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  • Journal article
    Sengupta S, Zhu Y, Hamady M, Xu Xet al., 2022,

    Evaluating the haemodynamic performance of endografts for complex aortic arch repair

    , Bioengineering, Vol: 9, ISSN: 2306-5354

    Thoracic endovascular aortic repair (TEVAR) of aortic aneurysms and dissections involving the arch has evolved over the last two decades. Compared to conventional surgical methods, endovascular repair offers a less invasive treatment option with lower risk and faster recovery. Endografts used in TEVAR vary in design depending on the procedure and application. Novel endografts (e.g., branched stent-graft) were developed to ensure perfusion of blood to the supra-aortic vessels, but their haemodynamic performance and long-term durability have not been adequately studied. This review focuses on the use of computational modelling to study haemodynamics in commercially available endografts designed for complex aortic arch repair. First, we summarise the currently adopted workflow for computational fluid dynamics (CFD) modelling, including geometry reconstruction, boundary conditions, flow models, and haemodynamic metrics of interest. This is followed by a review of recently (2010-present) published CFD studies on complex aortic arch repair, using both idealized and patient-specific models. Finally, we introduce some of the promising techniques that can be potentially applied to predict post-operative outcomes.

  • Journal article
    Manchester E, Pirola S, Salmasi MY, O'Regan D, Athanasiou T, Xu Xet al., 2022,

    Evaluation of computational methodologies for accurate prediction of wall shear stress and turbulence parameters in a patient-specific aorta

    , Frontiers in Bioengineering and Biotechnology, Vol: 10, ISSN: 2296-4185

    Background: Recent studies suggest that blood flow in main arteries is intrinsically disturbed, even under healthy conditions. Despite this, many computational fluid dynamics (CFD) analyses of aortic haemodynamics make the assumption of laminar flow, and best practices surroundingappropriate modelling choices are lacking. This study aims to address this gap by evaluating different modelling and post-processing approaches in simulations of a patient-specific aorta. Methods: Magnetic resonance imaging (MRI) and 4D flow MRI from a patient with aortic valve stenosis were used to reconstruct the aortic geometry and derive patient-specific inlet and outlet boundary conditions. Three different computational approaches were considered based on assumed laminar or assumed disturbed flow states including low-resolution laminar (LR-laminar),high-resolution laminar (HR-Laminar) and large-eddy simulation (LES). Each simulation was ran for 30 cardiac cycles and post-processing was conducted on either the final cardiac cycle, or using a phase-averaged approach which utilised all 30 simulated cycles. Model capabilities were evaluated in terms of mean and turbulence-based parameters. Results: All simulation types, regardless of post-processing approach could correctly predict velocity values and flow patterns throughout the aorta. Lower resolution simulations could not accurately predict gradient-derived parameters including wall shear stress and viscous energy loss (largest differences up to 44.6% and 130.3%, respectively), although phase-averagingthese parameters improved predictions. The HR-Laminar simulation produced more comparable results to LES with largest differences in wall shear stress and viscous energy loss parameters up to 5.1% and 11.6%, respectively. Laminar-based parameters were better estimated thanturbulence-based parameters.Conclusions: Our findings suggest that well-resolved laminar simulations can accurately predict many laminar-based parameters in disturbed flo

  • Journal article
    Gu B, Huang Y, Manchester E, Hughes A, Thom S, Chen R, Xu XYet al., 2022,

    Multiphysics modelling and simulation of thrombolysis via activated platelet-targeted nanomedicine

