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

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

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

Patient-specific analysis of ascending thoracic aortic aneurysm with the living heart human model

In ascending thoracic aortic aneurysms (ATAAs), aneurysm kinematics are driven by ventricular traction occurring every heartbeat, increasing the stress level of dilated aortic wall. Aortic elongation due to heart motion and aortic length are emerging as potential indicators of adverse events in ATAAs; however, simulation of ATAA that takes into account the cardiac mechanics is technically challenging. The objective of this study was to adapt the realistic Living Heart Human Model (LHHM) to the anatomy and physiology of a patient with ATAA to assess the role of cardiac motion on aortic wall stress distribution. Patient-specific segmentation and material parameter estimation were done using preoperative computed tomography angiography (CTA) and ex vivo biaxial testing of the harvested tissue collected during surgery. The lumped-parameter model of systemic circulation implemented in the LHHM was refined using clinical and echocardiographic data. The results showed that the longitudinal stress was highest in the major curvature of the aneurysm, with specific aortic quadrants having stress levels change from tensile to compressive in a transmural direction. This study revealed the key role of heart motion that stretches the aortic root and increases ATAA wall tension. The ATAA LHHM is a realistic cardiovascular platform where patient-specific information can be easily integrated to assess the aneurysm biomechanics and potentially support the clinical management of patients with ATAAs

Vascular remodeling after endovascular treatment: quantitative analysis of medical images with a focus on aorta

In the last years, the convergence of advanced imaging techniques and endovascular procedures has revolutionized the practice of vascular surgery. However, regardless the anatomical district, several complications still occur after endovascular treatment and the impact of endovascular repair on vessel morphology remains unclear. Starting from this background, the aim of this thesis is to ll the gaps in the eld of vessel remodeling after endovascular procedure. Main focus of the work will be the repair of the aorta and, in particular thoracic and thoracoabdominal treatments. Furthermore an investigation of the impact of endovascular repair on femoro-popliteal arterial segment will be reported in the present work. Analyses of medical images will been conducted to extract anatomical geometric features and to compare the changes in morphology before treatment and during follow-up. After illustrating in detail the aims and the outline of the dissertation in Chapter 1, Chapter 2 will concern the anatomy and the physiology of the aorta along with the main aortic pathologies and the related surgical treatments. Subsequently, an overview of the medical image techniques for segmentation and vessel geometric quantication will be provided. Chapter 3 will introduce the concept of remodeling of the aorta after endovascular procedure. In particular, two types of aortic remodeling will be considered. On one side remodeling can be seen as the shrinkage of the aneurysmal sac or false lumen thrombosis. On the other side, aortic remodeling could be seen as the changes in the aortic morphology following endograft placement which could lead to complications. Chapter 4 will illustrate a study regarding the analysis of medical images to measure the geometrical changes in the pathological aorta during follow-up in patients with thoracoabdominal aortic aneurysms treated with endovascular procedure using a novel uncovered device, the Cardiatis Multilayer Flow Modulator. Chapter 5 will focus on the geometrical remodeling of the aortic arch and descending aorta in patients who underwent hybrid arch treatment to treat thoracic aneurysms. The goal of the work is to develop a pipeline for the processing of pre-operative and post-operative Computed Tomography images in order to detect the changes in the aortic arch physiological curvature due to endograft insertion. Chapter 6 will focuse on the use of 3D printing technology as valuable tool to support patient's follow-up. In particular, we report a case of a patient originally treated with endovascular procedure for type B aortic dissection and which experimented several complications during follow-up. 3D printing technology is used to show the remodeling of the aortic vasculature during time. Chapter 7 will concern patient-specic nite element simulations of aortic endovascular procedure. In particular, starting from a clinical case where complication developed during followup, the predictive value of computational simulations will be shown. Chapter 8 will illustrate a study concerning the evaluation of morphological changes of the femoro-popliteal arterial segment due to limb exion in patients undergoing endovascular treatment of popliteal artery aneurysms

Irregular anatomical features can alter hemodynamics in Takayasu arteritis

Objective Takayasu arteritis (TA) is a difficult disease to deal with because there are neither reliable clinical signs, laboratory biomarkers, nor a single noninvasive imaging technique that can be used for early diagnosis and disease activity monitoring. Knowledge of aortic hemodynamics in TA is lacking. This study aimed to fill this gap by assessing hemodynamics in patients with TA using image-based computational fluid dynamics (CFD) simulations. Methods Eleven patients with TA were included in the present study. Patient-specific geometries were reconstructed from either clinical aortic computed tomography angiography or magnetic resonance angiography studies and coupled with physiological boundary conditions for CFD simulations. Key anatomical and hemodynamic parameters were compared with a control group consisting of 18 age- and sex-matched adults without TA who had healthy aortas. Results Compared with controls, patients with TA had significantly higher aortic velocities (0.9 m/s [0.7, 1.1 m/s] vs 0.6 m/s [0.5, 0.7 m/s]; P = .002), maximum time-averaged wall shear stress (14.2 Pa [9.8, 20.9 Pa] vs 8.0 Pa [6.2, 10.3 Pa]; P = .004), and maximum pressure drops between the ascending and descending aorta (36.9 mm Hg [29.0, 49.3 mm Hg] vs 28.5 mm Hg [25.8, 31.5 mm Hg]; P = .004). These significant hemodynamic alterations in patients with TA might result from abnormal anatomical features including smaller arch diameter (20.0 mm [13.8, 23.3 mm] vs 25.2 mm [23.3, 26.8 mm]; P = .003), supra-aortic branch diameters (21.9 mm [18.5, 24.6 mm] vs 25.7 mm [24.3, 28.3 mm]; P = .003) and descending aorta diameter (14.7 mm [12.2, 16.8 mm] vs 22.5 mm [19.8, 24.0 mm]; P < .001). Conclusions CFD analysis reveals hemodynamic changes in the aorta of patients with TA. The applicability of CFD technique coupled with standard imaging assessments in predicting disease progression of such patients will be explored in future studies. Future large cohort study with outcome correlation is also warranted. Clinical Relevance Based on patient-specific computational fluid dynamics simulations, the present retrospective study revealed significant difference in aortic hemodynamics between the patients with and without Takayasu arteritis (TA). To the best of our knowledge, this study is the first to evaluate hemodynamic conditions within TA, demonstrating the potential of computational flow modeling in capturing abnormal hemodynamic forces, such as high wall shear stress, resulted from irregular morphological changes. In the future, assessing the hemodynamic parameters within patients with TA during the prestenotic period, together with longitudinal computational fluid dynamics studies may allow better monitoring and management of TA

