Mixed formulation of the one-dimensional equilibrium model for elastic stents

In this paper we formulate and analyze the mixed formulation of the one-dimensional equilibrium model of elastic stents. The model is based on the curved rod model for the inextensible and ushearable struts and is formulated in the weak form in \v{C}ani\'{c} and Tamba\v{c}a, 2012. It is given by a system of ordinary differential equations at the graph structure. In order to numerically treat the model using finite element method the mixed formulation is plead for. We obtain equivalence of the weak and the mixed formulation by proving the Babuska--Brezzi condition for the stent structure

An investigation into the influence of stent strut thickness on in-stent restenosis using the finite element method

In-stent restenosis represents the major limitation for stenting procedures. In-stent restenosis is the renarrowing o f the artery lumen within a stent predominantly due to excessive growth of neointimal hyperplasia. Clinical studies have found that stent design is a key determinant in the propensity o f stents to cause restenosis, indicating a vital link between the biomechanics o f stents and the development of the disease. The ISAR-STEREO Trial specifically assessed the effect o f strut thickness on restenosis outcome and found that for the same stent design, a thinner strut stent was associated with a significant reduction o f angiographic and clinical restenosis compared to the same stent with a thicker strut. The main objective o f this study is to use the finite element method to simulate these stenting procedures, and to examine the stresses induced within the stented arterial vessel walls by the stents, thus enabling the mechanical stimuli for in-stent restenosis to be identified. Finite element models o f thin and thick strut stents were developed and the stents were deployed in various stenosed vessel geometries such that the stresses induced within the stented vessels by the two stents could be compared. The stresses were examined at the end o f stent deployment, to determine the mechanical stimuli for acute damage, and again at stent unloading, to determine the long term stimuli for in-stent restenosis. The stress analyses were used to determine the level of vascular injury caused to the artery by different strut thickness stents. The finite element studies successfully identified differences between the mechanical loading of the arterial tissue in the vessels stented with the two different stents. The higher restenosis rate of the thicker strut stent, reported in the ISAR-STEREO clinical study, was found to be the result of the higher luminal gain achieved by the thicker strut stent, due to the lower recoil of the stent structure when both stents were expanded to the same initial lumen diameter. Further stenting analyses, however, found that the thicker strut stent resulted in a lower percentage of volume stressed at high levels compared with the thinner strut stent when it was expanded to the same final lumen diameter. This suggests that a thicker strut stent may in fact have the potential to be expanded to an optimal diameter whereby the in-stent restenosis is minimised. Therefore, it is proposed that the use of preclinical testing tools, such as finite element modelling, could be used to predetermine the deployment protocol and optimum luminal gain of a particular stent design in order to minimise the mechanical stimuli for in-stent restenosis

In Vitro and Computational Analyses of Blood Flow at Aortoiliac Bifurcation for Patients with Atherosclerotic Plaque Treated with Endovascular Procedures

This research has developed an appropriate approach allowing for more accurate assessment of haemodynamic changes following implantation of endovascular stent graft to treat patients with occlusive aortoiliac disease. Two different endovascular techniques involving the use of different types of stent grafts were analysed and compared with regard to haemodynamics associated with these techniques. Results improved understanding of the flow characteristics of these endovascular techniques

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 Prediction of the Haemodynamic Impact of Coronary Stent Implantation

