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

Study of the relation between blood flow and the age-dependent localisation of early atherosclerosis

Atherosclerosis develops non-uniformly within the arterial system and the distribution of lesions has been observed to change with age. This thesis investigates the concept that the patchiness of the disease is related to local variations in blood flow. Based on the insights from a systematic literature review, a novel study was designed to analyse the relation between haemodynamic factors and age-dependent atherogenesis in the thoracic aorta of rabbits. Arterial geometries were reconstructed by micro-Computed Tomography of vascular corrosion casts, with particular attention to the anatomical accuracy of the dataset. Blood flow was simulated in these geometries using a spectral/hp element method. Distributions of traditional shear-related metrics were calculated and both qualitatively and quantitatively compared to maps of lesion prevalence. In addition, a time-averaged transverse wall shear stress was introduced. A geometric analysis of the dataset of rabbit thoracic aortas revealed a significant change with age in the degree of aortic taper. The geometric changes could explain age-related differences in flow characteristics, in particular in the extent of Dean-type vortical structures into the descending aorta and the strength of a dorsal streak of high shear. The comparative analysis of shear and lesion distributions did not unequivocally support the theory that lesions occur in regions of low shear. The novel haemodynamic metric, in combination with current metrics, enabled an improved identification of zones of multi-directional disturbed flow. In conclusion, this thesis adds to the understanding of the relation between blood flow and early atherosclerosis, and provides tools for use in future studies