PIV-based Investigation of Hemodynamic Factors in Diseased Carotid Artery Bifurcations with Varying Plaque Geometries

Ischemic stroke is often a consequence of complications due to clot formation (i.e. thrombosis) at the site of an atherosclerotic plaque developed in the internal carotid artery. Hemodynamic factors, such as shear-stress forces and flow disturbances, can facilitate the key mechanisms of thrombosis. Atherosclerotic plaques can differ in the severity of stenosis (narrowing), in eccentricity (symmetry), as well as inclusion of ulceration (wall roughness). Therefore, in terms of clinical significance, it is important to investigate how the local hemodynamics of the carotid artery is mediated by the geometry of plaque. Knowledge of thrombosis-associated hemodynamics may provide a basis to introduce advanced clinical diagnostic indices that reflect the increased probability of thrombosis and thus assist with better estimation of stroke risk, which is otherwise primarily assessed based on the degree of narrowing of the lumen. A stereoscopic particle image velocimetry (stereo-PIV) system was configured to obtain instantaneous full-field velocity measurements in life-sized carotid artery models. Extraction of the central-plane and volumetric features of the flow revealed the complexity of the stenotic carotid flow, which increased with increasing stenosis severity and changed with the symmetry of the plaque. Evaluation of the energy content of two models of the stenosed carotid bifurcation provided insight on the expected level of flow instabilities with potential clinical implications. Studies in a comprehensive family of eight models ranging from disease-free to severely stenosed (30%, 50%, 70% diameter reduction) and with two types of plaque symmetry (concentric or eccentric), as well as a single ulcerated stenosed model, clearly demonstrated the significance of plaque geometry in marked alteration of the levels and patterns of downstream flow disturbances and shear stress. Plaque eccentricity and ulceration resulted in enhanced flow disturbances. In addition, shear-stress patterns in those models with eccentric stenosis were suggestive of increased thrombosis potential at the post-stenotic recirculation zone compared to their concentric counterpart plaques

Investigation of Flow Disturbances and Multi-Directional Wall Shear Stress in the Stenosed Carotid Artery Bifurcation Using Particle Image Velocimetry

Hemodynamics and shear forces are associated with pathological changes in the vascular wall and its function, resulting in the focal development of atherosclerosis. Flow complexities that develop in the presence of established plaques create environments favourable to thrombosis formation and potentially plaque rupture leading to stroke. The carotid artery bifurcation is a common site of atherosclerosis development. Recently, the multi-directional nature of shear stress acting on the endothelial layer has been highlighted as a risk factor for atherogenesis, emphasizing the need for accurate measurements of shear stress magnitude as well direction. In the absence of comprehensive patient specific datasets numerical simulations of hemodynamics are limited by modeling assumptions. The objective of this thesis was to investigate the relative contributions of various factors - including geometry, rheology, pulsatility, and compliance – towards the development of disturbed flow and multi-directional wall shear stress (WSS) parameters related to the development of atherosclerosis An experimental stereoscopic particle image velocimetry (PIV) system was used to measure instantaneous full-field velocity in idealized asymmetrically stenosed carotid artery bifurcation models, enabling the extraction of bulk flow features and turbulence intensity (TI). The velocity data was combined with wall location information segmented from micro computed tomography (CT) to obtain phase-averaged maps of WSS magnitude and direction. A comparison between Newtonian and non-Newtonian blood-analogue fluids demonstrated that the conventional Newtonian viscosity assumption underestimates WSS magnitude while overestimating TI. Studies incorporating varying waveform pulsatility demonstrated that the levels of TI and oscillatory shear index (OSI) depend on the waveform amplitude in addition to the degree of vessel constriction. Local compliance resulted in a dampening of disturbed flow due to volumetric capacity of the upstream vessel, however wall tracking had a negligible effect on WSS prediction. While the degree of stenosis severity was found to have a dominant effect on local hemodynamics, comparable relative differences in metrics of flow and WSS disturbances were found due to viscosity model, waveform pulsatility and local vessel compliance

