Luca Augsburger

Effect of Flow Diverter Porosity on Intraaneurysmal Blood Flow

AbstractBackground and Purpose:Growth and rupture, the two events that dominate the evolution of an intracranial aneurysm, are both dependent on intraaneurysmal flow. Decrease of intraaneurysmal flow is considered an attractive alternative for treating intracranial aneurysms by minimally invasive techniques. Such modification can be achieved by inserting stents or flow diverters alone. In the present paper, the effect of different commercial and innovative flow diverters’ porosity was studied in intracranial aneurysm models.Material and Methods:Single and stent-in-stent combination of Neuroform II as well as single and stent-in-stent combination of a new innovative, low-porosity, intracranial stent device (D1, D2, D1 + D2) were inserted in models of intracranial aneurysms under shear-driven flow and inertia-driven flow configurations. Steady and pulsating flow rates were applied using a blood-like fluid. Particle image velocimetry was used to measure velocity vector fields in the aneurysm midplane along the vessel axis. Flow and vorticity patterns, velocity and vorticity magnitudes were quantified and their value compared with the same flows in absence of the flow diverter.Results:In absence of flow diverters, a solid-like rotation could be observed in both shear-driven and inertia-driven models under steady and pulsatile flow conditions. The flow effects due to the insertion of low-porous devices such as D1 or D2 provoked a complete alteration of the flow patterns and massive reduction of velocity or vorticity magnitudes, whereas the introduction of clinically adopted high-porous devices provoked less effect in the aneurysm cavity. As expected, results showed that the lower the porosity the larger the reduction in velocity and vorticity within the aneurysm cavity. The lowest-porosity device combination (D1 and D2) reached an averaged reduction of flow parameters of 80% and 88% under steady and pulsatile flow conditions, respectively. The reduction in mean velocity and vorticity was much more significant in the shear-driven flows as compared to the inertia-driven flows.Conclusion:Although device porosity is the main parameter influencing flow reduction, other parameters such as device design and local flow conditions may influence the level of flow reduction within intracranial aneurysms.ZusammenfassungHintergrund und Ziel:Die zwei wichtigsten Faktoren für die Entwicklung intrazerebraler Aneurysmen, nämlich Wachstum und Ruptur, hängen vom intraaneurysmatischen Blutfluss ab. Eine Verminderung des intraaneurysmatischen Blutflusses durch minimalinvasive Techniken wird als attraktive Behandlungsmethode erachtet. Eine solche Modifikation des Blutflusses kann durch das Einbringen eines Stents oder „flow diverter“ allein erzielt werden. In der vorliegenden Arbeit untersuchten die Autoren den Effekt der Porosität verschiedener handelsüblicher und innovativer „flow diverters“ an Modellen intrakranieller Aneurysmen.Material und Methodik:Sowohl einzelne oder Stent-in-Stent-Kombinationen des Neuroform II (NF) als auch einzelne oder Stent-in-Stent-Kombinationen von neuen innovativen, niedrigporösen intrakraniellen Stents (D1, D2, D1 + D2) wurden in Modellen intrakranieller Aneurysmen mit Eigenschaften von „shear-driven“ und „inertia-driven“ Fluss platziert. Flächen mit Geschwindigkeitsvektoren in der mittleren Ebene des Aneurysmas parallel zur Achse des Gefäßes wurden mit Hilfe der „particle image velocimetry“ (PIV) ermittelt. Eigenschaften von Fluss und Verwirbelungen, Geschwindigkeit und Ausmaß von Verwirbelungen wurden gemessen und mit Messwerten des gleichen Modells ohne „flow diverter“ verglichen.Ergebnisse:Ohne „flow diverter“ konnte eine beständige Rotation in beiden – „shear-driven“ und „inertia-driven“ – Flussmodellen beobachtet werden. Die Auswirkungen nach Platzierung eines niedrigporösen Modells wie D1 oder D2 riefen eine komplette Änderung der Flusseigenschaften und eine massive Verringerung der Geschwindigkeit und des Ausmaßes von Verwirbelungen hervor, wohingegen die Platzierung klinisch angewendeter hochporöser Modelle geringere Auswirkungen auf die Kavität des Aneurysmas hatte. Erwartungsgemäß haben die Ergebnisse gezeigt: Je kleiner die Porosität ist, desto größer sind die Auswirkungen auf Blutflussgeschwindigkeit und Verwirbelungen im Aneurysma. Die Kombination mit der geringsten Porosität (D1 und D2) erzielte eine durchschnittliche Reduktion der Flussparameter um 80% bzw. 88% bei konstanten und pulsatilen Flüssen. Die Verminderung von mittlerer Geschwindigkeit und von Verwirbelungen war beim „shear-driven“ Fluss deutlich signifikanter als beim „inertia-driven“ Fluss.Schlussfolgerung:Obwohl die Porosität der wichtigste Parameter zur Senkung des Flusses ist, können andere Parameter wie das Design des jeweiligen Modells oder lokale Flusseigenschaften die Wirksamkeit der Flussreduktion in intrakraniellen Aneurysmen beeinflussen.

