Multiaxial pulsatile dynamics of the thoracic aorta and impact of thoracic endovascular repair

Purpose: The thoracic aorta is a highly mobile organ whose dynamics are altered by thoracic endovascular aorta repair (TEVAR). The aim of this study was to quantify cardiac pulsatility-induced multi-axial deformation of the thoracic aorta before and after descending aortic TEVAR. Methods: Eleven TEVAR patients (8 males and 3 females, age 57–89) underwent retrospective cardiac-gated CT angiography before and after TEVAR. 3D geometric models of the thoracic aorta were constructed, and lumen centerlines, inner and outer surface curves, and cross-sections were extracted to measure aortic arclength, centerline, inner surface, and outer surface longitudinal curvatures, as well as cross-sectional effective diameter and eccentricity for the ascending and stented aortic portions. Results: From pre- to post-TEVAR, arclength deformation was increased at the ascending aorta from 5.9 \ub1 3.1 % to 8.8 \ub1 4.4 % (P < 0.05), and decreased at the stented aorta from 7.5 \ub1 5.1 % to 2.7 \ub1 2.5 % (P < 0.05). Longitudinal curvature and diametric deformations were reduced at the stented aorta. Centerline curvature, inner surface curvature, and cross-sectional eccentricity deformations were increased at the distal ascending aorta. Conclusions: Deformations were reduced in the stented thoracic aorta after TEVAR, but increased in the ascending aorta near the aortic arch, possibly as a compensatory mechanism to maintain overall thoracic compliance in the presence of reduced deformation in the stiffened stented aorta

Geometric Modeling of Thoracic Aortic Surface Morphology - Implications for Pathophysiology and Clinical Interventions

Vascular disease risk factors such as hypertension, hyperlipidemia and old age are all\ua0results of modern-day lifestyle, and these diseases are getting more and more common. One\ua0treatment option for vascular diseases such as aneurysms and dissections is endovascular\ua0aortic repair introduced in the early 1990s. This treatment uses tubular fabric covered\ua0metallic structures (endografts) that are implanted using a minimally invasive approach\ua0and placed to serve as an articial vessel in a damaged portion of the vasculature. To ensure\ua0that the interventions are successful, the endograft must be placed in the correct location,\ua0and designed to sustain the hostile biological, chemical, and mechanical conditions in the\ua0body for many years. This is an interaction that goes both ways, and keeping in mind\ua0that the endograft is a foreign object placed in the sensitive vascular system, it is also\ua0important that it does not disrupt the native conditions more than necessary.This thesis presents a segmentation and quantication methodology to accurately\ua0describe the complex morphology and motion of diseased blood vessels in vivo through a\ua0natural and intuitive description of their luminal surfaces. After methodology validation,\ua0a series of important clinical applications are performed, all based on non-invasive imaging.\ua0Firstly, it is shown that explicit surface curvature quantication is necessary when\ua0compared to relying solely on centerline curvature and estimation methods. Secondly, it is\ua0shown that endograft malapposition severity can be predicted from preoperative geometric\ua0analysis of thoracic aortic surfaces. Thirdly, a multiaxial dynamics analysis of cardiac\ua0induced thoracic aortic surface motion shows how thoracic endovascular aortic repair\ua0affects the deformations of the dierent portions of the thoracic aorta. Fourthly, the helical\ua0propagation pattern of type B aortic dissection is determined, and two distinct modes of\ua0chirality are revealed, i.e., achiral and right-handed chiral groups. Finally, the effects of\ua0thoracic endovascular aortic repair on helical and cross-sectional morphology of type B\ua0dissections are investigated revealing how acuity and chirality affects the alteration due to\ua0intraluminal lining with endografts. Thus, the work presented in this thesis contributes\ua0by adding knowledge about pathology and pathophysiology through better geometric\ua0description of surface conditions of diseased thoracic aortas. This gives clinicians insights\ua0to use in their treatment planning and provides more nuanced boundary conditions for\ua0endograft manufacturers. Comprehensive knowledge about diseases, better treatment\ua0planning, and better devices are all crucial in order to improve the outcomes of performed\ua0interventions and ultimately the quality of life for the treated patients

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

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

4D flow imaging of the thoracic aorta: is there an added clinical value?