    , Pharmaceutical Research, Vol: 39, Pages: 41-56, ISSN: 0724-8741

    Purpose:This study establishes a multiphysics simulation platform for both conventional and targeted thrombolysis using tissue plasminogen activator (tPA). Based on our computational results, the effects of therapeutic parameters on the dynamics of thrombolysis and the risk of side effects are investigated.Methods:The model extends our previously developed one-dimensional(1D) mathematical models for fibrinolysis by incorporating targeted thrombolysis. It consists of two parts: (i) a coupled mathematical model of systemic pharmacokinetics (PK) and pharmacodynamics (PD) and local PD in a 1D occluded artery, and (ii) a mechanistic model for a targeted thrombolytic system via activated platelet-targeted tPA-loaded nanovesicles (tPA-NV), with model parameters derived from our in vitro experiments. A total of 16 therapeutic scenarios are simulated by varying the clot location and composition as well as the dosing regimen with free tPA or tPA-NV.Results:Our simulation results indicate that tPA-NV offers several advantages over free tPA for thrombolysis. It reduces systemic exposure of tPA, thereby minimising the risk of bleeding complications. Simulations with different tPA-NV doses reveal that tPA-NV at 10% of the recommended dose can be as effective as the standard regimen with the full recommended dose of free tPA, demonstrating the potential of our tPA-NV as a new thrombolytic strategy with a reduced tPA dose. Moreover, faster recanalisation can be achieved with tPA-NV, especially for platelet-rich(or fibrin-poor) clots.Conclusions:Our simulation platform for thrombolysis with well-tuned model parameters can be used to evaluate and optimise treatment regimens of existing and new thrombolytic therapies via benefit/risk assessment under various therapeutic scenarios.

  • Journal article
    Sengupta S, Hamady M, Xu X-Y, 2022,

    Haemodynamic analysis of branched endografts for complex aortic arch repair

    , Bioengineering, Vol: 9, Pages: 1-17, ISSN: 2306-5354

    This study aims to investigate the haemodynamic response induced by implantation of a double-branched endograft used in thoracic endovascular aortic repair (TEVAR) of the aortic arch. Anatomically realistic models were reconstructed from CT images obtained from patients who underwent TEVAR using the RelayPlus double-branched endograft implanted in the aortic arch. Two cases (Patient 1, Patient 2) were included here, both patients presented with type A aortic dissection before TEVAR. To examine the influence of inner tunnel branch diameters on localised flow patterns, three tunnel branch diameters were tested using the geometric model reconstructed for Patient 1. Pulsatile blood flow through the models was simulated by numerically solving the Navier–Stokes equations along with a transitional flow model. The physiological boundary conditions were imposed at the model inlet and outlets, while the wall was assumed to be rigid. Our simulation results showed that the double-branched endograft allowed for the sufficient perfusion of blood to the supra-aortic branches and restored flow patterns expected in normal aortas. The diameter of tunnel branches in the device plays a crucial role in the development of flow downstream of the branches and thus must be selected carefully based on the overall geometry of the vessel. Given the importance of wall shear stress in vascular remodelling and thrombus formation, longitudinal studies should be performed in the future in order to elucidate the role of tunnel branch diameters in long-term patency of the supra-aortic branches following TEVAR with the double-branched endograft.

  • Journal article
    Manchester E, Roi D, Gu B, Xu X, Lobotesis Ket al., 2021,

    Modelling combined intravenous thrombolysis and mechanical thrombectomy in acute ischaemic stroke: Understanding the relationship between stent retriever configuration and clot lysis mechanisms

    , Life, Vol: 11, ISSN: 2075-1729

    Background: Combined intravenous thrombolysis and mechanical thrombectomy (IVT-MT) is a common treatment in acute ischaemic stroke, however the interaction between IVT and MT from a physiological standpoint is poorly understood. In this pilot study, we conduct numerical simulations of combined IVT-MT with various idealised stent retriever configurations to evaluate performance in terms of complete recanalisation times and lysis patterns. Methods: A 3D patient-specific geometry of a terminal internal carotid artery with anterior and middle cerebral arteries is reconstructed, and a thrombus is artificially implanted in the MCA branch. Various idealised stent retriever configurations are implemented by varying stent diameter and stent placement, and a configuration without a stent retriever provides a baseline for comparison. A previously validated multi-level model of thrombolysis is used, which incorporates blood flow, drug transport, and fibrinolytic reactions within a fibrin thrombus. Results: Fastest total recanalisation was achieved in the thrombus without a stent retriever, with lysis times increasing with stent retriever diameter. Two mechanisms of clot lysis were established: axial and radial permeation. Axial permeation from the clot front was the primary mechanism of lysis in all configurations, as it facilitated increased protein binding with fibrin fibres. Introducing a stent retriever channel allowed for radial permeation, which occurred at the fluid-thrombus interface, although lysis was much slower in the radial direction because of weaker secondary velocities. Conclusions: Numerical models can be used to better understand the complex physiological relationship between IVT and MT. Two different mechanisms of lysis were established, providing a basis towards improving the efficacy of combined treatments.

