497 research outputs found

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

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    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

    Numerical and experimental haemodynamic studies of stenotic coronary arteries

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    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

    High-frequency fluctuations in post-stenotic patient specific carotid stenosis fluid dynamics : a computational fluid dynamics strategy study

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    PurposeScreening of asymptomatic carotid stenoses is performed by auscultation of the carotid bruit, but the sensitivity is poor. Instead, it has been suggested to detect carotid bruit as neck's skin vibrations. We here take a first step towards a computational fluid dynamics proof-of-concept study, and investigate the robustness of our numerical approach for capturing high-frequent fluctuations in the post-stenotic flow. The aim was to find an ideal solution strategy from a pragmatic point of view, balancing accuracy with computational cost comparing an under-resolved direct numerical simulation (DNS) approach vs. three common large eddy simulation (LES) models (static/dynamic Smagorinsky and Sigma).MethodWe found a reference solution by performing a spatial and temporal refinement study of a stenosed carotid bifurcation with constant flow rate. The reference solution dwas compared against LES for both a constant and pulsatile flow.ResultsOnly the Sigma and Dynamic Smagorinsky models were able to replicate the flow field of the reference solution for a pulsatile simulation, however the computational cost of the Sigma model was lower. However, none of the sub-grid scale models were able to replicate the high-frequent flow in the peak-systolic constant flow rate simulations, which had a higher mean Reynolds number.ConclusionsThe Sigma model was the best combination between accuracy and cost for simulating the pulsatile post-stenotic flow field, whereas for the constant flow rate, the under-resolved DNS approach was better. These results can be used as a reference for future studies investigating high-frequent flow features

    Analysis of aortic-valve blood flow using computational fluid dynamics

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

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    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

    The importance of blood rheology in patient-specific computational fluid dynamics simulation of stenotic carotid arteries

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    The initiation and progression of atherosclerosis, which is the main cause of cardiovascular diseases, correlate with local haemodynamic factors such as wall shear stress (WSS). Numerical simulations such as computational fluid dynamics (CFD) based on medical imaging have been employed to analyse blood flow in different arteries with and without luminal stenosis. Patient-specific CFD models, however, have assumptions on blood rheology. The differences in the calculated haemodynamic factors between different rheological models have not been fully evaluated. In this study, carotid magnetic resonance imaging (MRI) was performed on six patients with different degrees of carotid stenosis and two healthy volunteers. Using the 3D reconstructed carotid geometries and the patient-specific boundary conditions, CFD simulations were performed by applying a Newtonian and four non-Newtonian models (Carreau, Cross, Quemada and Power-law). WSS descriptors and pressure gradient were analysed and compared between the models. The differences in the maximum and the average oscillatory shear index between the Newtonian and the non-Newtonian models were lower than 12.7% and 12%, respectively. The differences in pressure gradient were also within 15%. The differences in the mean time-averaged WSS (TAWSS) between the Newtonian and Cross, Carreau and Power-law models were lower than 6%. In contrast, a higher difference (26%) was found in Quemada. For the low TAWSS, the differences from the Newtonian to the non-Newtonian models were much larger, in the range of 0.4–31% for Carreau, 3–22% for Cross, 5–51% for Quemada and 10–41% for Power-law. The study suggests that the assumption of a Newtonian model is reasonable when the overall flow pattern or the mean values of the WSS descriptors are investigated. However, the non-Newtonian model is necessary when the low TAWSS region is the focus, especially for arteries with severe stenosis

    Uncertainty-aware Visualization in Medical Imaging - A Survey

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    Medical imaging (image acquisition, image transformation, and image visualization) is a standard tool for clinicians in order to make diagnoses, plan surgeries, or educate students. Each of these steps is affected by uncertainty, which can highly influence the decision-making process of clinicians. Visualization can help in understanding and communicating these uncertainties. In this manuscript, we aim to summarize the current state-of-the-art in uncertainty-aware visualization in medical imaging. Our report is based on the steps involved in medical imaging as well as its applications. Requirements are formulated to examine the considered approaches. In addition, this manuscript shows which approaches can be combined to form uncertainty-aware medical imaging pipelines. Based on our analysis, we are able to point to open problems in uncertainty-aware medical imaging
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