Benedict Verhegghe

Reproducibility of haemodynamical simulations in a subject-specific stented aneurysm model - A report on the Virtual Intracranial Stenting Challenge 2007

This paper presents the results of the Virtual Intracranial Stenting Challenge (VISC) 2007, an international initiative whose aim was to establish the reproducibility of state-of-the-art haemodynamical simulation techniques in subject-specific stented models of intracranial aneurysms (IAs). IAs are pathological dilatations of the cerebral artery walls, which are associated with high mortality and morbidity rates due to subarachnoid haemorrhage following rupture. The deployment of a stent as flow diverter has recently been indicated as a promising treatment option, which has the potential to protect the aneurysm by reducing the action of haemodynamical forces and facilitating aneurysm thrombosis. The direct assessment of changes in aneurysm haemodynamics after stent deployment is hampered by limitations in existing imaging techniques and currently requires resorting to numerical simulations. Numerical simulations also have the potential to assist in the personalized selection of an optimal stent design prior to intervention. However, from the current literature it is difficult to assess the level of technological advancement and the reproducibility of haemodynamical predictions in stented patient-specific models. The VISC 2007 initiative engaged in the development of a multicentre-controlled benchmark to analyse differences induced by diverse grid generation and computational fluid dynamics (CFD) technologies. The challenge also represented an opportunity to provide a survey of available technologies currently adopted by international teams from both academic and industrial institutions for constructing computational models of stented aneurysms. The results demonstrate the ability of current strategies in consistently quantifying the performance of three commercial intracranial stents, and contribute to reinforce the confidence in haemodynamical simulation, thus taking a step forward towards the introduction of simulation tools to support diagnostics and interventional planning.

A Novel Simulation Strategy for Stent Insertion and Deployment in Curved Coronary Bifurcations: Comparison of Three Drug-Eluting Stents

The introduction of drug-eluting stents (DES) has reduced the occurrence of restenosis in coronary arteries. However, restenosis remains a problem in stented coronary bifurcations. This study investigates and compares three different second generation DESs when being implanted in the curved main branch of a coronary bifurcation with the aim of providing better insights into the related changes of the mechanical environment. The 3D bifurcation model is based on patient-specific angiographic data that accurately reproduce the in vivo curvatures of the vessel segments. The layered structure of the arterial wall and its anisotropic mechanical behavior are taken into account by applying a novel algorithm to define the fiber orientations. An innovative simulation strategy considering the insertion of a folded balloon catheter over a guide wire is proposed in order to position the stents within the curved vessel. Straightening occurs after implantation of all stents investigated. The resulting distributions of the wall stresses are strongly dependent on the stent design. Using a parametric modeling approach, two design modifications, which reduce the predicted maximum values of the wall stress, are proposed and analyzed.

Carotid artery stenting simulation: From patient-specific images to finite element analysis

The outcome of carotid artery stenting (CAS) depends on a proper selection of patients and devices, requiring dedicated tools able to relate the device features with the target vessel. In the present study, we use finite element analysis to evaluate the performance of three self-expanding stent designs (laser-cut open-cell, laser-cut closed-cell, braided closed-cell) in a carotid artery (CA). We define six stent models considering the three designs in different sizes and configurations (i.e. straight and tapered), evaluating the stress induced in the vessel wall, the lumen gain and the vessel straightening in a patient-specific CA model based on computed angiography tomography (CTA) images. For the considered vascular anatomy and stents, the results suggest that: (i) the laser-cut closed-cell design provides a higher lumen gain; (ii) the impact of the stent configuration and of the stent oversizing is negligible with respect to the lumen gain and relevant with respect to the stress induced in the vessel wall; (iii) stent design, configuration and size have a limited impact on the vessel straightening. The presented numerical model represents a first step towards a quantitative assessment of the relation between a given carotid stent design and a given patient-specific CA anatomy.