Four-dimensional (4D) flow MRI has emerged as a powerful non-invasive technique in cardiovascular imaging, enabling to analyse in vivo complex flow dynamics models by quantifying flow parameters and derived features. Deep knowledge of aortic flow dynamics is fundamental to better understand how abnormal flow patterns may promote or worsen vascular diseases. In the perspective of an increasingly personalized and preventive medicine, growing interest is focused on identifying those quantitative functional features which are early predictive markers of pathological evolution. The thoracic aorta and its spectrum of diseases, as the first area of application and development of 4D flow MRI and supported by an extensive experimental validation, represents the ideal model to introduce this technique into daily clinical practice. The purpose of this review is to describe the impact of 4D flow MRI in the assessment of the thoracic aorta and its most common affecting diseases, providing an overview of the actual clinical applications and describing the potential role of derived advanced hemodynamic measures in tailoring follow-up and treatment

Modeling of Intraluminal Surfaces of Thoracic Aortas

Vascular diseases are getting more and more common as a result of modern-day lifestyle and the fact that the population is getting older. One of the newest treatments for vascular diseases such as aneurysms and dissections is endovascular repair with endografting. This treatment uses a fabric covered metallic structure that is implanted using a minimally invasive approach to serve as an artificial vessel in a damaged region. To ensure that the interventions are successful, the endograft must be placed in the correct location, and be designed to sustain the hostile biological, chemical, and mechanical conditions in the body for many years.To accurately describe the complex mechanical conditions of the intraluminal surfaces of diseased blood vessels inside the body, this thesis presented a segmentation and quantification methodology for a natural and intuitive vessel surface description. The thesis also included some important clinical applications, all based on non-invasive temporal imaging. The results emphasized the need for explicit surface curvature quantification, as compared to relying solely on centerline curvature and estimation methods. Methods for preoperative prediction of endograft malapposition severity based on geometric analysis of thoracic aortic surfaces were introduced. Finally, a multiaxial dynamic analysis of cardiac induced thoracic aortic surface deformation showed how a thoracic endovascular aortic repair is a↵ecting the deformations of the thoracic aorta.Thus, the work presented in this thesis contributes by giving surgeons a tool to use in their treatment planning to minimize complications. Moreover, this method provides more nuanced boundary conditions so that endograft manufacturers can improve their designs to improve the quality of life for the treated patients

Computational analysis of the hemodynamic performance of novel endovascular and surgical procedures for complex aortic diseases

Novel branched stent-grafts (BSG) have been developed for endovascular repair of complex thoracic aortic aneurysms (TAA) involving the aortic arch or thoracoabdominal aorta, but their haemodynamic performance has not been adequately studied. In addition, surgical replacement of the ascending aorta with a Dacron graft remains the gold standard for type A aortic dissection (TAAD), although 12% of patients are at risk of aortic rupture due to further dilatation of the residual dissected aorta. The underlying mechanisms for progressive aortic dilatation following TAAD repair are poorly understood, but haemodynamic and biomechanical factors are believed to play an important role. Therefore, the present study aims to provide more insights into the haemodynamics in novel BSGs developed for treating complex aortic diseases, and a comprehensive evaluation of flow and biomechanical conditions in post-surgery TAADs by means of state-of-the-art computational methods. The first part of this thesis focuses on evaluating the haemodynamic performance of novel BSG designs, including thoracoabdominal branch endoprosthesis (TAMBE) and dual-branched thoracic endograft. Haemodynamics in idealised and patient-specific BSG models has been analysed by examining side branch outflow waveforms, wall shear stress related indices, and displacement forces, in order to assess their long-term durability. The numerical results show that all the stent-graft models examined in this study are capable of providing normal blood perfusion to side vessels, and are at low risk of in-stent thrombosis and device migration. Furthermore, it has been shown that geometric variations in TAMBE do not affect the key haemodynamic results, indicating its potential suitability for a variety of visceral artery anatomies. Comparisons of dual-branched thoracic endograft models with different inner tunnel diameters suggest that BSGs with larger inner tunnel diameters than the respective vessels would be preferred. Comparisons between the pre- and post-intervention models show that insertion of a dual-branched stent-graft significantly alters the flow pattern in the aortic arch, some of which may have a detrimental effect in the long term, thus requiring follow-up studies. The second part of the thesis provides a comprehensive analysis of the haemodynamic and biomechanical conditions in surgically repaired TAAD. Geometric and haemodynamic parameters have been analyzed and compared between the group of patients with stable aortic diameter and another group with progressive aortic dilatation. The number of re-entry tears (6±5 vs 2±1;P= 0.02) and luminal pressure difference (1.3 ±1 vs 11.7 ±14.6 mmHg;P= 0.001) have been identified as potential predictors of progressive aortic dilatation in TAAD patients following surgical repair. This is an important finding and can potentially assist clinicians to make the most appropriate choice or surgical plan for individual patients. Based on the finite element analysis of four patient-specific cases, there are no clear differences in biomechanical parameters between the stable and unstable groups. Furthermore, a preliminary fluid-solid interaction (FSI) simulation performed on a single TAAD model has demonstrated the important influence of wall compliance on pressures in the true and false lumen. Compared to a rigid wall model, the FSI simulation results show a reduction in systolic pressure by up to 10 mmHg and a slight increase in diastolic pressure. However, pressures in the true and false lumen are affected in the same way, so that the luminal pressure difference remains the same between the rigid and FSI models.Open Acces

