99 research outputs found
Hemodynamics in the Stenosed Carotid Bifurcation with Plaque Ulceration
The presence of irregular plaque surface morphology or ulceration of the atherosclerotic lesion has been identified as an independent risk factor for ischemic stroke. Doppler ultrasound (DUS) is the most commonly performed non-invasive technique used to assess patients suspected of having carotid artery disease, but currently does not incorporate the diagnosis of plaque ulceration. Advanced Doppler analyses incorporating quantitative estimates of flow disturbances may result in diagnostic indices that identify plaque ulcerative conditions. A technique for the fabrication of DUS-compatible flow phantoms was developed, using a direct-machining method that is amenable to comprehensive DUS investigations. In vitro flow studies in an ensemble of matched model vessel geometries determined that ulceration as small as 2 mm can generate significant disturbances in the downstream flow field in a moderately stenosed carotid artery, which are detectable using the DUS velocity-derived parameter turbulence intensity (TI) measured with a clinical system. Further experimental results showed that distal TI was significantly elevated (P \u3c 0.001) due to proximal plaque ulceration in the mild and moderately stenosed carotid bifurcation (30%, 50%, 60% diameter reduction), and also increased with stenosis severity. Pulsatile computational fluid dynamics (CFD) models, with simulated particle tracking, demonstrated enhanced flow disruption of the stenotic jet and slight elevations in path-dependent shear exposure parameters in a stenosed carotid bifurcation model with ulceration. In addition, CFD models were used to evaluate the DUS index TI using finite volume sampling
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
In vitro biomodels in stenotic arteries to perform blood analogues flow visualizations and measurements: a review
Cardiovascular diseases are one of the leading causes of death globally and the most common pathological process is atherosclerosis. Over the years, these cardiovascular complications have been extensively studied by applying in vivo, in vitro and numerical methods (in silico). In vivo studies represent more accurately the physiological conditions and provide the most realistic data. Nevertheless, these approaches are expensive, and it is complex to control several physiological variables. Hence, the continuous effort to find reliable alternative methods has been growing. In the last decades, numerical simulations have been widely used to assess the blood flow behavior in stenotic arteries and, consequently, providing insights into the cardiovascular disease condition, its progression and therapeutic optimization. However, it is necessary to ensure its accuracy and reliability by comparing the numerical simulations with clinical and experimental data. For this reason, with the progress of the in vitro flow measurement techniques and rapid prototyping, experimental investigation of hemodynamics has gained widespread attention. The present work reviews state-of-the-art in vitro macro-scale arterial stenotic biomodels for flow measurements, summarizing the different fabrication methods, blood analogues and highlighting advantages and limitations of the most used techniques.This work has been supported by FCT – Fundação para a
Ciência e Tecnologia within the R&D Units Project Scope:
UIDB/00319/2020, UIDB/04077/2020, UIDB/00690/2020,
UIDB/04436/2020 and NORTE-01-0145-FEDER-030171,
NORTE-01-0145-FEDER-029394 funded by COMPETE2020,
NORTE 2020, PORTUGAL 2020, Lisb@2020 and FEDER.info:eu-repo/semantics/publishedVersio
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
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
3D printed biomodels for flow visualization in stenotic vessels: an experimental and numerical study
Atherosclerosis is one of the most serious and common forms of cardiovascular disease and a major cause of death and disability worldwide. It is a multifactorial and complex disease that promoted several hemodynamic studies. Although in vivo studies more accurately represent the physiological conditions, in vitro experiments more reliably control several physiological variables and most adequately validate numerical flow studies. Here, a hemodynamic study in idealized stenotic and healthy coronary arteries is presented by applying both numerical and in vitro approaches through computational fluid dynamics simulations and a high-speed video microscopy technique, respectively. By means of stereolithography 3D printing technology, biomodels with three different resolutions were used to perform experimental flow studies. The results showed that the biomodel printed with a resolution of 50 μm was able to most accurately visualize flow due to its lowest roughness values (Ra = 1.8 μm). The flow experimental results showed a qualitatively good agreement with the blood flow numerical data, providing a clear observation of recirculation regions when the diameter reduction reached 60%.This work was supported by FCT-Fundacao para a Ciencia e Tecnologia within the R&D Units Project Scope: UIDB/00319/2020, UIDB/04077/2020, and NORTE-01-0145-FEDER-030171, funded by COMPETE2020, NORTE 2020, PORTUGAL 2020, and FEDER. This project received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 798014. This project received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 828835
TURBULENCE ACCUMULATION AND AVERAGE IN THE SYMMETRICALLY AND ASYMMETRICALLY STENOSED CAROTID BIFURCATION
Ischemic stroke due to atherosclerotic disease has been studied widely in the recent past. Most studies focus on either the correlation between stroke risk and stenosis severity (narrowing of the plaque in the vessel) or mechanisms affecting platelet activation and aggregation. Shear stress has been identified as a strong indicator for platelet activation/aggregation, resulting in both thrombus formation and plaque growth. This has subsequently been correlated with regions of elevated turbulence.
Doppler ultrasound offers a method of characterizing these flow disturbances using a well-established parameter—turbulence intensity (Tl), which is the root mean squared deviation in the spectral mean velocity. Using an in-house in vitro flow system, Doppler spectra are obtained at each of over 1000, 1-mm3 isotropically spaced sites in the central plane of seven Teflon phantoms simulating varying degrees of arterial disease. An average of Tl over a 25 mm2 region of interest, as well as the volume of Tl and the cumulative Tl over the internal carotid artery showed that downstream turbulence increased significantly with both stenosis severity (30% - 650% increase) and plaque asymmetry (10% - 30% increase)
Blood Supply to the Brain via the Carotid Arteries: Examining Obstructive and Sclerotic Disorders using Theoretical and Experimental Models
Stroke remains one of the leading causes of death in North America. Approximately half of all ischemic episodes are a direct result of carotid artery disease, which can be categorized into either obstructive or sclerotic disease. Obstructive disease is a result of plaque development that imposes a direct limitation on the physical space available for blood flow. Sclerotic disease involves the hardening of the arteries as is often a result of aging and disease. While the impact of vessel stiffening is not as obvious, it does interfere with wave propagation. Effects of obstructive and sclerotic disease were studied using a lumped parameter model that was designed to match an experimental in vitro flow loop. Mild to moderate stenosis had minimal impact on blood supply to the brain. Both stiffness of the carotid artery and severe stenosis ( 70%) had a significant reduction on blood supply to the brain (p\u3c0.01)
Carotid Atheroma Rupture Observed In Vivo and FSI-Predicted Stress Distribution Based on Pre-rupture Imaging
Atherosclerosis at the carotid bifurcation is a major risk factor for stroke. As mechanical forces may impact lesion stability, finite element studies have been conducted on models of diseased vessels to elucidate the effects of lesion characteristics on the stresses within plaque materials. It is hoped that patient-specific biomechanical analyses may serve clinically to assess the rupture potential for any particular lesion, allowing better stratification of patients into the most appropriate treatments. Due to a sparsity of in vivo plaque rupture data, the relationship between various mechanical descriptors such as stresses or strains and rupture vulnerability is incompletely known, and the patient-specific utility of biomechanical analyses is unclear. In this article, we present a comparison between carotid atheroma rupture observed in vivo and the plaque stress distribution from fluid–structure interaction analysis based on pre-rupture medical imaging. The effects of image resolution are explored and the calculated stress fields are shown to vary by as much as 50% with sub-pixel geometric uncertainty. Within these bounds, we find a region of pronounced elevation in stress within the fibrous plaque layer of the lesion with a location and extent corresponding to that of the observed site of plaque rupture
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