Haemodynamics analysis of carotid artery stenosis and carotid artery stenting

Carotid stenosis is a local narrowing of the carotid artery, and is usually found in the internal carotid artery. The presence of a high-degree stenosis in a carotid artery may provoke transition from laminar to turbulent flow during part of the cardiac cycle. Turbulence in blood flow can influence haemodynamic parameters such as velocity profiles, shear stress and pressure, which are important in wall remodelling. Patients with severe stenosis could be treated with a minimally invasive clinical procedure, carotid artery stenting (CAS). Although CAS has been widely adopted in clinical practice, the complication of in-stent restenosis (ISR) has been reported after CAS. The incidence of ISR is influenced by stent characteristics and vessel geometry, and correlates strongly with regions of neointimal hyperplasia (NH). Therefore, the main purpose of this study is to provide more insights into the haemodynamics in stenosed carotid artery and in post-CAS geometries via computational simulation. The first part of the thesis presents a computational study on flow features in a stenotic carotid artery bifurcation using two computational approaches, large eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) incorporating the Shear Stress Transport model with the γ-Reθ transition (SST-Tran) models. The computed flow patterns are compared with those measured with particle image velocimetry (PIV). The results show that both SST-Tran and LES can predict the PIV results reasonably well, but LES is more accurate especially at locations distal to the stenosis where flow is highly disturbed. The second part of the thesis is to determine how stent strut design may influence the development of ISR at the carotid artery bifurcation following CAS. Key parameters that can be indicative of ISR are obtained for different stent designs and compared; these include low and oscillating wall shear stress (WSS), high residence time, and wall stress. A computationally efficient methodology is employed to reproduce stent strut geometry. This method facilitates the accurate reconstruction of actual stent geometry and details of strut configuration and its inclusion in the fluid domain. Computational simulations for flow patterns and low-density lipoprotein (LDL) transport are carried out in order to investigate spatial and temporal variations of WSS and LDL accumulation in the stented carotid geometries. Furthermore, finite element (FE) analysis is performed to evaluate the wall stress distribution with different stent designs. The results reveal that the closed-cell stent design is more likely to create atheroprone and procoagulant flow conditions, causing larger area to be exposed to low wall shear stress (WSS), elevated oscillatory shear index, as well as to induce higher wall stress compared to the open-cell stent design. This study also demonstrates the suitability of SST-Tran and LES models in capturing the presence of complex flow patterns in post-stenotic region.Open Acces

Modal Analysis of Blood Flow in Brain Aneurysms

The evolution of blood flow is vital in understanding the pathogenesis of brain aneurysms. Several past studies have shown evidence for a turbulent inflow jet at the aneurysm neck. Although there is a great need for analyzing inflow jet dynamics in clinical practice, data summarized in noninvasive modalities such as Magnetic Resonance Imaging or Computed Tomography are usually limited by spatial and temporal resolutions, and thus cannot account for the hemodynamics. In this paper, Dynamic Mode Decomposition (DMD) is used to pinpoint the dominant modes of the inflow jet in patient-specific models of sidewall aneurysms. This paper aims to prove that the dynamic modes are inflow jet interaction with the distal wall in addition to the hemodynamics of the parent artery. Our work indicates that DMD is an essential tool for analyzing blood flow patterns of brain aneurysms and is a promising tool to be used in in vivo context

