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. (C) 2008 Elsevier Ltd. All rights reserved.

Fluid mechanics of cerebral aneurysms and effects of intracranial stents on cerebral aneurysm flow

Luca Augsburger

This thesis focuses on intracranial aneurysms and particularly on the effect of blood flow in the three major phases that characterize an aneurysm: initiation, growth and rupture. As it stands today, blood flow is the major driver of this disease. The understanding of the complex relationships between aneurysm hemodynamics and aneurysm wall at the level of the endothelial cells helps to model the impact of flow on aneurysm wall during the aneurysm life cycle. Blood pressure is a parameter that impacts with magnitudes of about one thousand fold larger than shear and is widely accepted to be the driver for aneurysm expansion and rupture. Its pulsatility provides also a stimulus for the formation of collagen fibers. However, the main sensors of the wall tissue that are sensitive to flow energy are located at the endothelial level, where shear translates into hormones and into biological signals that are given to the vessel wall. The analysis of shear is therefore crucial to give some insight into the potential biological behavior of the aneurysm wall and may allow for having a sight into the aneurysm evolution potential and rupture risk. Shear can be deduced from the detailed investigation of aneurysm flow. Flow patterns and magnitudes correlate well with aneurysm rupture risk and are consequently useful for risk assessment. The prediction of flow stream lines and the quantification of flow magnitude are of interest to understand the potential evolution of the vessel to an aneurysm. As to treat the disease and to remediate and improve the local aneurysm flow, correction of blood flow may induce reverse remodeling of the diseased wall, intending at healing and normalizing the wall structure. Currently, in current clinical practice, the use of endovascular flow diverters has already shown to be of interest. However, the effort to provide scientifically valid explanations and to understand the principles used by such an approach remains. This objective in mind and in the present clinical neurovascular research context, computer modeling has become increasingly interesting to visualize and integrate the multiple factors that are each responsible for parts of the complexity to understand intracranial aneurysm disease. Indeed, computer modeling and validation efforts lead to a better understanding and control of the difficulty to capture steps of evolution or of reverse remodeling. By extension, computer modeling can also include the calculation of flow effects on perforating arteries caged by flow diverters or the initial phase of aneurysm clotting. The first part of the thesis describes the clinical problem of intracranial aneurysms and details methodologies developed to assess aneurysm hemodynamics. A vast review of the literature is presented. References to numerous publications and context of the thesis in regard to current state of research are presented. A part from computer modeling, in vitro setups allow reproducing the physiological conditions and enable to test novel endovascular devices. The second chapter describes the in vitro stent test bed developed for flow assessment in intracranial aneurysms. Flow changes in two aneurysm models and the influence of flow diverters' porosity are investigated. In the third chapter, we present an initiative called the "1st Virtual Intracranial Stenting Challenge (VISC)". The VISC objectives were first to assess state-of-the-art numerical approaches to the simulation of blood flow in one stented cerebral aneurysm, second to illustrate the ability of a given stent design at reducing blood flow patterns in a patient-specific geometry and finally, to evaluate the numerical approaches on a standardized benchmark. The fourth chapter presents the influence of segmentation on geometrical parameters and on hemodynamics in cerebral aneurysms. As the reconstruction of aneurysms is based on gray-level threshold selection, such operation which usually performed by a trained radiology technician is consequently a man-dependant operation. The problem resulting in this case is that several aneurysm geometries may point to the same original aneurysm dataset. In the fifth chapter, the effects of one flow diverter are simulated using CFD based on a porous medium approach in two patient-specific intracranial aneurysm geometries. The advantage of simulating the flow diverter as a porous medium consists in reducing the number of elements and therefore the computational power. Simulation results are then compared with the real stent simulation. The described work achieved in this thesis put in evidence the need to further investigate the relationship between local aneurysm hemodynamics alteration and the aneurysm wall biological response. In vitro investigations including human cells would allow to further understand this complex mechanism. The objectives described at the beginning of the work are reached: a method allowing the quantification the aneurysm hemodynamics and their relation with aneurysm rupture risk. The quantification of the expected effects of a given endovascular device prior to the clinical intervention has also been made possible.

EPFL2008

Intracranial Stents Being Modeled as a Porous Medium: Flow Simulation in Stented Cerebral Aneurysms

Luca Augsburger, Philippe Reymond, Nikolaos Stergiopulos

Intracranial aneurysms may be treated by flow diverters, alternatively to stents and coils combination. Numerical simulation allows the assessment of the complex nature of aneurismal flow. Endovascular devices present a rather dense and fine strut network, increasing the complexity of the meshing. We propose an alternative strategy, which is based on the modeling of the device as a porous medium. Two patient-specific aneurysm data sets were reconstructed using conventional clinical setups. The aneurysms selection was done so that intra-aneurismal flow was shear driven in one and inertia driven in the other. Stents and their porous medium analog were positioned at the aneurysm neck. Physiological flow and standard boundary conditions were applied. The comparison between both approaches was done by analyzing the velocity, vorticity, and shear rate magnitudes inside the aneurysm as well as the wall shear stress (WSS) at the aneurysm surface. Simulations without device were also computed. The average flow reduction reaches 76 and 41% for the shear and inertia driven flow models, respectively. When comparing the two approaches, results show a remarkable similarity in the flow patterns and magnitude. WSS, iso-velocity surfaces and velocity on a trans-sectional plane are in fairly good agreement. The root mean squared error on the investigated parameters reaches 20% for aneurysm velocity, 30.6% for aneurysm shear rate, and 47.4% for aneurysm vorticity. It reaches 20.6% for WSS computed on the aneurysm surface. The advantages of this approach reside in its facility to implement and in the gain in computational time. Results predicted by the porous medium approach compare well with the real stent geometry model and allow predicting the main effects of the device on intra-aneurismal flow, facilitating thus the analysis.

Springer US2011

Flow diversion treatment: intra-aneurismal blood flow velocity and WSS reduction are parameters to predict aneurysm thrombosis

Luca Augsburger, Philippe Reymond, Daniel Ruefenacht

To evaluate the haemodynamic changes induced by flow diversion treatment in cerebral aneurysms, resulting in thrombosis or persisting aneurysm patency over time. Eight patients with aneurysms at the para-ophthalmic segment of the internal carotid artery were treated by flow diversion only. The clinical follow-up ranged between 6 days and 12 months. Computational fluid dynamics (CFD) analysis of pre- and post-treatment conditions was performed in all cases. True geometric models of the flow diverter were created and placed over the neck of the aneurysms by using a virtual stent-deployment technique, and the device was simulated as a true physical barrier. Pre- and post-treatment haemodynamics were compared, including mean and maximal velocities, wall-shear stress (WSS) and intra-aneurysmal flow patterns. The CFD study results were then correlated to angiographic follow-up studies. Mean intra-aneurysmal flow velocities and WSS were significantly reduced in all aneurysms. Changes in flow patterns were recorded in only one case. Seven of eight aneurysms showed complete occlusion during the follow-up. One aneurysm remaining patent after 1 year showed no change in flow patterns. One aneurysm rupturing 5 days after treatment showed also no change in flow pattern, and no change in the maximal inflow velocity. Relative flow velocity and WSS reduction in and of itself may result in aneurysm thrombosis in the majority of cases. Flow reductions under aneurysm-specific thresholds may, however, be the reason why some aneurysms remain completely or partially patent after flow diversion.