Elsevier

Journal of Biomechanics

Volume 41, Issue 10, 19 July 2008, Pages 2069-2081
Journal of Biomechanics

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

https://doi.org/10.1016/j.jbiomech.2008.04.035Get rights and content

Abstract

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.

Introduction

Intracranial aneurysms (IAs) are pathological dilatations of the cerebral artery walls, which mostly occur at the Circle of Willis. The major complication of IAs is their rupture, which causes subarachnoid haemorrhage (SAH). The incidence of IAs has been estimated to be between 1% and 8% among western populations, with the majority of IAs being dormant over life. However, even though IA rupture is rather infrequent (9 per 100,000 (de Rooij et al., 2007)), when an IA has ruptured mortality and morbidity rates are exceedingly high (Tomasello et al., 1998; Wardlaw and White, 2000). When an IA is diagnosed and a patient needs to be protected from aneurysm rupture or re-bleeding, a number of treatment options are considered to isolate the aneurysm from the cerebral circulation and to re-establish a physiological flow passage into the parent artery. These include either open surgery and clipping of the aneurysm, or endovascular treatment (EVT), performed primarily by coil embolization. Surgical treatment involves craniotomy and may be associated with vasospasm, infection, brain oedema and higher health care costs. EVT is minimally invasive and involves the of deployment of platinum coils into the aneurysm after the placement of a microcatheter guided through the arterial system. The objective of EVT is to shield the aneurysmal wall and reduce the blood flow into the aneurysm sac, thus progressively inducing aneurysmal flow stasis, thrombus formation and aneurysm occlusion. Although widely accepted and increasingly used, coil embolization of IAs is not a risk-free procedure either, and may be associated with coil compaction, recanalization and aneurysmal re-growth, necessitating re-treatment. In addition, efficacy of coil embolization is limited in wide-neck aneurysms, which are also difficult to treat surgically. Intracranial stenting may be used to aid aneurysm coiling (stent-assisted coiling) providing a scaffold to hold the coils inside the aneurysm sac while being deployed and thus achieve higher packing density.

It is assumed that, depending on their design, stents will also cause a reduction of the intra-aneurysmal flow. This aspect has recently gained more interest as stents can be used alone as flow diverters. Stents are flexible, self-expanding or balloon-expandable porous tubular meshes made of stainless steel or other alloys such as Nitinol. They are expanded into the arterial lumen across the aneurysmal orifice to deviate blood flow away from the aneurysm sac. Ideally, stents should provide sufficient haemodynamical resistance to aneurysmal inflow and outflow to promote thrombosis and stabilization without the need of accessing the fragile aneurysmal cavity. Preliminary clinical experiences with stent-based flow diversion have been reported in Ahn et al. (2006), Benndorf et al. (2001), Vanninen et al. (2003) and Zenteno et al. (2005). Progressive aneurysmal thrombosis induced by single or double stenting was observed for wide-neck aneurysms and complex vertebrobasilar aneurysms. Parent vessel geometry and stent design were suggested to play an important role in the reduction of aneurysmal flow intensity and in the potential success of IA treatment.

Extensive analyses of aneurysmal haemodynamical changes induced by stent deployment have been provided by both in vitro and numerical studies. Stent-induced aneurysmal rheological changes (Ohta et al., 2005), characterization of aneurysmal flow activity reduction due to a stent (Hirabayashi et al., 2003) and influence of stent porosity on aneurysmal flow (Wakhloo and Leiber, 1997; Yu and Zhao, 1999) have been described for idealized aneurysm models. The influence of parent vessel curvature has also been analysed in Meng et al. (2006). When the same idealized aneurysm model was considered, straight and curved parent vessels provided significant differences in the potential of thrombotic occlusion created by a specific stent. These observations suggest the importance of considering patient-specific anatomy to assess the performance of a specific stent design and study its relation with clinical events and post-implant complications. Advanced image processing and geometrical modelling techniques have been recently combined with computational fluid dynamics (CFD) approaches to generate detailed haemodynamical descriptions in patient-specific anatomical models (Cebral et al., 2005a, Cebral et al., 2005b; Jou et al., 2004; Shojima et al., 2004; Steinman et al., 2003). Image-based haemodynamical modelling also provides a desirable framework for stent treatment planning and for the exploration of new stent designs. However, from the current literature it is difficult to assess the level of technological advancement and the reproducibility of image-based haemodynamical simulation techniques when applied to stented patient-specific models. The description of patient-specific IA flow alterations occurring after stent implantation involves the combination of complex geometrical modelling tasks such as vascular anatomy reconstruction from medical images and virtual in situ stent deployment. In addition, the construction of computational grids and the numerical reconstruction of the blood flow field within the aneurysm and around stent struts represent a far from trivial technical challenge. Cebral and Löhner (2005) recently proposed a novel technique to generate hybrid unstructured grids that are suitable for haemodynamical simulation in stented geometries. The technique was applied by Appanaboyina et al. (2007) to a patient-specific model involving an aneurysm of the internal carotid artery (ICA). Difficulties in achieving analogous results with more widely used body-conforming grids were also reported. Stuhne and Steinman (2004) investigated the influence of body-conforming grid resolution on the convergence of haemodynamical simulation in idealized stented models. The potential complexity of body-conforming grid generation in stented patient-specific models was identified. The authors also argued that a proper integration of geometrical processing, mesh generation and CFD analysis techniques is the key to the reliability of the simulation output.

