Simulation of arterial stenosis incorporating fluid-structural interaction and non-Newtonian blood flow

The aim of this study is to investigate the fluid-structural response to pulsatile Newtonian and non-Newtonian blood flow through an axisymmetric stenosed vessel using FLOTRAN and ANSYS. This is to provide a basic understanding of atherosclerosis. The flow was set to be laminar and follows a sinusoidal waveform. The solid model was set to have isotropic elastic properties. The Fluid-Structural Interaction (FSI) coupling was two-way and iterative. Rigid and Newtonian cases were investigated to provide an understanding on the effects of incorporating FSI into the model. The wall expansion was found to decrease the axial velocity and increase the recirculation effects of the flow. To validate the models and methods used, the results were compared with the study by Lee and Xu [2002] and Ohja et al [1989]. Close comparisons were achieved, suggesting the models used were valid. Two non-Newtonian models were investigated with FSI: Carreau and Power Law models. The Carreau model fluid behaviour was very close to the Newtonian model. The Power Law model produced significant difference in viscosity, velocity and wall shear stress distributions. Pressure distribution for all models was similar. In order to quantify the changes, Importance Factor (IG) was introduced to determine the overall non-Newtonian effects at two regions: the entire flow model and about the vessel wall. The Carreau model showed reasonable values of IG whereas the Power Law model showed excessive values. Transient and geometrical effects were found to affect the Importance Factor. The stress distributions for all models were found to be similar. Highest stress occurred at the shoulders of the stenosis where a stress concentration occurred due to sharp corners of the geometry and large bending moments. The highest stresses were in the axial direction. Notable circumferential stress was found at the ends of the vessel. Carreau model produced slightly higher stresses than the other models. Wall stresses were found to be primarily influenced by internal pressure, rather than wall shear stresses

Influence of blood pressure and rheology on oscillatory shear index and wall shear stress in the carotid artery

Atherosclerosis is a localized complication dependent on both the rheology and the arterial response to blood pressure. Fluid–structure interaction (FSI) study can be effectively used to understand the local haemodynamics and study the development and progression of atherosclerosis. Although numerical investigations of atherosclerosis are well documented, research on the influence of blood pressure as a result of the response to physio–social factors like anxiety, mental stress, and exercise is scarce. In this work, a three-dimensional (3D) Fluid–Structure Interaction (FSI) study was carried out for normal and stenosed patient-specific carotid artery models. Haemodynamic parameters such as Wall Shear Stress (WSS) and Oscillatory Shear Index (OSI) are evaluated for normal and hypertension conditions. The Carreau–Yasuda blood viscosity model was used in the FSI simulations, and the results are compared with the Newtonian model. The results reveal that high blood pressure increases the peripheral resistance, thereby reducing the WSS. Higher OSI occurs in the region with high flow recirculation. Variation of WSS due to changes in blood pressure and blood viscosity is important in understanding the haemodynamics of carotid arteries. This study demonstrates the potential of FSI to understand the causes of atherosclerosis due to altered blood pressures

Multiscale Fluid-Structure Interaction Models Development and Applications to the 3D Elements of a Human Cardiovascular System

Cardiovascular diseases (CVD) are the number one cause of death of humans in the United States and worldwide. Accurate, non-invasive, and cheaper diagnosis methods have always been on demand as cardiovascular monitoring increase in prevalence. The primary causes of the various forms of these CVDs are atherosclerosis and aneurysms in the blood vessels. Current noninvasive methods (i.e., statistical/medical) permit fairly accurate detection of the disease once clinical symptoms are suggestive of the existence of hemodynamic disorders. Therefore, the recent surge of hemodynamics models facilitated the prediction of cardiovascular conditions. The hemodynamic modeling of a human circulatory system involves varying levels of complexity which must be accounted for and resolved. Pulse-wave propagation effects and high aspect-ratio segments of the vasculature are represented using a quasi-one-dimensional (1D), non-steady, averaged over the cross-section models. However, these reduced 1D models do not account for the blood flow patterns (recirculation zones), vessel wall shear stresses and quantification of repetitive mechanical stresses which helps to predict a vessel life. Even a whole three-dimensional (3D) modeling of the vasculature is computationally intensive and do not fit the timeline of practical use. Thus the intertwining of a quasi 1D global vasculature model with a specific/risk-prone 3D local vessel ones is imperative. This research forms part of a multiphysics project that aims to improve the detailed understanding of the hemodynamics by investigating a computational model of fluid-structure interaction (FSI) of in vivo blood flow. First idealized computational a 3D FSI artery model is configured and executed in ANSYS Workbench, forming an implicit coupling of the blood flow and vessel walls. Then the thesis focuses on an approach developed to employ commercial tools rather than in-house mathematical models in achieving multiscale simulations. A robust algorithm is constructed to combine stabilization techniques to simultaneously overcome the added-mass effect in 3D FSI simulation and mathematical difficulties such as the assignment of boundary conditions at the interface between the 3D-1D coupling. Applications can be of numerical examples evaluating the change of hemodynamic parameters and diagnosis of an abdominal aneurysm, deep vein thrombosis, and bifurcation areas