  • Journal article
    Xu X, Kan X, Ma T, Lin J, Wang L, Dong Zet al., 2021,

    Patient-specific simulation of stent-graft deployment in type B aortic dissection: model development and validation

    , Biomechanics and Modeling in Mechanobiology, Vol: 20, Pages: 2247-2258, ISSN: 1617-7940

    Thoracic endovascular aortic repair (TEVAR) has been accepted as the mainstream treatment for type B aortic dissection, but post-TEVAR biomechanical-related complications are still a major drawback. Unfortunately, the stent-graft (SG) configuration after implantation and biomechanical interactions between the SG and local aorta are usually unknown prior to a TEVAR procedure. The ability to obtain such information via personalized computational simulation would greatly assist clinicians in pre-surgical planning. In this study, a virtual SG deployment simulation framework was developed for the treatment for a complicated aortic dissection case. It incorporates patient-specific anatomical information based on pre-TEVAR CT angiographic images, details of the SG design, and the mechanical properties of the stent wire, graft and dissected aorta. Hyperelastic material parameters for the aortic wall were determined based on uniaxial tensile testing performed on aortic tissue samples taken from type B aortic dissection patients. Pre-stress conditions of the aortic wall and the action of blood pressure were also accounted for. The simulated post-TEVAR configuration was compared with follow-up CT scans, demonstrating good agreement with mean deviations of 5.8% in local open area and 4.6 mm in stent strut position. Deployment of the SG increased the maximum principal stress by 24.30 KPa in the narrowed true lumen but reduced the stress by 31.38 KPa in the entry tear region where there was an aneurysmal expansion. Comparisons of simulation results with different levels of model complexity suggested that pre-stress of the aortic wall and blood pressure inside the stent-graft should be included in order to accurately predict the deformation of the deployed SG

  • Journal article
    Kan X, Ma T, Dong Z, Xu Xet al., 2021,

    Patient-specific virtual stent-graft deployment for Type B aortic dissection: a pilot study of the impact of stent-graft length

    , Frontiers in Physiology, Vol: 12, ISSN: 1664-042X

    Thoracic endovascular aortic repair (TEVAR) has been accepted as a standard treatment option for complicated type B aortic dissection. Distal stent-graft induced new entry (SINE) is recognized as one of the main post-TEVAR complications, which can lead to fatal prognosis. Previous retrospective cohort studies suggested that short stent-graft (SG) length (<165 mm) might correlate with increased risk of distal SINE. However, the influence of SG length on changes in local biomechanical conditions before and after TEVAR is unknown. In this paper, we aim to address this issue using a virtual SG deployment simulation model developed for application in type B aortic dissection. Our model incorporates detailed SG design and hyperelastic behaviour of the aortic wall. By making use of patient-specific geometry reconstructed from pre-TEVAR computed tomography angiography (CTA) scan, our model can predict post-TEVAR SG configuration and wall stress. Virtual SG deployment simulations were performed on a patient who underwent TEVAR with a short SG (158 mm in length), mimicking the actual clinical procedure. Further simulations were carried out on the same patient geometry but with different SG lengths (183 mm and 208 mm) in order to evaluate the effect of SG length on changes in local stress in the treated aorta.

  • Conference paper
    Yuan X, Kan X, Xu XY, Nienaber Cet al., 2021,

    Identifying and quantifying the 4D motion of aortic root

    , 70th Annual Scientific Session and Expo of the American-College-of-Cardiology (ACC), Publisher: Elsevier, Pages: 1832-1832, ISSN: 0735-1097