Fast virtual deployment of self-expandable stents: Method and in vitro evaluation for intracranial aneurysmal stenting

INTRODUCTION Minimally invasive treatment approaches, like the implantation of percutaneous stents, are becoming more popular every day for the treatment of intracranial aneurysms. The outcome of such treatments is related to factors like vessel and aneurysm geometry, hemodynamic conditions and device design. For this reason, having a tool for assessing stenting alternatives beforehand is crucial. METHODOLOGY The Fast Virtual Stenting (FVS) method, which provides an estimation of the configuration of intracranial stents when released in realistic geometries, is proposed in this paper. This method is based on constrained simplex deformable models. The constraints are used to account for the stent design. An algorithm for its computational implementation is also proposed. The performance of the proposed methodology was contrasted with real stents released in a silicone phantom. RESULTS In vitro experiments were performed on the phantom where a contrast injection was performed. Subsequently, corresponding Computational Fluid Dynamics (CFD) analyzes were carried out on a digital replica of the phantom with the virtually released stent. Virtual angiographies are used to compare in vitro experiments and CFD analysis. Contrast time-density curves for in vitro and CFD data were generated and used to compare them. CONCLUSIONS Results of both experiments resemble very well, especially when comparing the contrast density curves. The use of FVS methodology in the clinical environment could provide additional information to clinicians before the treatment to choose the therapy that best fits the patient.

Treatment of Renal Artery Aneurysm With the Multilayer Stent

Purpose: To describe a new type of stent consisting of a 3-dimensional (3D) braided tube made of 2 interconnected layers without any covering to treat a renal artery aneurysm. Case Report: A 78-year-old hypertensive man with multiple comorbidities was incidentally found to have a large (28-×30 mm) saccular aneurysm in the main right renal artery involving the inferior renal artery. Via a percutaneous femoral approach, a 6–3×0-mm Multilayer stent was deployed easily in front of the aneurysm neck covering the inferior renal artery. Blood flow inside the sac was immediately and significantly reduced. All the renal artery branches remained patent. Blood pressure returned to normal after the procedure. At 6 months, angiography showed complete shrinkage of the aneurysm wall; all the inferior renal artery branches remained patent. Conclusion: The 3D multilayer fluid modulating stent concept appears to be a viable alternative for renal aneurysm exclusion. A larger study is underway to evaluate this new stent in other peripheral aneurysms.

Effect of flow diversion treatment on very small ruptured aneurysms

BACKGROUNDRuptured aneurysms of < 2 mm are not amenable to endovascular coiling and therefore pose a significant treatment challenge. OBJECTIVETo test recently introduced flow diverters that allow endovascular reconstruction via another method and may represent a new treatment option for such lesions. PATIENTS AND METHODSThree female patients presented with acute subarachnoid hemorrhage. An aneurysm of < 2 mm was identified in all patients as the cause of bleeding. The aneurysms were located at the C2 segment of the internal carotid in 2 patients and on the basilar bifurcation in the other. All patients had failed early endovascular treatment attempts. Flow diversion with the SILK flow diverter was offered as an alternative in each patient. RESULTSSILK deployment successfully eliminated the aneurysms in all 3 instances. One of the aneurysms was excluded from contrast material visualization immediately after stent deployment. Transient thrombotic complication was observed in the case of the basilar artery aneurysm. It resolved with the administration of intraarterial tirofiban. There was no treatment-related morbidity, and none of the aneurysms reruptured after SILK implantation during a clinical follow-up of at least 4 months (range, 4–10 months). Imaging follow-up showed complete vessel remodeling in all cases. CONCLUSIONFlow diversion treatment prevented rebleeding during the follow-up period. Reverse remodeling of the concerned vascular segment with delayed disappearance of the aneurysm was observed in each case.