Longitudinale Evaluierung der Aortenhämodynamik bei Patienten mit Aortenklappenpathologie mittels vierdimensionaler kardiovaskulärer Magnetresonanztomographie

Background Four-Dimensional Flow Magnetic Resonance Imaging (4D flow MRI) provides a non-invasive assessment of aortic hemodynamics and insight into blood flow patterns and wall shear stress (WSS). Aortic valve stenosis (AS) and bicuspid aortic valves (BAV) cause substantial changes in blood flow patterns and elevated and asymmetrically distributed WSS in the ascending aorta (1,2,3,4). The AS and BAV patients exhibited more helical, vortical, and eccentric flow (1,2). This study aimed to understand better the evolution of aortic hemodynamics in aortic valve pathologies. Methods After receiving ethical approval and informed consent, 20 patients from the primary studies (1,3) were reevaluated using cardiovascular MRI. There were 14 patients with BAV and 6 with stenotic tricuspid aortic valves (TAV). Between primary (P) and follow-up (FU) examination, 1 TAV and 6 BAV underwent aortic valve replacement (AVR). The patients were divided into 2 groups: non-operated (NOP) (n=13) and operated (OP) (n=7). The average duration from P to FU was 4 years. The mean age at FU was 73.3±4.4 years for OP and 57.5±15.9 for NOP. MRI studies were performed on 1.5 T (n=4) and 3.0 T (n=16) systems. Aortic valve morphology, LV function, and mass were assessed using ECG-gated breath-hold SSFP cine imaging. Aortic dimensions were measured using non-contrast magnetic resonance angiography in FU and axial SSFP imaging of the thorax in P. Standard cardiac MRI analysis was performed using cvi42. Thoracic aorta 4D flow MRI using ECG gating and respiratory navigators was acquired. Hemodynamic parameters were evaluated, including net flow, peak systolic velocity, WSS, and helical and vortical flow patterns. Results NOP and OP groups significantly differed in age and BMI. After surgery, LV mass and WSS magnitude were significantly decreased in OP in FU. WSS was significantly correlated with peak systolic velocity, helical flow, and LV mass. Peak velocity in the ascending aorta and aortic arch of OP declined significantly in FU but remained constant in NOP. There was a significant increase in net flow in certain aortic planes in OP in FU. In some cases, the helical and vortical flow patterns changed.Hintergrund Die vierdimensionale Fluss-Magnetresonanztomographie (4D-Flow-MRT) ermöglicht eine nicht-invasive Beurteilung der Aortenhämodynamik und bietet Einblick in die Blutflussmuster und die Wandschubspannung (WSS). Aortenklappenstenose (AS) und bikuspide Aortenklappen (BAV) verursachen erhebliche Veränderungen der Blutflussmuster und erhöhte und asymmetrisch verteilte WSS in der Aorta ascendens (1,2,3,4). Ziel der Studie war es, die Entwicklung der Aortenhämodynamik bei Aortenklappenpathologien besser zu verstehen. Methodik Nach Erhalt der ethischen Genehmigung und informierten Zustimmung wurden 20 Patienten aus den Primärstudien (1,3) mittels CMR erneut untersucht. Es handelte sich um 14 Patienten mit BAV und 6 mit stenotischen trikuspidalen Aortenklappen (TAV). Zwischen der Erst- (P) und Nachuntersuchung (FU) unterzogen sich 7 Patienten einem Aortenklappenersatz (AVR). Die Patienten wurden in 2 Gruppen eingeteilt: nicht operiert (NOP) und operiert (OP). Die Dauer von P bis FU betrug circa 4 Jahre. Die MRT-Untersuchungen wurden mit 1,5 T und 3,0 T Systemen durchgeführt. Die Aortenklappenmorphologie, LV-Funktion und Masse wurden mittels SSFP-Cine-Bildgebung beurteilt. Die Aortenabmessungen wurden mittels kontrastfreier Magnetresonanzangiographie bei FU und axialer SSFP-Bildgebung des Thorax bei P gemessen. Die Standardanalyse der kardialen MRT wurde mit cvi42 durchgeführt. Die 4D-Flow-MRT der Aorta wurde mit EKG-Gating und Atemnavigator durchgeführt. Es wurden hämodynamische Parameter ausgewertet, darunter Nettofluss, systolische Spitzengeschwindigkeit, WSS sowie helikale und vortikale Flussmuster. Ergebnisse Die NOP- und OP unterschieden sich signifikant in Alter und BMI. Nach der AVR waren die LV-Masse und die WSS-Größe bei OP in FU signifikant verringert. Die WSS korrelierte signifikant mit der systolischen Spitzengeschwindigkeit, dem helikalen Fluss und der LV-Masse. Die Spitzengeschwindigkeit in der Aorta ascendens und im Aortenbogen der OP nahm in FU signifikant ab. Der Nettofluss in bestimmten Aortenebenen war bei OP in FU signifikant erhöht. In einigen Fällen veränderten sich die helikalen und vortikalen Flussmuster. Fazit Aortenklappenpathologien können die Hämodynamik des linken Ventrikels und der Aorta beeinträchtigen. Die AVR verbessert die Parameter. Die hämodynamischen Veränderungen können zur Aortendilatation und Dissektion beitragen. In dieser Studie wurde festgestellt, dass das WSS der Aorta in Gebieten mit der höchsten Dissektionshäufigkeit höher war. Das Management von Aortenerkrankungen sucht nach neuen Parametern, die über die Aortengröße hinausgehen, um chirurgische Entscheidungen zu verbessern und Dissektionen vorherzusagen. Wir schlagen vor, Kriterien, die nicht die Größe betreffen, wie das WSS, als potenziellen Parameter für die individuelle Behandlung von Aortenerkrankungen zu entwickeln. Letztlich haben wir Hypothesen für weitere, umfassendere Studien aufgestellt, um unsere Ergebnisse zu validiere