Numerical and experimental haemodynamic studies of stenotic coronary arteries

Dissertação de mestrado integrado em Engenharia Biomédica (área de especialização em Biomateriais, Reabilitação e Biomecânica)Cardiovascular diseases remain the most frequent cause of mortality worldwide and constitute a major healthcare challenge. Among them, coronary artery disease causes nearly half of the deaths and, thus it is of great interest to better understand its development and effects. This disease is characterized by the narrowing (stenosis) of coronary arteries due to plaque deposition at the arterial wall, a pathological process known as atherosclerosis. This dissertation aimed to study the hemodynamics in stenotic coronary arteries, in order to get a deeper understanding of the effects of this pathology on the blood flow behavior. For this purpose, both numerical and experimental studies were conducted using idealized models. The numerical research was carried out using Ansys® software by means of computational fluid dynamics which applies the finite volume method. The experimental approach was performed using a high-speed video microscopy system, to visualize and investigate the blood flow in the in vitro stenotic biomodels. Initially, the influence of roughness in flow visualizations was studied, and the best biomodel was the one printed with the lowest resolution having been, therefore, the selected to perform the hemodynamic studies. To compare those results with numerical data, the flow was set to be laminar and stationary and the fluid was considered Newtonian. In general, the numerical and experimental results were in good agreement, not only in the prediction of the flow behavior with the appearance of recirculation zones in the post-stenotic section, but also in the velocity profiles. In a posterior phase, a pulsatile inlet condition was applied to compare the use of laminar and turbulent assumptions, using the SST k- model. The results obtained allowed to conclude that the second one is more appropriate to simulate the blood flow. Subsequently, the main differences in hemodynamics were examined considering blood as a Newtonian and non-Newtonian fluid (Carreau model). For these models, the differences were very slight in terms of velocity fields, but more significant for the wall shear stress measurements, with the Newtonian model predicting lower values. The remaining simulations were performed using the Carreau model and a transient inlet flow, having observed an increase in the velocities and wall shear stress values with the degree of stenosis, which is associated with a greater risk of thrombosis.As doenças cardiovasculares continuam a ser a causa mais frequente de mortalidade em todo o mundo e constituem um grande desafio para a saúde. Entre elas, a doença arterial coronariana causa quase metade das mortes e, portanto, é de enorme interesse entender melhor o seu desenvolvimento e efeitos. Esta doença é caracterizada pelo estreitamento (estenose) das artérias coronárias devido à deposição de placas na parede arterial, um processo patológico conhecido como aterosclerose. Esta dissertação teve como objetivo estudar a hemodinâmica nas artérias coronárias estenóticas, a fim de obter uma compreensão mais profunda dos efeitos desta patologia no comportamento do fluxo sanguíneo. Para tal, foram realizados estudos numéricos e experimentais, utilizando modelos idealizados. A investigação numérica foi realizada no software Ansys®, através da dinâmica computacional dos fluidos, que aplica o método dos volumes finitos. A abordagem experimental foi realizada utilizando um sistema de microscopia de vídeo de alta velocidade, para visualizar e investigar o fluxo sanguíneo nos biomodelos estenóticos in vitro. Inicialmente, estudou-se a influência da rugosidade nas visualizações do escoamento, e o melhor biomodelo foi o impresso com menor resolução tendo sido, portanto, o selecionado para a realização dos estudos hemodinâmicos. Para comparar esses resultados com dados numéricos, o escoamento foi definido como laminar e estacionário e o fluído foi considerado Newtoniano. Em geral, os resultados numéricos e experimentais foram concordantes, não só na previsão do comportamento do fluxo com aparecimento de zonas de recirculação na zona pós-estenótica, mas também nos perfis de velocidade. Numa fase posterior, foi aplicada uma condição de entrada pulsátil para comparar o uso de simulações de natureza laminar e turbulenta, usando o modelo SST k-. Os resultados obtidos permitiram concluir que a segunda é mais apropriado para simular o fluxo sanguíneo. Posteriormente, foram examinadas as principais diferenças hemodinâmicas, considerando o sangue como fluído Newtoniano e não-Newtoniano (modelo de Carreau). Para estes modelos, as diferenças foram muito pequenas nos perfis de velocidade, mas mais significativas nas tensões de corte na parede medidas, com o modelo Newtoniano a prever valores mais baixos. As restantes simulações foram realizadas usando o modelo de Carreau e um escoamento de entrada transiente, tendo-se observado um aumento dos valores das velocidades e da tensão de corte na parede com o grau de estenose, o que está associado a um maior risco de trombose

Development of a haemodynamic model for improving clinical treatment of vascular disease

Atherosclerosis is a chronic artery disease that leads to heart attack and stroke; affecting millions of people worldwide. It tends to develop in locations where disturbed flow patterns occur, such as the carotid artery, left coronary artery and abdominal aorta. The causative factors leading to atherosclerosis still remain relatively poorly understood. Conventional diagnosis of arterial disease relies on a combination of history, clinical examination and clinical imaging derived from CT, MRI, etc. To address some of the important factors related to arterial haemodynamics, Computational Fluid Dynamics (CFD) studies were performed on in-vitro models using physiologically relevant conditions. The flow disturbances in terms of wall shear stress and oscillatory shear index were examined. Based on the current research, new insights from a haemodynamics point of view were provided. This study aims to enrich and complement the current arterial disease research, and contribute to promoting the diagnosis accuracy and efficiency in the future. This thesis is composed by six parts of work. Firstly, a comprehensive literature review was performed to identify the research gaps between the current relevant numerical studies with real clinical application. Secondly, the proposed CFD model was validated with published experimental work using particle image velocimetry (PIV) approach. A downstream impedance model was then developed to improve numerical simulation accuracy for image-based artery bifurcations. The numerical results were correlated with a clinical indicator to provide relevant findings for treating physicians. Lastly, a fully fluid-structure interaction (FSI) modelling over left coronary artery models with different bifurcation angles was conducted. The relationship between the mechanical force (first principle stress), the hemodynamic force (wall shear stress), and the bifurcation angle was analysed. In summary, this thesis developed a new downstream artery impedance model, and converted the numerical simulation results into clinical indicators, which can improve the current simulation accuracy and contribute more meaningful results to assist a better clinical diagnosis. A FSI simulation was performed over left coronary artery bifurcation models. The bifurcation angle influence on atherosclerosis progression was addressed. The left circumflex side bifurcation shoulder was found to be more vulnerable in developing atherosclerosis

A locally conservative Galerkin approach for subject-specific biofluid dynamics.