    BackgroundThe motion of aortic root due to heart traction was previously suggested to contribute to proximal aortic dissection. The 4D motion of the aorta is recognisable with dynamic image acquisition (multiphase ECG-gated contrast-enhanced CT). However, both displacement and rotation in quantitative terms still remain unknown. The objective is to investigate the motion of aortic root from dynamic CT images in quantitative terms.Methods40 patients’ dynamic CT images for coronary assessment have been identified from PACS at Royal Brompton and Harefield hospital. All images were acquired under the same scanning protocol and no aortopathy had been identified. The scans were triggered by ECG and consist of 10 evenly spaced phases (0%-90%) in a cardiac cycle. The end diastolic phase (0%) was used as reference phase and the three commissures of leaflets were manually marked to identify the plane of sinotubular junction (STJ) by image post-processing software. A patient-specific coordinate system was created at the centre of STJ with the Z-axis parallel to the local longitudinal direction. Both the ostia of the left and right coronary were chosen as landmarks and traced at each phase. The coordinates of the two coronary ostia were transferred to the patient-specific coordinate system to quantify the motion normal to STJ plane (out-plane), the motion within STJ plane (in-plane) and the twist motion.ResultsA total of 40 patients enrolled for this study with a mean age 65±12, and 14 patients were male (35%). The out-plane motion was recorded the largest displacement with 10.03±2.90 and 9.30±2.36 mm referenced by the left and right coronary ostium, respectively. The mean downward movement of aortic root is 9.10±2.38 mm. The STJ in-plane motion was 7.56±3.01 and 6.65±2.74 mm for left coronary ostium, compared with 6.65±2.74 and 6.54±2.51 mm for right coronary ostium. The twisting of the aortic root is 10.78±4

  • Journal article
    Manchester E, Pirola S, Salmasi M, O'Regan D, Athanasiou T, Xu Xet al., 2021,

    Analysis of turbulence effects in a patient-specific aorta with aortic valve stenosis

    , Cardiovascular Engineering and Technology, Vol: 12, Pages: 438-453, ISSN: 1869-408X

    Blood flow in the aorta is often assumed laminar, however aortic valve pathologies may induce transition to turbulence and our understanding of turbulence effects is incomplete. The aim of the study was to provide a detailed analysis of turbulence effects in aortic valve stenosis (AVS).Methods:Large-eddy simulation (LES) of flow through a patient-specific aorta with AVS was conducted. Magnetic resonance imaging (MRI) was performed and used for geometric reconstruction and patient-specific boundary conditions. Computed velocity field was compared with 4D flow MRI to check qualitative and quantitative consistency. The effect of turbulence was evaluated in terms of fluctuating kinetic energy, turbulence-related wall shear stress (WSS) and energy loss.Results:Our analysis suggested that turbulence was induced by a combination of a high velocity jet impinging on the arterial wall and a dilated ascending aorta which provided sufficient space for turbulence to develop. Turbulent WSS contributed to 40% of the total WSS in the ascending aorta and 38% in the entire aorta. Viscous and turbulent irreversible energy losses accounted for 3.9 and 2.7% of the total stroke work, respectively.Conclusions:This study demonstrates the importance of turbulence in assessing aortic haemodynamics in a patient with AVS. Neglecting the turbulent contribution to WSS could potentially result in a significant underestimation of the total WSS. Further work is warranted to extend the analysis to more AVS cases and patients with other aortic valve diseases.

  • Journal article
    Armour C, Guo B, Pirola S, Saitta S, Liu Y, Dong Z, Xu Xet al., 2021,

    The influence of inlet velocity profile on predicted flow in type B aortic dissection

    , Biomechanics and Modeling in Mechanobiology, Vol: 20, Pages: 481-490, ISSN: 1617-7940

    In order for computational fluid dynamics to provide quantitative parameters to aid in the clinical assessment of type B aortic dissection, the results must accurately mimic the hemodynamic environment within the aorta. The choice of inlet velocity profile (IVP) therefore is crucial; however, idealised profiles are often adopted, and the effect of IVP on hemodynamics in a dissected aorta is unclear. This study examined two scenarios with respect to the influence of IVP—using (a) patient-specific data in the form of a three-directional (3D), through-plane (TP) or flat IVP; and (b) non-patient-specific flow waveform. The results obtained from nine simulations using patient-specific data showed that all forms of IVP were able to reproduce global flow patterns as observed with 4D flow magnetic resonance imaging. Differences in maximum velocity and time-averaged wall shear stress near the primary entry tear were up to 3% and 6%, respectively, while pressure differences across the true and false lumen differed by up to 6%. More notable variations were found in regions of low wall shear stress when the primary entry tear was close to the left subclavian artery. The results obtained with non-patient-specific waveforms were markedly different. Throughout the aorta, a 25% reduction in stroke volume resulted in up to 28% and 35% reduction in velocity and wall shear stress, respectively, while the shape of flow waveform had a profound influence on the predicted pressure. The results of this study suggest that 3D, TP and flat IVPs all yield reasonably similar velocity and time-averaged wall shear stress results, but TP IVPs should be used where possible for better prediction of pressure. In the absence of patient-specific velocity data, effort should be made to acquire patient’s stroke volume and adjust the applied IVP accordingly.