The objective of this paper is to present the results of the Virtual Intracranial Stenting Challenge (VISC) 2007,1 an international initiative aimed at establishing the reproducibility of state-of-the-art haemodynamical simulation techniques in subject-specific stented aneurysm models and, at the same time, offering a survey of available technologies in this specific application. The initiative engaged in the development of a controlled benchmark to analyse differences induced by diverse grid generation and CFD technologies, while standardizing conditions such as geometrical modelling, stent deployment and simulation boundary conditions. International teams from both academic and industrial institutions leading in CFD solutions were invited to participate in the challenge and an organizing committee was constituted to monitor, support and finally analyse the reproducibility of the simulation outputs. This paper is intended to illustrate the methodology adopted for the comparison of simulation strategies and to investigate the reproducibility of CFD solutions in describing the performance of three commercial stents. We believe that the report of this experience will trigger similar initiatives to compare current modelling and simulation tools in practical settings and will thus contribute to their widespread use and acceptance by non-expert end-user communities.

Section snippets

The Virtual Intracranial Stenting Challenge 2007

The VISC 2007 was launched on the occasion of the European Society of Neuroradiology and 3rd Intracranial Stent (ICS) meeting, held in Geneva, Switzerland, in September 2006. The challenge was designed to achieve the highest participation within the CFD simulation community taking into account the diversity of available CFD techniques. International teams and networks were contacted for feedback, and a formal deadline for expression of interest was set for November 2006.

Two main phases

Comparison of simulation outputs

The aneurysmal flow structure is first described by the isovelocity surfaces (v=0.1 m/s) reported in Fig. 4 for both unstented and stented S2 models. A clear region of inflow and outflow with a main vortical structure inside the dome of the aneurysm characterize the aneurysmal haemodynamics. The same structure is preserved after the deployment of stent 2, which, due to high porosity in the aneurysmal inflow region, has only a limited impact on inflow reduction. The depiction of regions of inflow

Discussion

The proposed haemodynamical simulations produced, in both stented and unstented models, comparable results across different techniques and simulation teams, thus supporting their reproducibility. Although differences in the magnitude of WSS and velocity were observed, WSS and velocity distribution and aneurysmal flow activity were reproducible across teams for both stented and unstented simulations. The comparison and performance evaluations focused on quantities that have been previously

Conflict of interest

All authors were fully involved in the study and preparation of the manuscript and the material within has not been and will not be submitted for publication elsewhere. Other contributors and sources of funding are detailed in the Acknowledgements section of the manuscript. No financial and personal relationships with other people or organizations that could inappropriately influence and/or bias the work were identified.

Acknowledgements

The VISC committee would like to thank Dr. Arani Bose, Penumbra Inc., San Leandro, CA, USA and Mr. Jesper Thyregod, William Cook Europe, Bjaeverskov, Denmark for their support. They also would like to thank Cook, Boston Scientific and Biotronik for the stent samples, Sebastien Devillers for his help in performing the micro CT scans and Dr. Teresa Sola Martínez and Dr. Elio Vivas Díaz from the Department of Therapeutic Neuroangiography JJ Merland of the Hospital General de Catalunya, Barcelona,

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