On the use of in vivo measured flow rates as boundary conditions for image-based hemodynamic models of the human aorta: implications for indicators of abnormal flow

The purpose of this study is to investigate how the imposition of personalized, non-invasively measured blood flow rates as boundary conditions (BCs) influences image-based computational hemodynamic studies in the human aorta. We extracted from 4D phase-contrast MRI acquisitions of a healthy human (1) the geometry of the thoracic aorta with supra-aortic arteries and (2) flow rate waveforms at all boundaries. Flow simulations were carried out, and the implications that the imposition of different BC schemes based on the measured flow rates have on wall shear stress (WSS)-based indicators of abnormal flow were analyzed. Our results show that both the flow rate repartition among the multiple outlets of the aorta and the distribution and magnitude of the WSS-based indicators are strongly influenced by the adopted BC strategy. Keeping as reference hemodynamic model the one where the applied BC scheme allowed to obtain a satisfactory agreement between the computed and the measured flow rate waveforms, differences in WSS-based indicators up to 49% were observed when the other BC strategies were applied. In conclusion, we demonstrate that in subject-specific computational hemodynamics models of the human aorta the imposition of BC settings based on non-invasively measured flow rate waveforms influences indicators of abnormal flow to a large extent. Hence, a BCs set-up assuring realistic, subject-specific instantaneous flow rate distribution must be applied when BCs such as flow rates are prescribed.

Patient-specific computational fluid dynamics: structured mesh generation from coronary angiography

Patient-specific simulations are widely used to investigate the local hemodynamics within realistic morphologies. However, pre-processing and mesh generation are time consuming, operator dependent, and the quality of the resulting mesh is often suboptimal. Therefore, a semi-automatic methodology for patient-specific reconstruction and structured meshing of a left coronary tree from biplane angiography is presented. Seven hexahedral grids have been generated with the new method (50,000–3,200,000 cells) and compared to nine unstructured tetrahedral grids with prismatic boundary layer (150,000–3,100,000 cells). Steady-state blood flow simulation using Computational Fluid Dynamics (CFD) has been used to calculate the Wall Shear Stress (WSS). Our results (99 percentile, area-weighted and local WSS values along a line) demonstrate that hexahedral meshes with respect to tetrahedral/prismatic meshes converge better, and for the same accuracy of the result, six times less cells and 14 times less computational time are required. Hexahedral meshes are superior to tetrahedral/prismatic meshes and should be preferred for the calculation of the WSS.

Intraluminal thrombus and risk of rupture in patient specific abdominal aortic aneurysm - FSI modelling

Recent numerical studies of abdominal aortic aneurysm (AAA) suggest that intraluminal thrombus (ILT) may reduce the stress loading on the aneurysmal wall. Detailed fluid structure interaction (FSI) in the presence and absence of ILT may help predict AAA rupture risk better. Two patients, with varied AAA geometries and ILT structures, were studied and compared in detail. The patient specific 3D geometries were reconstructed from CT scans, and uncoupled FSI approach was applied. Complex flow trajectories within the AAA lumen indicated a viable mechanism for the formation and growth of the ILT. The resulting magnitude and location of the peak wall stresses was dependent on the shape of the AAA, and the ILT appeared to reduce wall stresses for both patients. Accordingly, the inclusion of ILT in stress analysis of AAA is of importance and would likely increase the accuracy of predicting AAA risk of rupture.

A computational method to assess the in vivo stresses and unloaded configuration of patient-specific blood vessels

In the modelling process of cardiovascular diseases, one often comes across the numerical simulation of the blood vessel wall. When the vessel geometry is patient-specific and is obtained in vivo via medical imaging, the stress distribution throughout the vessel wall is unknown. However, simulating the full physiological pressure load inside the blood vessel without incorporating the in vivo stresses will result in an inaccurate stress distribution and an incorrect deformation of the vessel wall. In this work a computational method is formulated to restore the zero-pressure geometry of patient-specific blood vessels, and to recover the in vivo stress field of the loaded structures at the moment of imaging. The proposed backward displacement method is able to solve the inverse problem iteratively using fixed point iterations. As only an update of the mesh is required, the formulation of this method allows for a straightforward implementation in combination with existing structural solvers, even if the structural solver is a black box.

