An Efficient Hemodynamic Workflow in Computational Surgery

For few decades, it has been shown that atherosclerosis is the cause of the majority of clinical cardiovascular diseases including peripheral arterial diseases. The diagnosis and treatment for vascular disease has evolved significantly over the past years considering the rapid advances in imaging technologies. In recent years, computational fluid dynamics has been increasingly used as a simulation tool for blood flows. Numerous researches connect wall shear stress quantities to endovascular diseases such as stenosis, aneurism, and atherosclerosis. A thorough knowledge of vascular anatomy and hemodynamic would be beneficial for understanding the development and progression of the disease, the therapeutic decision process and follow up. The objective of this dissertation is to propose a computational fluid dynamic framework that includes: Understanding how streamline efficiently hemodynamic simulation for main arteries to produce database for clinical study/Providing some confidence estimate on numerical results/Extending the state of the art of clinical study by including motion and particles analysis.Computer Science, Department o

Severity parameter and global importance factor of non-newtonian models in 3D reconstructed human left coronary artery

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.The capabilities and limitations of various molecular viscosity models, when testing Left Coronary Artery (LCA) tree, were analyzed via: molecular viscosity, local and global non-Newtonian importance factors, Wall Shear Stress (WSS) and Wall Shear Stress Gradient (WSSG). Seven non-Newtonian molecular viscosity models, plus the Newtonian one, were compared. Dense grid of 620000 nodes located, mostly, at near to low WSS flow regions (endothelium regions) is needed for current LCA application. The WSS distribution yields a consistent LCA pattern for nearly all non-Newtonian models. High molecular viscosity, low WSS low WSSG values appear at proximal LCA regions at the outer walls of the major bifurcation. The global importance factor for the non-Newtonian power law model yields 76.7% (non-Newtonian flow), while for the Generalized power law model this value is 6.1% (Newtonian flow). The capabilities of the applied non-Newtonian law models appear at low strain rates. The Newtonian blood flow treatment is considered to be a good approximation at mid-and high-strain rates. In general, the non-Newtonian power law and the Generalized power law blood viscosity models are considered to approximate the molecular viscosity and WSS calculations in a more satisfactory way

Validation of CFD simulations of cerebral aneurysms with implication of geometric variations

Background. Computational fluid dynamics (CFD) simulations using medical-image-based anatomical vascular geometry are now gaining clinical relevance. This study aimed at validating the CFD methodology for studying cerebral aneurysms by using particle image velocimetry (PIV) measurements, with a focus on the effects of small geometric variations in aneurysm models on the flow dynamics obtained with CFD. Method of Approach. An experimental phantom was fabricated out of silicone elastomer to best mimic a spherical aneurysm model. PIV measurements were obtained from the phantom and compared with the CFD results from an ideal spherical aneurysm model (S1). These measurements were also compared with CFD results, based on the geometry reconstructed from three-dimensional images of the experimental phantom. We further performed CFD analysis on two geometric variations, S2 and S3, of the phantom to investigate the effects of small geometric variations on the aneurysmal flow field. Results. We found poor agreement between the CFD results from the ideal spherical aneurysm model and the PIV measurements from the phantom, including inconsistent secondary flow patterns. The CFD results based on the actual phantom geometry, however, matched well with the PIV measurements. CFD of models S2 and S3 produced qualitatively similar flow fields to that of the phantom but quantitatively significant changes in key hemodynamic parameters such as vorticity, positive circulation, and wall shear stress. Conclusion. CFD simulation results can closely match experimental measurements as long as both are performed on the same model geometry. Small geometric variations on the aneurysm model can significantly alter the flow-field and key hemodynamic parameters. Since medical images are subjected to geometric uncertainties, image-based patient-specific CFD results must be carefully scrutinized before providing clinical feedback