Arterial restenosis limits the effectiveness of coronary stenting. Restenosis is caused by excessive tissue growth which is stimulated by arterial injury and alterations to the arterial WSS. The altered WSS results from stent-induced disturbances to the natural haelnodynamics of the artery. Recent numerical studies have predcted only minor digerences in altered WSS between different stent designs using a commonly employed threshold assessment technique. While it is possible that there are only minor differences, it is more likely that the assessment technique is incapable of fully elucidating the altered WSS created by stent implantation. This thesis proposes a methodology that involves a more complete level of investigation into the stentinduced alterations to the WSS by incorporating the full suite of WSS-based variables: WSS, WSS gradient (WSSG), WSS angle gradient (WSSAG) and oscillatory shear index (OSI). Each of these variables highlights a different type of alteration to the arterial WSS that could lead to excessive tissue growth. The four variables are analysed quantitatively and qualitatively using statistical methods to assess the effect of the stent implantation. The methodology is applied to three stents with contrasting designs: the Palinaz-Schatz (PS), the Gianturco-Roubin II (GR-11) and the Bx-Velocity (Bx) stents. From the results, the sients are ranked (best to worst) for WSS: GR-11, PS, Bx (Cohen\u27s d: -0.01, -0.6131, for WSSG: PS, Bx, GR-I1 (d: 0.159,0.764), for WSSAG: PS GR-I1 Bx (d: 0.213, 0.082), and for OSI: PS, GR- 11, Bx (d: 0.3 15, 0.380). The proposed method of analysis is shown to elucidate the alterations to the WSS created by the stents to a far greater level than with the previously used threshold technique. This method of stent assessment could be utilised to minimise WSS alterations at the design stage of future bare metal, as well as permanent and bioabsorbable drug-eluting coronary stents

A Deep Segmentation Network of Stent Structs Based on IoT for Interventional Cardiovascular Diagnosis

[EN] The Internet of Things (IoT) technology has been widely introduced to the existing medical system. An eHealth system based on IoT devices has gained widespread popularity. In this article, we propose an IoT eHealth framework to provide an autonomous solution for patients with interventional cardiovascular diseases. In this framework, wearable sensors are used to collect a patient's health data, which is daily monitored by a remote doctor. When the monitoring data is abnormal, the remote doctor will ask for image acquisition of the patient's cardiovascular internal conditions. We leverage edge computing to classify these training images by the local base classifier; thereafter, pseudo-labels are generated according to its output. Moreover, a deep segmentation network is leveraged for the segmentation of stent structs in intravascular optical coherence tomography and intravenous ultrasound images of patients. The experimental results demonstrate that remote and local doctors perform real-time visual communication to complete telesurgery. In the experiments, we adopt the U-net backbone with a pretrained SeResNet34 as the encoder to segment the stent structs. Meanwhile, a series of comparative experiments have been conducted to demonstrate the effectiveness of our method based on accuracy, sensitivity, Jaccard, and dice.This work was supported by the National Key Research and Development Program of China (Grant no. 2020YFB1313703), the National Natural Science Foundation of China (Grant no. 62002304), and the Natural Science Foundation of Fujian Province of China (Grant no. 2020J05002).Huang, C.; Zong, Y.; Chen, J.; Liu, W.; Lloret, J.; Mukherjee, M. (2021). A Deep Segmentation Network of Stent Structs Based on IoT for Interventional Cardiovascular Diagnosis. IEEE Wireless Communications. 28(3):36-43. https://doi.org/10.1109/MWC.001.2000407S364328

Searching for improvement

Engineering design can be thought of as a search for the best solutions to engineering problems. To perform an effective search, one must distinguish between competing designs and establish a measure of design quality, or fitness. To compare different designs, their features must be adequately described in a well-defined framework, which can mean separating the creative and analytical parts of the design process. By this we mean that a distinction is drawn between coming up with novel design concepts, or architectures, and the process of detailing or refining existing design architecture. In the case of a given design architecture, one can consider the set of all possible designs that could be created by varying its features. If it were possible to measure the fitness of all designs in this set, then one could identify a fitness landscape and search for the best possible solution for this design architecture. In this Chapter, the significance of the interactions between design features in defining the metaphorical fitness landscape is described. This highlights that the efficiency of a search algorithm is inextricably linked to the problem structure (and hence the landscape). Two approaches, namely, Genetic Algorithms (GA) and Robust Engineering Design (RED) are considered in some detail with reference to a case study on improving the design of cardiovascular stents