Breigh N. Roszelle

Flow diverter effect on cerebral aneurysm hemodynamics: an in vitro comparison of telescoping stents and the Pipeline

IntroductionFlow diverting devices and stents can be used to treat cerebral aneurysms too difficult to treat with coiling or craniotomy and clipping. However, the hemodynamic effects of these devices have not been studied in depth. The objective of this study was to quantify and understand the fluid dynamic changes that occur within bifurcating aneurysms when treated with different devices and configurations.MethodsTwo physical models of bifurcating cerebral aneurysms were constructed: an idealized model and a patient-specific model. The models were treated with four device configurations: a single low-porosity Pipeline embolization device (PED) and one, two, and three high-porosity Enterprise stents deployed in a telescoping fashion. Particle image velocimetry was used to measure the fluid dynamics within the aneurysms; pressure was measured within the patient-specific model.ResultsThe PED resulted in the greatest reductions in fluid dynamic activity within the aneurysm for both models. However, a configuration of three telescoping stents reduced the fluid dynamic activity within the aneurysm similarly to the PED treatment. Pressure within the patient-specific aneurysm did not show significant changes among the treatment configurations; however, the pressure difference across the untreated vessel side of the model was greatest with the PED.ConclusionTreatment with stents and a flow diverter led to reductions in aneurysmal fluid dynamic activity for both idealized and patient-specific models. While the PED resulted in the greatest flow reductions, telescoping high-porosity stents performed similarly and may represent a viable treatment alternative in situations where the use of a PED is not an option.

In vitro and in silico study of intracranial stent treatments for cerebral aneurysms: effects on perforating vessel flows

Background Many cerebral aneurysms can be treated effectively with intracranial stents. Unfortunately, stents can occlude perforating vessels near the treatment site which can decrease cerebral perfusion and increase the risk of stroke. Methods Particle image velocimetry was used to investigate the effects of intracranial stents on flows in perforators near a treated aneurysm. In Phase 1 of the study, different stent configurations were deployed into an idealized physical model of a sidewall aneurysm with perforating vessels. The configurations investigated were the Pipeline embolization device (PED) and one, two and three telescoping Neuroform stents. In Phase 2 of the study a single Neuroform stent was deployed so that the stent struts directly occluded the perforating vessel. Results In Phase 1 of the study it was found that even three telescoping stents affected perforating vessel flow less than a single PED under pulsatile conditions (average reduction 32.7% vs 46.5%). Results from Phase 2 indicated that the location of the occluding strut across the perforating vessel orifice had a greater impact on perforating vessel flow than the percentage occlusion. Conclusion The findings of this study show that the use, configuration and positioning of intracranial stents can all have considerable influence on flow in affected perforating vessels near treated cerebral aneurysms.

An In Vitro Study of Pulsatile Fluid Dynamics in Intracranial Aneurysm Models Treated with Embolic Coils and Flow Diverters

Although coil embolization is one of the most effective treatments for intracranial aneurysms (ICAs), the procedure is often unsuccessful. For example, an ICA may persist after coil embolization if deployed coils fail to block the flow of blood into the aneurysm. Unfortunately, the specific flow changes that are effected by embolic coiling (and other endovascular therapies) are poorly understood, which creates a barrier to the design and execution of optimal treatments in the clinic. We present an in vitro pulsatile flow study of treated basilar tip aneurysm models that elucidates relationships between controllable treatment parameters and clinically important post-treatment fluid dynamics. We also compare fluid dynamic performance across embolic coils and more recently proposed devices (e.g., the Pipeline Embolization Device) that focus on treating ICAs by diverting rather than blocking blood flow. In agreement with previous steady flow studies, coil embolization-reduced velocity magnitude at the aneurysmal neck by greater percentages for a narrow-neck aneurysm, and reduced flow into aneurysms by greater percentages at lower parent vessel flow rates. However, flow diversion reduced flow into a wide-neck aneurysm more so than coil embolization, regardless of flow conditions. Finally, results also showed that for the endovascular devices we examined, treatment effects were generally less dramatic under physiologic pulsatile flow conditions as compared to steady flow conditions. The fluid dynamic performance data presented in this study represent the first direct in vitro comparison of coils and flow diverters in aneurysm models, and provide a novel, quantitative basis to aid in designing endovascular treatments toward specific fluid dynamic outcomes.

