Intra-aneurysmal Hemodynamics Research Articles

Ongoing Geometric Remodeling of the Parent Artery After Flow-Diverter Stent Reconstruction in Cerebral Aneurysms: The Device Design Matters

Configuration changes of the parent artery (PA) after flow-diverter (FD) stent reconstruction, caused by the bending force of the device, may have an additional role in aneurysm occlusion as a result of the secondary alteration of intra-aneurysmal hemodynamics related to the geometry alteration of the vessel. To determine the degree of PA deformation and aneurysm occlusion rates after deployment of 2 different types of FD. Patients treated with 2 different designs of cobalt-chromium braid (48 and 64 wire braid) structure FD were subject to analysis. Vascular angle changes at the level of the reconstructed segment immediately after FD deployment and at 1 year follow-up were measured and the potential relationship with aneurysmal occlusion rate was analyzed. Forty-two patients harboring 48 aneurysms were included in the present study. The aneurysms were divided into side wall (85.4%) and bifurcation types (14.6%). Twenty-six aneurysms were treated using the Pipeline FD (48 wire braid; 54.2%) and 22 using the Evolve FD (64 wire braid; 45.8%). Of the 48 aneurysms, 42 (87.5%) met the primary end point of complete occlusion at 12 months. The median postdeployment angle change was 7.04°± 4.59° for the Pipeline and 5.05°± 2.49° for the Evolve, whereas the median 12 months follow-up angle change was 15.49°± 10.99° and 10.01°± 8.83°, respectively. PA angle changes were significantly higher in the bifurcation group compared with the side wall group both during procedure and at 12 months follow-up. Angle change had a statistically nonsignificant association with complete aneurysm occlusion. PA deformation starts immediately after deployment and remodeling continues for 1 year after. Aneurysms located in the vessel bifurcation were more prone to PA straightening after FD deployment than were side wall aneurysms. Furthermore, Pipeline seemed to be more prone to inducing vascular deformation, compared with Evolve.

CFD to Quantify Idealized Intra-Aneurysmal Blood Flow in Response to Regular and Flow Diverter Stent Treatment

Cerebral aneurysms are pathological dilatations of the vessels supplying the brain. They carry a certain risk of rupture, which in turn, results in a high risk of mortality and morbidity. Flow diverters (FDs) are high-density meshed stents which are implanted in the vessel segment harboring an intracranial aneurysm to cover the entrance of the aneurysm, thus reducing the blood flow into the aneurysm, promoting thrombosis formation and stable occlusion, which prevents rupture or growth of the aneurysm. In the present study, the blood flow in an idealized aneurysm, treated with an FD stent and a regular stent (RS), were modeled and analyzed considering their design, surface area porosity, and flow reduction to investigate the quantitative and qualitative effect of the stent on intra-aneurysmal hemodynamics. CFD simulations were conducted before and after treatment. Significant reductions were observed for most hemodynamic variables with the use of stents, during both the peak systolic and late diastolic cardiac cycles. FD reduces the intra-aneurysmal wall shear stress (WSS), inflow, and aneurysmal flow velocity, and increases the turnover time when compared to the RS; therefore, the possibility of aneurysm thrombotic occlusion is likely to increase, reducing the risk of rupture in cerebral aneurysms.

Impact of patient-specific inflow boundary conditions on intracranial aneurysm hemodynamics

Abstract For hemodynamic simulations of intracranial aneurysms boundary conditions (BC) are required. In most cases, these are not patient-specific and thus do not reflect the real flow conditions in the patient. This study investigates the influence of patient-specific inflow BC on intra-aneurysmal hemodynamics. The focus lies on gender and age variations of the patients. To asses the impact, four different inflow curves representing the velocity profile of the inflow over one cardiac cycle is modeled. These four inflow BC are varied in the simulations of each aneurysm from selected subgroups. From the results of the simulations, the hemodynamic parameters are determined for each inflow BC and the percent differences between inflow BC are determined. The results show that the hemodynamic parameters are not robust to varying inflow BC. It can be seen that age has more influence on the hemodynamic parameters than gender. This study demonstrates the dependence of valid hemodynamic parameters on realistic inflow BC. Thus, if available, patient-specific inflow curves are recommended.