  • Journal article
    Yuan X, Kan X, Xu XY, Nienaber CA, Yuan X, Kan X, Xu X, Nienaber Cet al., 2020,

    Finite element modeling to predict procedural success of thoracic endovascular aortic repair in type A aortic dissection

    , JTCVS Techniques, Vol: 4, Pages: 40-47, ISSN: 2666-2507

    ObjectiveThoracic endovascular aortic repair (TEVAR) is recommended for type B aortic dissection and recently has even been used in selected cases of proximal (Stanford type A) aortic dissections in scenarios of prohibitive surgical risk. However, mechanical interactions between the native aorta and stent-graft are poorly understood, as some cases ended in failure. The aim of this study is to explore and better understand biomechanical changes after TEVAR and predict the result via virtual stenting.MethodsA case of type A aortic dissection was considered inoperable and selected for TEVAR. The procedure failed due to stent-graft migration even with precise deployment. A novel patient-specific virtual stent-graft deployment model based on finite element method was employed to analyze TEVAR-induced changes under such conditions. Two landing positions were simulated to investigate the reason for stent-graft migration immediately after TEVAR and explore options for optimization.ResultsSimulation of the actual procedure revealed that the proximal bare metal stent pushed the lamella into the false lumen and led to further stent-graft migration during the launch phase. An alternative landing position has reduced the local deformation of the dissection lamella and avoided stent-graft migration. Higher maximum principal stress (>20 KPa) was found on the lamella with deployment at the actual position, while the alternative strategy would have reduced the stress to <5 KPa.ConclusionsVirtual stent-graft deployment simulation based on finite element model could be helpful to both predict outcomes of TEVAR and better plan future endovascular procedures.

  • Journal article
    Chong MY, Gu B, Chan BT, Ong ZC, Xu XY, Lim Eet al., 2020,

    Effect of intimal flap motion on flow in acute type B aortic dissection by using fluid-structure interaction.

    , International Journal for Numerical Methods in Biomedical Engineering, Vol: 36, Pages: 1-22, ISSN: 1069-8299

    A monolithic, fully coupled fluid-structure interaction (FSI) computational framework was developed to account for dissection flap motion in acute type B aortic dissection (TBAD). Analysis of results included wall deformation, pressure, flow, wall shear stress (WSS), von. Mises stress and comparison of hemodynamics between rigid wall and FSI models. Our FSI model mimicked realistic wall deformation that resulted in maximum compression of the distal true lumen (TL) by 21.4%. The substantial movement of intimal flap mostly affected flow conditions in the false lumen (FL). Flap motion facilitated more flow entering the FL at peak systole, with the TL to FL flow split changing from 88:12 in the rigid model to 83:17 in the FSI model. There was more disturbed flow in the FL during systole (5.8% FSI vs. 5.2% rigid) and diastole (13.5% FSI vs. 9.8% rigid), via a λ2  -criterion. The flap-induced disturbed flow near the tears in the FSI model caused an increase of local WSS by up to 70.0% during diastole. This resulted in a significant reduction in the size of low time-averaged WSS (TAWSS) regions in the FL (113.11 cm2 FSI vs. 177.44 cm2 rigid). Moreover, the FSI model predicted lower systolic pressure, higher diastolic pressure, and hence lower pulse pressure. Our results provided new insights into the possible impact of flap motion on flow in aortic dissections, which are particularly important when evaluating hemodynamics of acute TBAD. This article is protected by copyright. All rights reserved.