Comparison Among Different High Porosity Stent Configurations: Hemodynamic Effects of Treatment in a Large Cerebral Aneurysm

Whether treated surgically or with endovascular techniques, large and giant cerebral aneurysms are particularly difficult to treat. Nevertheless, high porosity stents can be used to accomplish stent-assisted coiling and even standalone stent-based treatments that have been shown to improve the occlusion of such aneurysms. Further, stent assisted coiling can reduce the incidence of complications that sometimes result from embolic coiling (e.g., neck remnants and thromboembolism). However, in treating cerebral aneurysms at bifurcation termini, it remains unclear which configuration of high porosity stents will result in the most advantageous hemodynamic environment. The goal of this study was to compare how three different stent configurations affected fluid dynamics in a large patient-specific aneurysm model. Three common stent configurations were deployed into the model: a half-Y, a full-Y, and a crossbar configuration. Particle image velocimetry was used to examine post-treatment flow patterns and quantify root-mean-squared velocity magnitude (VRMS) within the aneurysmal sac. While each configuration did reduce VRMS within the aneurysm, the full-Y configuration resulted in the greatest reduction across all flow conditions (an average of 56% with respect to the untreated case). The experimental results agreed well with clinical follow up after treatment with the full-Y configuration; there was evidence of thrombosis within the sac from the stents alone before coil embolization was performed. A computational simulation of the full-Y configuration aligned well with the experimental and in vivo findings, indicating potential for clinically useful prediction of post-treatment hemodynamics. This study found that applying different stent configurations resulted in considerably different fluid dynamics in an anatomically accurate aneurysm model and that the full-Y configuration performed best. The study indicates that knowledge of how stent configurations will affect post-treatment hemodynamics could be important in interventional planning and demonstrates the capability for such planning based on novel computational tools.

Hands-on integrated CFD educational interface for introductory fluids mechanics

The development, implementation, and evaluation of an effective curriculum for students to learn integrated computational fluid dynamics (CFD) and experimental fluid dynamics (EFD) is described. The CFD objective is to teach students CFD methodology and procedures through a step-by-step CFD process using a CFD educational interface for hands-on student experience. The EFD objective is to teach students use of modern facilities, measurement systems including ePIV and Flowcoach, and uncertainty analysis (UA), following a step-by-step EFD process for fluids engineering experiments. Students analyse and relate CFD and EFD results to fluid physics and classroom lectures, including teamwork and presentation of results. Implementation is described based on results for an introductory level fluid mechanics course, which includes integrated CFD and EFD laboratories for the same geometries and conditions. An independent evaluation investigates and reports the learning outcomes and the effectiveness of the CFD educational interface, ePIV, Flowcoach and CFD and EFD laboratories.

In Vitro Investigation of a New Thin Film Nitinol-Based Neurovascular Flow Diverter

Fusiform and wide-neck cerebral aneurysms can be challenging to treat with conventional endovascular or surgical procedures. Recently, flow diverters have been developed that treat such aneurysms by diverting flow away from the sac rather than direct occlusion. The Pipeline Embolization Device (PED), which embodies a single-layer braided design, is best known among available flow diverters. While the device has demonstrated success in recent trials, late aneurysmal rupture after PED treatment has been a concern. More recently, a new generation of dual-layer devices has emerged that includes a novel Hyperelastic Thin Film Nitinol (HE-TFN)-covered design. In this study, we compare fluid dynamic performance between the PED and HE-TFN devices using particle image velocimetry (PIV). The PED has a pore density of 12.5-20 pores/mm2 and a porosity of 65-70%. The two HE-TFN flow diverters have pore densities of 14.75 pores/mm2 and 40 pores/mm2, and porosities of 82% and 77%, respectively. Conventional wisdom suggests that the lower porosity PED would decrease intraaneurysmal flow to the greatest degree. However, under physiologically realistic pulsatile flow conditions, average drops in root-mean-square velocity (VRMS) within the aneurysm of an idealized physical flow model were 42.8-73.7% for the PED and 68.9-82.7% for the HE-TFN device with the highest pore density. Interestingly, examination of collateral vessel flows in the same model also showed that the HE-TFN design allowed for greater collateral perfusion than the PED. Similar trends were observed under steady flow conditions in the idealized model. In a more clinically realistic scenario wherein an anatomical aneurysm model was investigated, the PED affected intraaneurysmal VRMS reductions of 64.3% and 56.3% under steady and pulsatile flow conditions, respectively. In comparison, the high pore density HE-TFN device reduced intraaneurysmal VRMS by 88% and 71.3% under steady and pulsatile flow conditions, respectively. The HE-TFN device also led to greater reductions in cross-neck flow than the PED. We attribute the superior performance of the HE-TFN device to higher pore density, which may play a more important role in modifying aneurysmal fluid dynamics than the conventional flow diverter design parameter of greatest general interest, absolute porosity. Lastly, the PED led to more elevated intraaneurysmal pressures after deployment, which provides insight into a potential mechanism for late rupture following treatment with the device.

O-012 The Effect of Pipeline Embolisation Device on Intra-Aneurysmal Pressures: In-Vitro Study