Virtual optimization of self-expandable braided wire stents

At present, the deployment of self-expandable braided stents has become a common and widely used minimally invasive treatment for stenotic lesions in the cardiovascular, gastrointestinal and respiratory system. To improve these revascularization procedures (e.g. increase the positioning accuracy) the optimal strategy lies in the further development of the stent design. In the context of optimizing braided stent designs, computational models can provide an excellent research tool complementary to analytical models. In this study, a finite element based modelling strategy is proposed to investigate and optimize the mechanics of braided stents. First a geometrical and finite element model of a braided Urolume endoprosthesis was built with the open source pyFormex design tool. The results of the reference simulation of the Urolume stent are in close agreement with both analytical and experimental data. Subsequently, a simplex-based design optimization algorithm automatically adjusts the reference Urolume geometry to facilitate precise positioning by reducing the foreshortening with 20% while maintaining the radial stiffness. Therefore, the proposed modelling strategy appears to be a promising optimization methodology in braided stent design.

Our capricious vessels: The influence of stent design and vessel geometry on the mechanics of intracranial aneurysm stent deployment.

There is a growing interest in virtual tools to assist clinicians in evaluating different procedures and devices for endovascular treatment. In the present study we use finite element analysis to investigate the influence of stent design and vessel geometry for stent assisted coiling of intracranial aneurysms. Nine virtual stenting procedures were performed: three nitinol stent designs ((i) an open cell stent resembling the Neuroform, (ii) a generic stiff and (iii) a more flexible closed cell design), were deployed in three patient-specific cerebral aneurysmatic vessels. We investigated the percentage of strut area covering the aneurysm neck, the straightening induced on the cerebrovasculature by the stent placement (quantified by the reduction in tortuosity), and stent apposition to the wall (quantified as the percentage of struts within 0.2mm of the vessel). The results suggest that the open cell design better covers the aneurysm neck (11.0±1.1%) compared to both the stiff (7.8±1.6%) and flexible (8.7±1.6%) closed cell stents, and induces less straightening of the vessel (-5.1±1.6% vs. -42.9±9.8% and -26.9±11.9% ). The open cell design has, however, less struts apposing well to the vessel wall (56.0±6.4%) compared to the flexible (73.4±4.6%) and stiff (70.4±5.1%) closed cell design. With the presented study, we hope to contribute to and improve aneurysm treatment, using a novel patient specific environment as a possible pre-operative tool to evaluate mechanical stent behavior in different vascular geometries.

Comparison of drug-eluting stent cell size using micro-CT: important data for bifurcation stent selection

AIMS The size of stent cells is an important design parameter influencing the resulting wall coverage and strut positioning of several bifurcation stenting techniques. Therefore, we report accurate geometrical information on stent cell size of five different stents with the key purpose to assist with stent selection for bifurcation lesions. METHODS AND RESULTS We developed an innovative technique to assess cell size with high accuracy based on micro-CT images. In addition, the size of the ostium was calculated for several idealised bifurcations with a cylindrical main and side branch. The cell circumferences of the investigated stents vary substantially, with values between 9.5 and 19.8 mm (or an equivalent maximal cell diameter between 3.0 and 6.3 mm). Comparison of these cell circumferences with the calculated ostium circumferences shows which stents should preferably be used for which bifurcations, as these stents have cells that can be sufficiently enlarged by dilating through the side. CONCLUSIONS The cell sizes of the investigated stents differ considerably. This should be taken into account when applying a particular bifurcation stenting technique. The risk on limited main or side branch patency due to obstructing stent struts can be minimised by using the reported data for stent selection.