Geometry and flow properties affect phase shift between pressure and shear stress waves in blood vessels

The phase shift between pressure and wall shear stress (WSS) has been associated with vascular diseases such as atherosclerosis and aneurysms. The present study aims to understand the effects of geometry and flow properties on the phase shift under the stiff wall assumption, using an immersed-boundary-lattice-Boltzmann method. For pulsatile flow in a straight pipe, the phase shift is known to increase with the Womersley number, but is independent of the flow speed (or the Reynolds number). For a complex geometry, such as a curved pipe, however, we find that the phase shift develops a strong dependence on the geometry and Reynolds number. We observed that the phase shift at the inner bend of the curved vessel and in the aneurysm dome is larger than that in a straight pipe. Moreover, the geometry affects the connection between the phase shift and other WSS-related metrics, such as time-averaged WSS (TAWSS). For straight and curved blood vessels, the phase shift behaves qualitatively similarly to and can thus be represented by the TAWSS, which is a widely used hemodynamic index. However, these observables significantly differ in other geometries, such as in aneurysms. In such cases, one needs to consider the phase shift as an independent quantity that may carry additional valuable information compared to well-established metrics

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

Investigation of vibrations at the skin surface caused by the flow disturbance in stenosed tubes

An occlusion in an artery (a stenosis) induces disturbances in the downstream flow and those disturbances produce mechanical waves that impinge on the vessel wall causing it to vibrate. These vibrations then travel through the soft tissues to the skin surface where they can be detected, thus providing a possible method for the non-invasive diagnosis of the underlying disease. Hence, in this study, the potential of measuring those disturbances was explored. Experiments were set-up to model the behaviour of carotid artery under different con-ditions. A 40:60 (by volume) glycerine-water solution was selected to simulate the vis-cosity of blood (approximately 4cP). A thin walled (250 μm - 350μm wall thickness) latex Penrose drain tube, with an external diameter of 6.35mm and a Young’s Modulus value of around 0.9MPa (for circumferential strains between 0.08 - 0.10), was used to mimic the carotid artery. Different severities (60%, 75% and 90% area reduction) of the stenoses, with a circular cross-section, were investigated. Two different types, axisym-metric and non-axisymmetric, were investigated to study the effects of asymmetry in the stenoses. The stenoses were 3D printed in an opaque VeroWhite material to mimic the occlusion. The stenosed artery was then embedded into a standardised neck phantom (filled with Parker Aquasonic 100 ultrasound gel) to mimic the soft tissues and the top of the phantom was sealed with a thin (50μm) polyurethane film (Platilon), with a stiffness value of approximately 21MPa, to simulate the skin. To detect the disturbance on the phantom surface, different equipment (including ac-celerometers, and a Laser Doppler Vibrometer [LDV]) were tested. The LDV proved to be the most reliable under all conditions and was chosen as the standard measurement method for all the phantom experiments. Having selected the appropriate materials and measurement techniques, the effects of flow rate, stenosis severity, stenosis symmetry and fluid viscosity were investigated. Both, steady and pulsatile flows were perfused through the phantom with flows ranging from 0-450ml/min for steady flow and 308-340ml/min for pulsatile flow. On modifying the viscosity of the fluid, it was seen that increasing the viscosity reduced the perturbations in the flow. This was expected due to the increased viscous forces in the flow, as the viscosity of the fluid increased. Furthermore, the experiments showed that, on increasing the flow rate, the stenosis severity, and/or introducing asymmetry in the stenosis, the post-stenotic perturbations in the flow were amplified and their zone of origin moved nearer to the stenosis. These features were confirmed by conducting bare tube experiments as well as some ultrasound scans in a modified phantom. On further investigation it was found that along with the positional dependence of these perturbations, their range of frequencies was increased with increasing flow rate, stenosis severity and/or stenosis asymmetry. In the phantom experiments the disturbances were barely detectable for an area reduc-tion of 60% and were weakly present at 75%. However, strong disturbances were seen for the highly (90%) stenosed tube. A possible cause of the unexpectedly small effect of the 75% stenosis was speculated to be the stenosis symmetry: in-vivo, atherosclerotic plaques are invariably not symmetrical. To show this, experiments were conducted with an asymmetric stenosis where higher level of disturbances were detected (even with the 75% stenosis severity), hence, emphasising the impact of stenosis symmetry. A preliminary computational simulation was also set-up to allow for future detailed modelling of the effects of changing the physical conditions on the signals arriving at the skin. The simulations (whose accuracy yet remains to be validated) showed similar effects of the increasing flow rates and the stenosis severity as the disturbances were amplified and moved nearer to the stenosis on increasing the value of either variable. Following this, an attempt was made to develop a fluid-structure interaction model to simulate the neck phantom and a sample simulation was set-up. This study developed a novel method for detecting the disturbances in the post-stenotic region, and the experimental results from this study suggest the feasibility of using LVD to infer the presence of a stenosis at an early stage before the symptoms are evident

