Elsevier

Journal of Biomechanics

Volume 48, Issue 12, 18 September 2015, Pages 3332-3340
Journal of Biomechanics

Finite element modeling of endovascular coiling and flow diversion enables hemodynamic prediction of complex treatment strategies for intracranial aneurysm

https://doi.org/10.1016/j.jbiomech.2015.06.018Get rights and content

Abstract

Endovascular interventions using coil embolization and flow diversion are becoming the mainstream treatment for intracranial aneurysms (IAs). To help assess the effect of intervention strategies on aneurysm hemodynamics and treatment outcome, we have developed a finite-element-method (FEM)-based technique for coil deployment along with our HiFiVS technique for flow diverter (FD) deployment in patient-specific IAs. We tested four clinical intervention strategies: coiling (1–8 coils), single FD, FD with adjunctive coils (1–8 coils), and overlapping FDs. By evaluating post-treatment hemodynamics using computational fluid dynamics (CFD), we compared the flow-modification performance of these strategies. Results show that a single FD provides more reduction in inflow rate than low packing density (PD) coiling, but less reduction in average velocity inside the aneurysm. Adjunctive coils add no additional reduction of inflow rate beyond a single FD until coil PD exceeds 11%. This suggests that the main role of FDs is to divert inflow, while that of coils is to create stasis in the aneurysm. Overlapping FDs decreases inflow rate, average velocity, and average wall shear stress (WSS) in the aneurysm sac, but adding a third FD produces minimal additional reduction. In conclusion, our FEM-based techniques for virtual coiling and flow diversion enable recapitulation of complex endovascular intervention strategies and detailed hemodynamics to identify hemodynamic factors that affect treatment outcome.

Introduction

Endovascular intervention is emerging as the predominant means for treating intracranial aneurysms (IAs). In coil embolization, coils deployed in the IA sac cause thrombotic occlusion of the aneurysm. In flow diversion, flow diverters (FDs), which are densely-braided, mesh-like stents deployed across the IA orifice, redirect blood inflow away from the IA, thereby causing its thrombotic occlusion and parent vessel reconstruction. Despite the success of endovascular interventions, poor treatment outcomes have also been reported: 30% of coiled IAs experience aneurysm recurrence (Raymond et al., 2003), and 24% of FD-treated IAs fail to completely occlude at 6 months (Brinjikji et al., 2013). Since these strategies work primarily by modifying aneurysmal flow, post-treatment hemodynamics may play a critical role in treatment outcome. Computational tools for simulating clinical interventions and hemodynamics in patient-specific IAs may help evaluate the impact of various intervention strategies and identify hemodynamic factors that affect treatment outcome.

Several coiling and FD modeling techniques have been reported, but most of them employed simplified representations of device geometries and deployment (Appanaboyina et al., 2009, Byun and Rhee, 2004, Cebral and Lohner, 2005, Dequidt et al., 2008, Kakalis et al., 2008, Larrabide et al., 2008, Morales et al., 2011c, Schirmer and Malek, 2010; Schirmer and Malek, 2010). It is unclear if they can capture the wide variation of deployed device configurations to enable accurate post-treatment hemodynamic analysis which could explain treatment outcome variations.

To simulate accurate FD deployment, we established a FEM-based high fidelity virtual stenting (HiFiVS) technique (Ma et al., 2012, Ma et al., 2013) and validated it in vitro (Ma et al., 2013). HiFiVS has allowed us to investigate the hemodynamic modifications of emerging clinical strategies for FD treatment, for the first time (Ma et al., 2014, Xiang et al., 2014).

Expanding this modeling capability to encompass most current IA intervention strategies, we introduce a new FEM-based coil deployment technique. It explicitly models pre-shaped coils and their deployment mechanics. Using this coiling method and HiFiVS, we simulated four classes of clinical intervention strategies in a patient-specific IA: coiling (1–8 coils), single FD, FD with adjunctive coils (1–8 coils), and overlapping FDs. By evaluating post-treatment hemodynamics via computational fluid dynamics (CFD), we assessed hemodynamic modifications following treatment. Results demonstrated that our FEM-based simulations recapitulate complex endovascular treatment strategies and enable post-treatment hemodynamic predictions that are supported by several clinical and in vitro deployment and flow studies.

Section snippets

Aneurysm model

As a test case, we used a patient-specific internal carotid artery (ICA) aneurysm, which in reality was treated with three overlapping Pipeline Embolization Devices (PED, Covidien, Irvine, CA) (3.25×14 mm, 3.25×12 mm, and 3.25×10 mm) and one adjunctive coil (7mm×30cm). The aneurysm and vascular geometry were reconstructed from 3D rotational angiography using the software VMTK (www.vmtk.org).

Workflow for coil deployment modeling

We implemented the FEM-based workflow in Abaqus/Explicit v6.12 (SIMULIA, Providence, RI). In this coiling

Coiling

The coils sparsely covered the neck plane and lied along the periphery of the aneurysm sac at low PDs (<11%), but coil neck coverage increased and coils distributed more evenly in the aneurysm sac at higher PD (Fig. 3A).

Single FD

The deployed FD had uniform mesh density across the IA orifice (Fig. 4A).

FD with adjunctive coils

We merged the deployment results for the coil embolization scenarios with the single FD to simulate FD with adjunctive coils (Fig. 3B).

Overlapping FDs

We simulated the overlapping FDs strategy for up to 3 FDs, starting

Discussion

Endovascular interventions ─ notably coil embolization and flow diversion ─ have become the predominant mode for treating IAs. Despite their success as minimally-invasive alternatives to open skull surgery, reports of IA recurrence and incomplete occlusion at treatment follow-up emphasize the uncertainty of treatment outcomes (Brinjikji et al., 2013, Raymond et al., 2003). Currently there is no way to predict endovascular intervention outcomes. Post-treatment hemodynamics is believed to be a

Conclusion

We have developed a new FEM-based coil deployment method. Combining it with the HiFiVS technique for FD deployment, we are able to recapitulate a range of current clinical intervention strategies including coiling, single FD, FD with adjunctive coils, and overlapping FDs. Our modeling techniques have allowed us to investigate the hemodynamic effect of these intervention strategies in patient-specific IAs.

Limitations

Our study has several limitations. First, both the aneurysm wall and the devices were assumed rigid. Second, only steady-state CFD simulations were conducted. Third, caution is required in interpreting the hemodynamic results since only one patient-specific IA was examined.

Conflict of interest statement

Damiano: None. Ma: None. Xiang: Awardee for the American Society for Quality Biomedical Division Dr. Richard J. Schlesinger grant and principal investigator of Brain Aneurysm Foundation grant. Siddiqui: Financial interests- Hotspur, Intratech Medical, StimSox, Valor Medical; Consultant- Codman & Shurtleff, Concentric Medical, ev3/Covidien Vascular Therapies, GuidePoint Global Consulting, Penumbra; Speakers’ bureau’s- Codman & Shurtleff, Genentech; Advisory board- Codman & Shurtleff; Honoraria-

Acknowledgments

This study was supported by Covidien (Grant no. VTGCC053012-009) and National Institutes of Health (R01 NS 091075). We gratefully thank Vincent Tutino and Nikhil Paliwal for their assistance in preparation of the figures and manuscript.

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