  • Journal article
    Xu X, Manchester E, 2020,

    The effect of turbulence on transitional flow in the FDA’s benchmark nozzle model using large-eddy simulation

    , International Journal for Numerical Methods in Biomedical Engineering, Vol: 36, Pages: 1-15, ISSN: 1069-8299

    The Food and Drug Administration's (FDA) benchmark nozzle model has been studied extensively both experimentally and computationally. Although considerable efforts have been made on validations of a variety of numerical models against available experimental data, the transitional flow cases are still not fully resolved, especially with regards to detailed comparison of predicted turbulence quantities with experimental measurements. This study aims to fill this gap by conducting large‐eddy simulations (LES) of flow through the FDA's benchmark model, at a transitional Reynolds number of 2000. Numerical results are compared to previous interlaboratory experimental results, with an emphasis on turbulence characteristics. Our results show that the LES methodology can accurately capture laminar quantities throughout the model. In the pre‐jet breakdown region, predicted turbulence quantities are generally larger than high resolution experimental data acquired with laser Doppler velocimetry. In the jet breakdown regions, where maximum Reynolds stresses occur, Reynolds shear stresses show excellent agreement. Differences of up to 4% and 20% are observed near the jet core in the axial and radial normal Reynolds stresses, respectively. Comparisons between viscous and Reynolds shear stresses show that peak viscous shear stresses occur in the nozzle throat reaching a value of 18 Pa in the boundary layer, whilst peak Reynolds shear stresses occur in the jet breakdown region reaching a maximum value of 87 Pa. Our results highlight the importance in considering both laminar and turbulent contributions towards shear stresses and that neglecting the turbulence effect can significantly underestimate the total shear force exerted on the fluid.

  • Journal article
    Jarral OA, Tan MKH, Salmasi MY, Pirola S, Pepper JR, O'Regan DP, Xu XY, Athanasiou Tet al., 2020,

    Phase-contrast magnetic resonance imaging and computational fluid dynamics assessment of thoracic aorta blood flow: a literature review

    , European Journal of Cardio-Thoracic Surgery, Vol: 57, Pages: 438-446, ISSN: 1010-7940

    The death rate from thoracic aortic disease is on the rise and represents a growing global health concern as patients are often asymptomatic before acute events, which have devastating effects on health-related quality of life. Biomechanical factors have been found to play a major role in the development of both acquired and congenital aortic diseases. However, much is still unknown and translational benefits of this knowledge are yet to be seen. Phase-contrast cardiovascular magnetic resonance imaging of thoracic aortic blood flow has emerged as an exceptionally powerful non-invasive tool enabling visualization of complex flow patterns, and calculation of variables such as wall shear stress. This has led to multiple new findings in the areas of phenotype-dependent bicuspid valve flow patterns, thoracic aortic aneurysm formation and aortic prosthesis performance assessment. Phase-contrast cardiovascular magnetic resonance imaging has also been used in conjunction with computational fluid modelling techniques to produce even more sophisticated analyses, by allowing the calculation of haemodynamic variables with exceptional temporal and spatial resolution. Translationally, these technologies may potentially play a major role in the emergence of precision medicine and patient-specific treatments in patients with aortic disease. This clinically focused review will provide a systematic overview of key insights from published studies to date.

  • Journal article
    Pirola S, Guo B, Menichini C, Saitta S, Fu W, Dong Z, Xu XYet al., 2019,

    4D flow MRI-based computational analysis of blood flow in patient-specific aortic dissection

    , IEEE Transactions on Biomedical Engineering, Vol: 66, Pages: 3411-3419, ISSN: 0018-9294

    OBJECTIVE: Computational hemodynamics studies of aortic dissections usually combine patient-specific geometries with idealized or generic boundary conditions. In this study we present a comprehensive methodology for simulations of hemodynamics in type B aortic dissection (TBAD) based on fully patient-specific BCs. METHODS: Pre-operative 4D flow magnetic resonance imaging (MRI) and Doppler-wire pressure measurements (pre- and post-operative) were acquired from a TBAD patient. These data were used to derive boundary conditions for computational modelling of flow before and after thoracic endovascular repair (TEVAR). Validations of the computational results were performed by comparing predicted flow patterns with pre-TEVAR 4D flow MRI, as well as pressures with in vivo measurements. RESULTS AND CONCLUSION: Comparison of instantaneous velocity streamlines showed a good qualitative agreement with 4D flow MRI. Quantitative comparison of predicted pressures with pressure measurements revealed a maximum difference of 11 mmHg (-9.7%). Furthermore, our model correctly predicted the reduction of true lumen pressure from 74/115 mmHg pre-TEVAR to 64/107 mmHg post-TEVAR (diastolic/systolic pressures at entry tear level), compared to the corresponding measurements of 72/118 mmHg and 64/114 mmHg. This demonstrates that pre-TEVAR 4D flow MRI can be used to tune boundary conditions for post-TEVAR hemodynamic analyses.