In this thesis, a parallel solver was developed for the modelling of blood flow through a number of patient-specific geometries. A locally conservative Galerkin (LCG) spatial discretisation was applied along with an artificial compressibility and characteristic based split (CBS) scheme to solve the 3D incompressible Navier-Stokes equations. The Spalart-Allmaras one equation turbulence model was also optionally employed. The solver was constructed using FORTRAN and the Message Passing Interface (MPI). Parallel testing demonstrated linear or better than linear speedup on hybrid patient-specific meshes. These meshes were unstructured with structured boundary layers. From the parallel testing it is clear that the significance of inter-processor communication is negligible in a three dimensional case. Preliminary tests on a short patient-specific carotid geometry demonstrated the need for ten or more boundary layer meshes in order to sufficiently resolve the peak wall shear stress (WSS) along with the peak time-averaged WSS. A time sensitivity study was also undertaken along with the assessment of the order of the real time step term. Three backward difference formulae (BDF) were tested and no significant difference between them was detected. Significant speedup was possible as the order of time discretisation increased however, making the choice of BDF important in producing a timely solution. Followed by the preliminary investigation, four more carotid geometries were investigated in detail. A total of six haemodynamic wall parameters have been brought together to analyse the regions of possible atherogenesis within each carotid. The investigations revealed that geometry plays an overriding influence on the wall parameter distribution. Each carotid artery displayed high time-averaged WSS at the apex, although the value increased significantly with a proximal stenosis. Two out of four meshes contained a region of low time-averaged WSS distal to the flow divider and within the largest connecting artery (internal or external carotid artery), indicating a potential region of atherosclerosis plaque formation. The remaining two meshes already had a stenosis in the corresponding region. This is in excellent agreement with other established works. From the investigations, it is apparent that a classification system of stenosis severity may be possible with potential application as a clinical diagnosis aid. Finally, the flow within a thoracic aortic aneurysm was investigated in order to assess the influence of a proximal folded neck. The folded neck had a significant effect on the wall shear stress, increasing by up to 250% over an artificially smoothed neck. High wall shear stresses may be linked to aneurysm rupture. Being proximal to the aneurysm, this indicated that local geometry should be taken into account when assessing the rupture potential of an aneurysm

Transitional pulsatile flows with stenosis in a two-dimensional channel

Although blood flows are mostly laminar, transition to turbulence and flow separations are observed at curved vessels, bifurcations, or constrictions. It is known that wall-shear stress plays an important role in the development of atherosclerosis as well as in arteriovenous grafts. In order to help understand the behavior of flow separation and transition to turbulence in post-stenotic blood flows, an experimental study of transitional pulsatile flow with stenosis was carried out using time-resolved particle image velocimetry and a microelectromechanical systems wall-shear stress sensor at the mean Reynolds number of 1750 with the Womersley number of 6.15. At the start of the pulsatile cycle, a strong shear layer develops from the tip of the stenosis, increasing the flow separation region. The flow at the throat of the stenosis is always laminar due to acceleration, which quickly becomes turbulent through a shear-layer instability under a strong adverse pressure gradient. At the same time, a recirculation region appears over the wall opposite to the stenosis, moving downstream in sync with the movement of the reattachment point. These flow behaviors observed in a two-dimensional channel flow are very similar to the results obtained previously in a pipe flow. We also found that the behavior in a pulsating channel flow during the acceleration phase of both 25% and 50% stenosis cases is similar to that of the steady flow, including the location and size of post-stenotic flow separation regions. This is because the peak Reynolds number of the pulsatile flow is similar to that of the steady flow that is investigated. The transition to turbulence is more dominant for the 50% stenosis as compared to the 25% stenosis, as the wavelet spectra show a greater broadening of turbulence energy. With an increase in stenosis to 75%, the accelerating flow is directed toward the opposite wall, creating a wall jet. The shear layer from the stenosis bifurcates as a result of this, one moving with the flow separation region toward the upper wall and the other with the wall jet toward the bottom wall. Low wall-shear stress fluctuations are found at two post-stenotic locations in the channel flow – one immediately downstream of the stenosis over the top wall (stenosis side) inside the flow separation region, and the other in the recirculation region on the bottom wall (opposite side of the stenosis)