Advanced Geometric Analyses in Vascular Disease and Interventions

The central function of the vascular system is the transportation and distribution of blood throughout the body. Occurrence of various forms of vascular disease, in which this central function is compromised, is associated with high mortality and morbidity rates. Vascular disease is an active and multifactorial process causing changes in local hemodynamics and blood vessel wall mechanics. The work presented here aims to advance the current knowledge on the role of local geometrical parameters in vascular disease and endovascular intervention modalities. In our first set of studies, we develop a computational framework to understand the role of aortic geometry in abdominal aortic aneurysm wall mechanics, with the purpose of predicting peak wall stress and the risk of rupture. In our next set of studies, we seek to understand possible associations between local hemodynamics and geometric variables in severe carotid artery stenosis (CAS). Using key geometric descriptors of CAS, we formulate stress-based prediction models that can be useful in disease progression risk stratification. In regard to geometric analyses in an endovascular device technology, studies are focused on identifying microstructurefunction relations in drug-coated balloon (DCB) therapy and developing upon current DCB excipient design. The DCB therapy is an emerging intervention procedure with a great scope for improvement, that integrates angioplasty with local drug delivery to restore lumen patency at atherosclerotic lesion sites. Taken together, results from these studies can vi contribute to clinical practices and healthcare market in capacities including disease severity diagnosis, surgical decision making, and endovascular therapy technologies