  • Journal article
    Saitta S, Pirola S, Piatti F, Votta E, Lucherini F, Pluchinotta F, Carminati M, Lombardi M, Geppert C, Cuomo F, Figueroa C, Xu X, Redaelli Aet al., 2019,

    Evaluation of 4D Flow MRI-based non-invasive pressure assessment in aortic coarctations

    , Journal of Biomechanics, Vol: 94, Pages: 13-21, ISSN: 0021-9290

    Severity of aortic coarctation (CoA) is currently assessed by estimating trans-coarctation pressure drops through cardiac catheterization or echocardiography. In principle, more detailed information could be obtained non-invasively based on space- and time-resolved magnetic resonance imaging (4D flow) data. Yet the limitations of this imaging technique require testing the accuracy of 4D flow-derived hemodynamic quantities against other methodologies.With the objective of assessing the feasibility and accuracy of this non-invasive method to support the clinical diagnosis of CoA, we developed an algorithm (4DF-FEPPE) to obtain relative pressure distributions from 4D flow data by solving the Poisson pressure equation. 4DF-FEPPE was tested against results from a patient-specific fluid-structure interaction (FSI) simulation, whose patient-specific boundary conditions were prescribed based on 4D flow data. Since numerical simulations provide noise-free pressure fields on fine spatial and temporal scales, our analysis allowed to assess the uncertainties related to 4D flow noise and limited resolution.4DF-FEPPE and FSI results were compared on a series of cross-sections along the aorta. Bland-Altman analysis revealed very good agreement between the two methodologies in terms of instantaneous data at peak systole, end-diastole and time-averaged values: biases (means of differences) were +0.4 mmHg, −1.1 mmHg and +0.6 mmHg, respectively. Limits of agreement (2 SD) were ±0.978 mmHg, ±1.06 mmHg and ±1.97 mmHg, respectively. Peak-to-peak and maximum trans-coarctation pressure drops obtained with 4DF-FEPPE differed from FSI results by 0.75 mmHg and −1.34 mmHg respectively. The present study considers important validation aspects of non-invasive pressure difference estimation based on 4D flow MRI, showing the potential of this technology to be more broadly applied to the clinical practice.

  • Journal article
    Guo B, Dong Z, Pirola S, Liu Y, Menichini C, Xu XY, Guo D, Fu Wet al., 2019,

    Dissection level within aortic wall layers is associated with propagation of type B aortic dissection: a swine model study

    , European Journal of Vascular and Endovascular Surgery, Vol: 58, Pages: 415-425, ISSN: 1078-5884

    OBJECTIVE: Haemodynamic and geometric factors play pivotal roles in the propagation of acute type B aortic dissection (TBAD). The aim of this study was to evaluate the association between dissection level within all aortic layers and the propagation of acute TBAD in porcine aorta. METHODS: Twelve pig acute TBAD models were created. In model A, the aortic wall tear was superficial and close to the intima (thin intimal flap), whereas in model B it was deep and close to the adventitia (thick intimal flap). Dissection propagation was evaluated using angiography or computed tomography scans, and the haemodynamic measurements were acquired using Doppler wires. Most pigs were followed up at 1, 3, 6, 12, 18, and up to 24 months; four animals were euthanised at three and six months, respectively (two from each group). RESULTS: Both models were successfully created. No statistical difference was observed for the median antegrade propagation distance intra-operatively between the two models (p = .092). At 24 months, the longitudinal propagation distance was significantly greater in model B than in model A (p = .016). No statistical difference in retrograde propagation was noted (p = .691). Over time, aortic wall dissection progressed most notably over the first three months in model A, whereas it continued over the first 12 months in model B. Flow velocity was significantly greater in the true lumen than in false lumen at the level of the primary tear (p = .001) and in the middle of the dissection (p = .004). The histopathological images at three and six months demonstrated the fibres were stretched linearly at the outside wall of false lumen in both models, while the depth of intimal tears developed to be superficial and similar at the distal dissection. CONCLUSION: In this swine model of TBAD, a deeper intimal tear resulted in greater dissection propagation.