Introduction Pipeline (PED) has become commonly used to treat cerebral aneurysms, studies have found that 1% of cases that use the device result in devastating subarachnoid haemorrhage due to aneurysm rupture. The cause of these complications is still unclear, and some hypothesise it may be due to the effect the PED has on the intra-aneurysmal pressure. Materials and Methods We performed in vitro studies on two patient-specific models of cerebral aneurysms (fig 1). While both aneurysms were similar in size they had different fundamental geometries, as the first was a basilar tip and the other a side wall. In order to measure the intra-aneurysmal pressure a 0.42 mm tap was drilled into the model and a 0.40 mm micro-catheter was thread into the hole and attached to a pressure transducer (Harvard Apparatus, Holliston, MA, USA). pressure measurements were acquired and collected with Labview Signal Express (National Instruments, Austin, TX, USA). Along with the intra-aneurysmal pressure, pressure measurements were also collected at the inlet and outlet of the model. In conjunction with the pressure measurements, particle image velocimetry, a flow visualisation technique, was used to observe the haemodynamic flow patterns within the aneurysm. Results Results from the sidewall aneurysm showed that the deployment of the PED led to a dampening of the pulsatile waveform within the aneurysm. However, the PED also led to an increase in average intra-aneurysmal pressure of over 7 mmHg (fig. 2). In contrast, the BTA model did not show an increase in intra-aneurysmal pressure. This difference is likely related to the fact that the sidewall aneurysm is a closed system (only one inlet and outlet), while the BTA has an additional untreated outflow. In fact the BTA model also showed an increase in pressure drop across the untreated vessel during PED deployment, which indicates that the blood flow is being directed in that direction. The flow visualisation patterns of both models indicated a reduction in the velocities within the aneurysm, indicating that the PED is leading to reductions in fluid dynamic activity, making the increase in pressure of even greater interest. Conclusion The results of this study, while preliminary, give insight into the intra-aneurysmal pressure changes associated after deployment of the PED. Our results indicate that the PED does lead to haemodynamic changes in pressure within the aneurysm that could lead to ruptured. Abstract O-012 Figure 1 Disclosures F. Gonzalez: None. B. Roszelle: None. H. Babiker: None. D. Frakes: None.

Comparison of the Effects of Embolic Coils and a Low Porosity Stent on Cerebral Aneurysm Fluid Dynamics

Wide-neck cerebral aneurysms are difficult to treat with embolic coils. Concerns over the stability of coils within the aneurysmal sac often lead to incomplete filling of the sac, which may cause recurrence [1]. To overcome this challenge, clinicians may deploy a high porosity stent in a staged process to act as a supporting bridge for coils. The stent is commonly deployed 6–8 week prior to coil embolization, which lengthens the treatment period [2].Copyright

The Pipeline Versus Telescoping Stents: In Vitro Flows in a Treated Cerebral Aneurysm Model

Cerebral aneurysms are a significant concern; they are found in 2% of the world population [1]. While endovascular treatments have become a successful option for patients with cerebral aneurysms, recurrence rates remain as high as 50% [2]. Accordingly, many interventional devices are being developed with the hope of increasing the success rate of endovascular aneurysm occlusion. One of these devices is the low-porosity Pipeline emboilzation device (PED). Compared to more traditional intracranial stents, the PED contains a higher ratio of metal to surface area. Previously, similar reductions in porosity were obtained by sequentially deploying multiple high-porosity stents inside of one another, which is known as “telescoping.” The hemodynamic effects of using a single low-porosity device, versus telescoped high-porosity stents, have not been investigated thoroughly. In this study flow was quantified for idealized and anatomical basilar tip aneurysm models. The models were treated with sequentially placed high-porosity stents and a single low-porosity PED.Copyright

Hands-On Integrated CFD Educational Interface and EFD/ePIV/Flowcoach Laboratories for Introductory Fluids Mechanics (Invited)

The development, implementation, and evaluation of an effective curriculum for students to learn integrated computational fluid dynamics (CFD) and experimental fluid dynamics (EFD) including ePIV and Flowcoach in introductory undergraduate level courses and laboratories is described. The CFD objective is to teach students from novice to expert users who are well prepared for engineering practice using a CFD Educational Interface for hands-on student experience, which mirrors actual engineering practice. The Educational Interface teaches CFD methodology and procedures through a step-by-step interactive implementation automating the CFD process. A hierarchical system of predefined active options facilitates use at introductory and intermediate levels, encouraging self-learning, and eases transition to using industrial CFD codes. The EFD objective is to teach students use of modern facilities, measurement systems, and uncertainty analysis (UA) following a step-bystep approach, which mirrors the “real-life” EFD process: setup facility; install model; setup equipment; setup data acquisition; perform calibrations; data acquisition, analysis and reduction; and UA, and comparison CFD and/or analytical fluid dynamics (AFD) results. Students conduct fluids engineering experiments using tabletop and modern facilities such as pipe stands and wind tunnels and modern measurement systems, including pressure transducers, pitot probes, load cells, ePIV, Flowcoach and computer data acquisition systems (Lab View) and data reduction. Students analyze and relate CFD and EFD results to fluid physics and classroom lectures, including teamwork and presentation of results in written and graphical form. Implementation is described based on results for an introductory level fluid mechanics course, which includes integrated CFD and EFD laboratories for the same geometries and conditions. The laboratories constitute one credit hour of a four credit hour one semester course and include tabletop kinematic viscosity experiment focusing on UA procedures and pipe and airfoil experiments focusing on integrated EFD and CFD. An independent evaluation investigates and reports the learning outcomes and the effectiveness of the CFD educational interface, ePIV, Flowcoach and CFD and EFD laboratories.