Hemodynamics of Diseased Coronary Arteries

Cardiovascular diseases are one of the main causes of death worldwide. A common cardiovascular disease is atherosclerosis, caused by plaque deposition on the arterial wall which leads to the obstruction of the blood ow, known as stenosis. Atherosclerosis can form in any part of the arterial system but it may have serious consequences when located in one of the coronary arteries which supply blood to the heart. Plaque formation inside a coronary artery influences the flow behaviour and leads to the development of turbulence structures with physiological consequences, such as pressure drop. Percutaneous coronary intervention (PCI) is one of the most common treatments for coronary artery diseases (CADs). There are several benefits of PCI over other alternative methods for treatment of CADs including lower risks of complications and much shorter recovery period. However, it can result in thrombosis and in-stent restenosis, which are the major drawbacks of coronary stent placement in patients with CADs. It was shown that the likelihood of occurrence of restenosis and thrombosis is a function of the wall shear stress (WSS) distribution. The motivation for the research presented in this thesis is to develop an understanding of the hemodynamics of stenosed and stented coronary arteries with an ultimate goal of improving patient outcomes. This can only be achieved if the effect of stenoses and stents on the flow behaviour in arteries is well-characterised. Hence, in this thesis the relationship between the shape of stenosis, stent pattern, the downstream transitional ow behaviour, and the hemodynamic parameters is investigated. The research presented in this thesis is focused on the development of an in-depth understanding of the hemodynamics of diseased coronary arteries. Extensive pressure drop measurements, visualisation of the flow using particle image velocimetry (PIV), and computational modelling of the flow were conducted. Attention was mainly given to the stenosed and stented coronary arteries by investigating their influence on the flow behaviour, including velocity profile, pressure drop, time-averaged and -dependent WSS, and turbulent kinetic energy. The need for modelling the temporal geometric variations of the coronary arteries during a cardiac cycle for the investigation of the hemodynamics is discussed. Temporal geometric variations of the coronary arteries during a cardiac cycle are classified as a superposition of the changes in the position, curvature and torsion of the coronary artery and the variations in lumen cross-sectional shape due to distensible wall motion induced by the pulse pressure and/or contraction of the myocardium in a cardiac cycle. A sensitivity analysis was conducted to evaluate the effects of temporal geometric variations of the coronary arteries on the pressure drop and WSS. The results show that neglecting the effects of temporal geometric variations results in less than 5% deviation of the time-averaged pressure drop and WSS values. However, they lead to an approximately 20% deviation in the temporal geometric variations of hemodynamic parameters, such as time-dependent WSS. Based on the presented discussion, the temporal geometric variations of coronary arteries were not modelled in this thesis and the focus was on modelling the flow dynamics to develop an in-depth understanding of the ow features inside the stenosed and stented coronary arteries. In the next stage of the research, a model incorporating the plaque geometry, the pulsatile inlet ow and the induced turbulence in a stenosed coronary artery was developed and validated against numerical and experimental data. The transitional ow behaviour was quantified by investigation of the changes in the turbulent kinetic energy. The results suggest that there is a high risk of the formation of a secondary stenosis at a downstream distance of equal to 10 times the artery diameter in the regions to the side and downstream of the initial stenosis due to existence of the recirculation zones and low shear stresses. The applicability of the obtained results was tested with a patient-specific stenosed coronary artery model. Furthermore, for the non-invasive determination of the pressure drop in a stenosed artery model a mathematical model incorporating different physical parameters such as blood viscosity, artery length and diameter, ow rate and ow profile, and shape and degrees of stenosis, was developed. Extensive experimental pressure measurements were conducted for a wide range of degrees and shapes of stenosis to form a database in the process of the development of this equation. The validity of the developed relationship was also tested for the stenosed coronary artery models with the physiological flow profile of the left and right coronary arteries by comparing the pressure drop obtained from the developed equation and those from the experimental measurements. Moreover, the effect of artery curvature on the pressure drop and fractional ow reserve (FFR) wa investigated. The results show that neglecting the effect of artery curvature results in under-estimation of pressure drop by about 25{35%. The developed equation can determine the pressure drop inside a stenosed coronary artery using the measurement of the flow profle inside the artery as well as the images of the stenosed coronary artery. In order to develop an understanding of the hemodynamic performance of coronary stents, the effect of stent design on the hemodynamics of stented arteries was investigated experimentally and numerically. An innovative PIV technique was implemented for the visualisation of the entire ow and the investigation of WSS within the stent struts without covering the region of interest inside a stented coronary artery model. This novel technique was based on the construction of a transparent stented artery using silicone cast in one piece, instead of inserting a metal or non-metallic stent inside a cast artery model, which are translucent and distort the field of view. The results show that WSS is strongly dependent on the design of the stent. It was also shown that the likelihood of occurrence of restenosis is strongly dependent on strut depth and thickness, the distance between two consecutive struts, and the shape of the connector between the struts. This thesis provides an improved understanding of the hemodynamics of diseased coronary arteries with an ultimate goal of improving patient outcomes. The findings will provide a basis for improvement of the most common CAD diagnostic and treatment methods. Based on the results of this research, the susceptible regions for the formation of a new stenosis downstream of the initial stenosis can be determined. Identification of these locations, which are a function of different physical and geometrical parameters, such as shape, degree and eccentricity of the initial stenosis, can provide the necessary information for prevention of the distal propagation of stenoses. Furthermore, the equation developed to evaluate FFR non-invasively in this research can be used as a gatekeeper to prevent unnecessary FFR procedures for all patients. This will result in better patient outcomes and reduce costs related to unnecessary invasive FFR which will benefit the health system. In addition, the results of this study provide a better understanding of the effect of stents on the flow which can be used to improve stent designs.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 202

Computational estimation of haemodynamics and tissue stresses in abdominal aortic aneurysms

'o e Abdominal aortic aneurysm is a vascular disease involving a focal dilation of the aorta. The exact cause is unknown but possibilities include infection and weakening of the connective tissue. Risk factors include a history of atherosclerosis, current smoking and a close relative with the disease. Although abdominal aortic aneurysm can affect anyone, it is most often seen in older men, and may be present in up to 5.9 % of the population aged 80 years. Biomechanical factors such as tissue stresses and shear stresses have been shown to play a part in aneurysm progression, although the specific mechanisms are still to be determined. The growth rate of the abdominal aortic aneurysm has been found to correlate with the peak stress in the aneurysm wall and the blood flow is thought to influence disease development. In order to resolve the connections between biology and biomechanics, accurate estimations of the forces involved are required. The first part of this thesis assesses the use of computational fluid dynamics for modelling haemodynamics in abdominal aortic aneurysms. Boundary conditions from the literature o