  • Journal article
    Menichini C, Pirola S, Guo B, Fu W, Dong Z, Xu XYet al., 2018,

    High wall stress may predict the formation of stent-graft-induced new entries after thoracic endovascular aortic repair

    , Journal of Endovascular Therapy, Vol: 25, Pages: 571-577, ISSN: 1526-6028

    PURPOSE: To explore the potential role of morphological factors and wall stress in the formation of stent-graft-induced new entries (SINE) based on computed tomography (CT) images after thoracic endovascular aortic repair (TEVAR). CASE REPORT: Two female patients aged 59 years (patient 1) and 44 years (patient 2) underwent TEVAR for type B dissection in the chronic (patient 1) or subacute (patient 2) phase. CT scans at 3-month follow-up showed varying degrees of false lumen thrombosis in both patients. At 14-month follow-up, a SINE was observed in patient 1 while the dissected aorta in the other patient remained stable. Morphological and finite element analyses were performed based on the first follow-up CT images. The computational results showed that the SINE patient had higher stent-graft tortuosity than the non-SINE patient and much higher wall stress in the region close to the distal SINE. CONCLUSION: This case study suggests that high stent-graft tortuosity can lead to high wall stress, which is potentially linked to the formation of SINE. Further large population-based studies are needed to confirm this preliminary finding.

  • Journal article
    Xu XY, Pirola S, Jarral O, O'Regan D, Asimakopoulos G, Anderson JR, Pepper J, Athanasiou Tet al., 2018,

    Computational study of aortic hemodynamics for patients with an abnormal aortic valve: the importance of secondary flow at the ascending aorta inlet

    , APL Bioengineering, Vol: 2, Pages: 026101-1-026101-14, ISSN: 2473-2877

    Blood flow in the aorta is helical, but most computational studies ignore the presence of secondary flow components at the ascending aorta (AAo) inlet. The aim of this study is to ascertain the importance of inlet boundary conditions (BCs) in computational analysis of flow patterns in the thoracic aorta based on patient-specific images, with a particular focus on patients with an abnormal aortic valve. Two cases were studied: one presenting a severe aortic valve stenosis and the other with a mechanical valve. For both aorta models, three inlet BCs were compared; these included the flat profile and 1D through-plane velocity and 3D phase-contrast magnetic resonance imaging derived velocity profiles, with the latter being used for benchmarking. Our results showed that peak and mean velocities at the proximal end of the ascending aorta were underestimated by up to 41% when the secondary flow components were neglected. The results for helical flow descriptors highlighted the strong influence of secondary velocities on the helical flow structure in the AAo. Differences in all wall shear stress (WSS)-derived indices were much more pronounced in the AAo and aortic arch (AA) than in the descending aorta (DAo). Overall, this study demonstrates that using 3D velocity profiles as inlet BC is essential for patient-specific analysis of hemodynamics and WSS in the AAo and AA in the presence of an abnormal aortic valve. However, predicted flow in the DAo is less sensitive to the secondary velocities imposed at the inlet; hence, the 1D through-plane profile could be a sufficient inlet BC for studies focusing on distal regions of the thoracic aorta.

  • Journal article
    Guo B, Pirola S, Guo D, Dong Z, Xu XY, Fu Wet al., 2018,

    Hemodynamic evaluation using four-dimensional flow magnetic resonance imaging for a patient with multichanneled aortic dissection

    , Journal of Vascular Surgery Cases and Innovative Techniques, Vol: 4, Pages: 67-71, ISSN: 2468-4287

    The hemodynamic function of multichanneled aortic dissection (MCAD) requires close monitoring and effective management to avoid potentially catastrophic sequelae. This report describes a 47-year-old man who underwent endovascular repair based on findings from four-dimensional (4D) flow magnetic resonance imaging of an MCAD. The acquired 4D flow data revealed complex, bidirectional flow patterns in the false lumens and accelerated blood flow in the compressed true lumen. The collapsed abdominal true lumen expanded unsatisfactorily after primary tear repair, which required further remodeling with bare stents. This case study demonstrates that hemodynamic analysis using 4D flow magnetic resonance imaging can help understand the complex pathologic changes of MCAD.

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