL EFFICACY: Stage 1.

Maroun A ，Scott MB ，Catania R ，Berhane H ，Jarvis K ，Allen BD ，Barker AJ ，Markl M ... -  《-》