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

Assessing the Near-Wall Hemodynamics in the Left Coronary Artery Using CFD

The objective of this thesis is to computationally investigate the flow mechanics and the near-wall hemodynamics associated with the different take-off angles in the left coronary artery of the human heart. From this study, we will be able to evaluate if the increase in the take-off angles of the left coronary artery will significantly increases or decrease the likelihood of plaque (atherosclerosis) buildup in the left coronary artery bifurcations. This study quantifies the effects of the varying take-off angles on the branches along the left anterior descending (LAD) of the left coronary artery using computational fluid dynamics (CFD) simulations. The study aims to compare five test cases of the different take off-angles of the left coronary artery (LCA) and four different branch angles between the LAD and the left circumflex (LCx). It also considered the branch angles of the coronary artery downstream the LAD. The idealized geometries used for this study were constructed in SolidWorks 2015 and imported as surface meshes into Star-CCM+, a commercially available CFD solver. In this study, the LCA inlet boundary conditions was set as a pulsatile mass flow inlet and flow split ratios were set for the outlets boundary conditions that are representations of a middle age man at rest. The nature of blood pulsatile flow characteristic was accounted for and the properties of blood which include the density (1,050Kg/m3) and dynamic viscosity (0.0046Pa) were obtained from previous research. The results from the simulations are compared using established scales for the parameters evaluated. The parameters evaluated were: (i) Oscillatory Shear Index (OSI); which quantifies the extent in which the blood flow changes direction as it flows (ii) Time Average Wall Shear Stress (TAWSS); which quantifies the average shear stress experienced by the wall of the artery and (ii) Relative Residence Time (RRT); which defined how long blood spends in a location during blood flow. These parameters are used to predict the likelihood of blood clots, atherosclerosis, endothelial damage, plaque formation, and aneurysm in the blood vessels. The data from the simulations were analyzed using functional macros to quantify and generate threshold values for the parameters. Computational Fluid Dynamics has gain more recognition in field of medicine because it has been used to obtain the various mechanic behaviors of most artificial implanted devices used for endovascular and cardiovascular treatments before these devices are used in patients’ treatment. This can be a useful insight in coronary stenting, solid and stress analysis of biodegradable stent and can also provide insight into stenting for more complex arterial networks like brain stent grafts. In addition, it is important to understand the hemodynamics of the LCA before carrying out stent graft or angioplasty procedures. This will help determine the effectiveness of the stent graft in the coronary artery

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

Multiscale Modeling of Hemodynamics in Human Vessel Network and Its Applications in Cerebral Aneurysms

Three-dimensional (3D) simulation of patient-specific morphological models has been widely used to provide the hemodynamic information of individual patients, such as wall shear stress (WSS), oscillatory shear index (OSI), and flow patterns, etc. Since patient-specific morphological segment was only restricted locally, boundary conditions (BCs) are required to implement the CFD simulation. Direct measurements of the flow and pressure waveforms were often required as input BCs for 3D CFD simulations of patient-specific models. However, as the morphology develops, the feedback from this topological deformation may lead to BCs being altered, and hence without this feedback, the flow characteristics of the morphology are only computed locally. A one-dimensional (1D) numerical model containing the entire human vessel network has been proposed to compute the global hemodynamics. In the meantime, experimental studies of blood flow in the patient-specific modeling of the circle of Willies (CoW) was conducted. The flow and pressure waveforms were quantified to validate the accuracy of the pure 1D model. This 1D model will be coupled with a 3D morphological model to account for the effects of the altered BCs. The proposed 1D-3D multi-scale modeling approach investigates how the global hemodynamic changes can be induced by the local morphological effects, and in consequence, may further result in altering of BCs to interfere with the solution of the 3D simulation. Validation of the proposed multi-scale model has also been made by comparing the solution of the flow rate and pressure waveforms with the experimental data and 3D numerical simulations reported in the literature. Moreover, the multi-scale model is extended to study a patient-specific cerebral aneurysm and a stenosis model. The proposed multi-scale model can be used as an alternative to current approaches to study intracranial vascular diseases such as an aneurysm, stenosis, and combined cases