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

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.

Methodologies to assess blood flow in cerebral aneurysms: current state of research and perspectives.

With intracranial aneurysms disease bringing a weakened arterial wall segment to initiate, grow and potentially rupture an aneurysm, current understanding of vessel wall biology perceives the disease to follow the path of a dynamic evolution and increasingly recognizes blood flow as being one of the main stakeholders driving the process. Although currently mostly morphological information is used to decide on whether or not to treat a yet unruptured aneurysm, among other factors, knowledge of blood flow parameters may provide an advanced understanding of the mechanisms leading to further aneurismal growth and potential rupture. Flow patterns, velocities, pressure and their derived quantifications, such as shear and vorticity, are today accessible by direct measurements or can be calculated through computation. This paper reviews and puts into perspective current experimental methodologies and numerical approaches available for such purposes. In our view, the combination of current medical imaging standards, numerical simulation methods and endovascular treatment methods allow for thinking that flow conditions govern more than any other factor fate and treatment in cerebral aneurysms. Approaching aneurysms from this perspective improves understanding, and while requiring a personalized aneurysm management by flow assessment and flow correction, if indicated.

Subtracted vortex centers path line method with cinematic angiography for measurement of flow speed in cerebral aneurysms

Abstract Background and purpose: The assessment of blood flow speed by imaging modalities is of increasing importance for endovascular treatment, such as stent implantation, of cerebral aneurysms. The subtracted vortex centers path line method (SVC method) utilizes image post-processing for determining flow quantitatively. In current practice, intra-aneurysmal flow in an in vitro model is visualized by laser sheet translumination and digitally recorded. In this study, we applied this method to cinematic angiography (CA), which is the preferred imaging method for endovascular interventions, to analyse hemodynamic changes. The SVC method was applied to the images and compared with results of the slipstream line method with colored fluid. Methods: A transparent tubular model was constructed of silicone which included an aneurysm 10 mm in diameter and having a 5 mm neck on a straight parent artery with a diameter of 3.5 mm. The model was integrated into a pulsatile circulation system. By CA, successive images at 25 frames/s with injection of contrast were obtained. Results and conclusion: Rotating vortexes of contrast, which advanced along the wall of the aneurysm, were observed in successive images of the aneurysm cavity. This phenomenon was also observed in the successive images with the slipstream line method. The speed of the vortex center was calculated and the results show that the vortex speed of CA was the same as that under the slipstream line method. This indicates the possibility of applying the SVC method to medical imaging equipment for analysis of the flow in aneurysms containing stent.

Structural properties of human cerebral arteries as assessed by a constituent-baised biomechanical model

Reference EPFL-CONF-179793View record in Web of Science Record created on 2012-07-04, modified on 2017-05-10

Cinematic Angiography for Measurements of Blood Flow in Cerebral Aneurysms With Stents

Background and Purpose: The assessment of blood flow speed by imaging modalities is important for endovascular treatments, such as stent implantation, of cerebral aneurysms. The subtracted vortex centers path line method (SVC method) is one of the ways of determining flow speed quantitatively using the image sequence. And a cinematic angiography (CA) is a high speed image acquisition system using X-ray and contrast media integrated in Digital Subtraction Angiography (DSA) for endovascular therapy. The combination of SVC and CA may useful for determining the blood flow speed during the operation using DSA. In this study, we applied this combination to analyze hemodynamic changes before and after stenting. Methods: A transparent tubular model was constructed of silicone which included an aneurysm 10 mm in diameter and having a 5 mm neck on a straight parent artery with a diameter of 3.5 mm. The model was integrated into a pulsatile circulation system. A double layer stent was placed in the parent artery on the aneurysm. By CA, successive images at 25 frames per second with injection of contrast were obtained. Results and conclusion: Rotating vortexes of contrast, which advanced along the wall of the aneurysm, were observed in successive images of the aneurysm cavity. The movement distance of the vortex center was measured and the results show that the vortex speed decrease after stenting. This indicates the possibility of applying the SVC method to medical imaging equipment for analysis of the flow in aneurysms containing stent.Copyright