Study of the Dissipation in Spiraling Vortical Structures

This work deals with study of swirling flows where the spiral vortical structure appears. The main relation is to flow seen in the draft tube cone of hydraulic turbines operated out of the design point (i.e. best efficiency point). In this cases large coherent vortex structure (vortex rope) appears and consequently high pressure pulsations are propagated to the whole machine system leading to possible restriction of turbine operation. This flow features are consequence of flow instability called vortex breakdown in case of Francis turbine operated at part load (flow rate lower than optimal one). The present study is carried out using simplified device of swirl generator in order to access similar flow conditions as can be found in real hydraulic turbines. Both the dynamic and dissipation effect of spiral vortex breakdown are investigated. The first part of thesis deals with spiral form of vortex breakdown. The experimentally measured velocity profiles (LDA) and wall static pressures are correlated with numerical simulations carried out using open-source CFD package OpenFOAM 2.2.2. The high speed camera recording of cavitating vortex core is used to obtain image ensemble for further post-processing. The dissipation effect of spiral vortex structure is in detail discussed based on computed flow fields. The second part of thesis is dedicated to the application of POD decomposition to the study of spatio-temporal features of spiral vortex dynamics. Firstly the POD is applied to the both the experimentally obtained image ensemble of cavitating vortex and numerically computed static pressure fields. Secondly the comprehensive analysis of spiral vortex mitigation effect by the axial water jet is analyzed. The collaborative study employing the swirl generator apparatus designed by the researchers from Politehnica University of Timisoara in Romania is performed and changes in spatio-temporal vortex dynamic are studied. In this study the numerical data (in a form of three-dimensional pressure and velocity fields) are obtained using commercial CFD software ANSYS Fluent R14.This work deals with study of swirling flows where the spiral vortical structure appears. The main relation is to flow seen in the draft tube cone of hydraulic turbines operated out of the design point (i.e. best efficiency point). In this cases large coherent vortex structure (vortex rope) appears and consequently high pressure pulsations are propagated to the whole machine system leading to possible restriction of turbine operation. This flow features are consequence of flow instability called vortex breakdown in case of Francis turbine operated at part load (flow rate lower than optimal one). The present study is carried out using simplified device of swirl generator in order to access similar flow conditions as can be found in real hydraulic turbines. Both the dynamic and dissipation effect of spiral vortex breakdown are investigated. The first part of thesis deals with spiral form of vortex breakdown. The experimentally measured velocity profiles (LDA) and wall static pressures are correlated with numerical simulations carried out using open-source CFD package OpenFOAM 2.2.2. The high speed camera recording of cavitating vortex core is used to obtain image ensemble for further post-processing. The dissipation effect of spiral vortex structure is in detail discussed based on computed flow fields. The second part of thesis is dedicated to the application of POD decomposition to the study of spatio-temporal features of spiral vortex dynamics. Firstly the POD is applied to the both the experimentally obtained image ensemble of cavitating vortex and numerically computed static pressure fields. Secondly the comprehensive analysis of spiral vortex mitigation effect by the axial water jet is analyzed. The collaborative study employing the swirl generator apparatus designed by the researchers from Politehnica University of Timisoara in Romania is performed and changes in spatio-temporal vortex dynamic are studied. In this study the numerical data (in a form of three-dimensional pressure and velocity fields) are obtained using commercial CFD software ANSYS Fluent R14.

High Frame Rate Ultrasound Particle Image Velocimetry for Estimating High Velocity Flow Patterns in the Left Ventricle

Echocardiographic determination of multi-component blood flow dynamics in the left ventricle remains a challenge. In this study we compare contrast enhanced, high frame rate (1000 fps) echo particle image velocimetry (ePIV) against optical particle image velocimetry (oPIV, gold standard), in a realistic left ventricular phantom. We find that ePIV compares well to oPIV, even for the high velocity inflow jet (normalized RMSE = 9 ±1%). Additionally, we perform the method of Proper Orthogonal Decomposition, to better qualify and quantify the differences between the two modalities. We show that ePIV and oPIV resolve very similar flow structures, especially for the lowest order mode with a cosine similarity index of 86%. The co