Hemodynamic Effects of Entry and Exit Tear Size in Aortic Dissection Evaluated with In Vitro Magnetic Resonance Imaging and Fluid-Structure Interaction Simulation

Understanding the complex interplay between morphologic and hemodynamic features in aortic dissection is critical for risk stratification and for the development of individualized therapy. This work evaluates the effects of entry and exit tear size on the hemodynamics in type B aortic dissection by comparing fluid-structure interaction (FSI) simulations with in vitro 4D-flow magnetic resonance imaging (MRI). A baseline patient-specific 3D-printed model and two variants with modified tear size (smaller entry tear, smaller exit tear) were embedded into a flow- and pressure-controlled setup to perform MRI as well as 12-point catheter-based pressure measurements. The same models defined the wall and fluid domains for FSI simulations, for which boundary conditions were matched with measured data. Results showed exceptionally well matched complex flow patterns between 4D-flow MRI and FSI simulations. Compared to the baseline model, false lumen flow volume decreased with either a smaller entry tear (-17.8 and -18.5 %, for FSI simulation and 4D-flow MRI, respectively) or smaller exit tear (-16.0 and -17.3 %). True to false lumen pressure difference (initially 11.0 and 7.9 mmHg, for FSI simulation and catheter-based pressure measurements, respectively) increased with a smaller entry tear (28.9 and 14.6 mmHg), and became negative with a smaller exit tear (-20.6 and -13.2 mmHg). This work establishes quantitative and qualitative effects of entry or exit tear size on hemodynamics in aortic dissection, with particularly notable impact observed on FL pressurization. FSI simulations demonstrate acceptable qualitative and quantitative agreement with flow imaging, supporting its deployment in clinical studies.Comment: Judith Zimmermann and Kathrin B\"aumler contributed equall

The geometric evolution of aortic dissections: Predicting surgical success using fluctuations in integrated Gaussian curvature

Clinical imaging modalities are a mainstay of modern disease management, but the full utilization of imaging-based data remains elusive. Aortic disease is defined by anatomic scalars quantifying aortic size, even though aortic disease progression initiates complex shape changes. We present an imaging-based geometric descriptor, inspired by fundamental ideas from topology and soft-matter physics that captures dynamic shape evolution. The aorta is reduced to a two-dimensional mathematical surface in space whose geometry is fully characterized by the local principal curvatures. Disease causes deviation from the smooth bent cylindrical shape of normal aortas, leading to a family of highly heterogeneous surfaces of varying shapes and sizes. To deconvolute changes in shape from size, the shape is characterized using integrated Gaussian curvature or total curvature. The fluctuation in total curvature (δK) across aortic surfaces captures heterogeneous morphologic evolution by characterizing local shape changes. We discover that aortic morphology evolves with a power-law defined behavior with rapidly increasing δK forming the hallmark of aortic disease. Divergent δK is seen for highly diseased aortas indicative of impending topologic catastrophe or aortic rupture. We also show that aortic size (surface area or enclosed aortic volume) scales as a generalized cylinder for all shapes. Classification accuracy for predicting aortic disease state (normal, diseased with successful surgery, and diseased with failed surgical outcomes) is 92.8±1.7%. The analysis of δK can be applied on any three-dimensional geometric structure and thus may be extended to other clinical problems of characterizing disease through captured anatomic changes

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

Study of a medical device to treat aortic dissection with Finite Element Analysis

Treballs Finals de Grau d'Enginyeria Biomèdica. Facultat de Medicina i Ciències de la Salut. Universitat de Barcelona. Curs: 2022-2023. Tutor/Director: Carmona Flores, Manuel, Soudah Prieto, EduardoThe aortic dissection is a cardiovascular disease that results from the rupture of the inner layer of the aorta. Type B aortic dissections commonly become a chronic disease with a high long-term morbidity and mortality rates. Current treatments include open surgery repair and thoracic endovascular aortic repair (TEVAR). However, new non-invasive treatments are being developed that favour the own regeneration of the tissue, avoiding the permanent presence of a foreign device in the body. This project focuses on the understanding of a new treatment with a medical device, an aortic patch, by in silico testing. The goal is to determine the performance of the patch in a simulated aortic dissection and then compare it with the current treatment with the stent graft (TEVAR), to determine if it would avoid the hypertension that can be caused by the stent. To do the first part, it was created a model of the aortic dissection, but due to complications with the simulation, this part of the project couldn’t be finished, and the performance of the patch in the aortic dissection couldn’t be determined. To do the second part three models were created: healthy aorta, aortic dissection with stent graft and aortic dissection with patch. A transient simulation was run for the three models and the pressure waveform was analyzed. The results show that the pressure in the stent graft model is higher, and the patch has a similar response to the healthy aorta. However, all the models presented hypertension (including the healthy aorta) and the differences between the models are too small to be concluding, so it cannot be assured that the patch is a better option than the stent graft to avoid causing hypertension in the aortic dissection treatment