Computational analysis of blood flow and stress patterns in the aorta of patients with Marfan syndrome

Personalised external aortic root support (PEARS) was designed to prevent progressive aortic dilatation, and the associated risk of aortic dissection, in patients with Marfan syndrome by providing an additional support to the aorta. The objective of this thesis was to understand the biomechanical implications of PEARS surgery as well as to investigate the altered haemodynamics associated with the disease and its treatment. Finite element (FE) models were developed using patient-specific aortic geometries reconstructed from pre and post-PEARS magnetic resonance (MR) images of three Marfan patients. The wall and PEARS materials were assumed to be isotropic, incompressible and linearly elastic. A static load on the inner wall corresponding to the patients’ pulse pressure was applied with a zero-displacement constraint at all boundaries. Results showed that peak aortic stresses and displacements before PEARS were located at the sinuses of Valsalva but following PEARS surgery, they were shifted to the aortic arch, at the intersection between the supported and unsupported aorta. The zero-displacement constraint at the aortic root was subsequently removed and replaced with downward motion measured from in vivo images. This revealed significant increases in the longitudinal wall stress, especially in the pre-PEARS models. Computational fluid dynamics (CFD) models were developed to evaluate flow characteristics. The correlation-based transitional Shear Stress Transport (SST-Tran) model was adopted to simulate potential transitional and turbulence flow during part of the cardiac cycle and flow waveforms derived from phase-contrast MR images were imposed at the inlets. Qualitative patterns of the haemodynamics were similar pre- and post-PEARS with variations in mean helicity flow index (HFI) of -10%, 35% and 20% in the post-PEARS aortas of the three patients. A fluid-structure interaction (FSI) model was developed for one patient, pre- and post-PEARS in order to examine the effect of wall compliance on aortic flow as well as the effect of pulsatile flow on wall stress. This model excluded the sinuses and was based on the laminar flow assumption. The results were similar to those obtained using the rigid wall and static structural models, with minor quantitative differences. Considering the higher computational cost of FSI simulations and the relatively small differences observed in peak wall stress, it is reasonable to suggest that static structural models would be sufficient for wall stress prediction. Additionally, aortic root motion had a more profound effect on wall stress than wall compliance. Further studies are required to assess the statistical significance of the findings outlined in this thesis. Recommendations for future work were also highlighted, with emphasis on model assumptions including material properties, residual stress and boundary conditions.Open Acces

Aortic Coarctation: Recent Developments in Experimental and Computational Methods to Assess Treatments for this Simple Condition

Coarctation of the aorta (CoA) is often considered a relatively simple disease, but long-term outcomes suggest otherwise as life expectancies are decades less than in the average population and substantial morbidity often exists. What follows is an expanded version of collective work conducted by the authors\u27 and numerous collaborators that was presented at the 1st International Conference on Computational Simulation in Congenital Heart Disease pertaining to recent advances for CoA. The work begins by focusing on what is known about blood flow, pressure and indices of wall shear stress (WSS) in patients with normal vascular anatomy from both clinical imaging and the use of computational fluid dynamics (CFD) techniques. Hemodynamic alterations observed in CFD studies from untreated CoA patients and those undergoing surgical or interventional treatment are subsequently discussed. The impact of surgical approach, stent design and valve morphology are also presented for these patient populations. Finally, recent work from a representative experimental animal model of CoA that may offer insight into proposed mechanisms of long-term morbidity in CoA is presented