How to perform intra-aneurysmal coil embolization after Pipeline deployment: a study from a hemodynamic viewpoint

BackgroundPipeline embolization device (PED) deployment combined with coil therapy for large complex intracranial aneurysms is effective and considered superior to PED deployment alone. However, the optimal strategy for use of...

Studying the effect of stent thickness and porosity on post-stent implantation hemodynamics

This study investigates the effect of stent thickness and stent porosity which are important factors determining the post-treatment intra-aneurysmal hemodynamics. The study uses computational fluid dynamics (CFD) to estimate the hemodynamic behaviour: flow velocity, pressure distributions, time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), besides relative residence time (RRT) blood flow distribution in a proposed stent and three other commercially available stents. The hemodynamic behaviour is compared between four different cases. In each case, each stent has the specific thickness and porosity values. The results show that the velocity magnitude inside the sac declined in thinner stents and lower porosity stents, TAWSS on the aneurysmal wall declined linearly in lower porosity stents, OSI and RRT increased obviously in thicker stents and higher porosity stents. Finally, the results conclude that the stent with the lowest thickness and porosity has the best performance that leads to post-stent thrombus formation and healing. However, the proposed stent design, a more porous construct, has a higher RRT compared to the used commercially available stents, which helps promote the thrombus growth inside the aneurysm sac.

Stent-Induced Vascular Remodeling in Two-Step Stent-Assisted Coiling Treatment of Brain Aneurysms: A Closer Look Into the Hemodynamic Changes During the Stent Healing Period.

Stenting has become an important adjunctive tool for assisting coil embolization in complex-shaped intracranial aneurysms. However, as a secondary effect, stent deployment has been related to both immediate and delayed remodeling of the local vasculature. Recent studies have demonstrated that this phenomenon may assume different roles depending on the treatment stage. However, the extent of such event on the intra-aneurysmal hemodynamics is still unclear; especially when performing two-step stent-assisted coiling (SAC). Therefore, we performed computational fluid dynamics (CFD) analysis of the blood flow in four bifurcation aneurysms focusing on the stent healing period found in SAC as a two-step maneuver. Our results show that by changing the local vasculature, the intra-aneurysmal hemodynamics changes considerably. However, even though changes do occur, they were not consistent among the cases. Furthermore, by changing the local vasculature not only the shear levels change but also the shear distribution on the aneurysm surface. Additionally, a geometric analysis alone can mislead the estimation of the novel hemodynamic environment after vascular remodeling, especially in the presence of mixing streams. Therefore, although the novel local vasculature might induce an improved hemodynamic environment, it is also plausible to expect that adverse hemodynamic conditions might occur. This could pose a particularly delicate condition since the aneurysm surface remains completely exposed to the novel hemodynamic environment during the stent healing period. Finally, our study emphasizes that vascular remodeling should be considered when assessing the hemodynamics in aneurysms treated with stents, especially when evaluating the earlier stages of the treatment process.

Recent Advances in Biomechanical Characterization of Thoracic Aortic Aneurysms.

Thoracic aortic aneurysm (TAA) is a focal enlargement of the thoracic aorta, but the etiology of this disease is not fully understood. Previous work suggests that various genetic syndromes, congenital defects such as bicuspid aortic valve, hypertension, and age are associated with TAA formation. Though occurrence of TAAs is rare, they can be life-threatening when dissection or rupture occurs. Prevention of these adverse events often requires surgical intervention through full aortic root replacement or implantation of endovascular stent grafts. Currently, aneurysm diameters and expansion rates are used to determine if intervention is warranted. Unfortunately, this approach oversimplifies the complex aortopathy. Improving treatment of TAAs will likely require an increased understanding of the biological and biomechanical factors contributing to the disease. Past studies have substantially contributed to our knowledge of TAAs using various ex vivo, in vivo, and computational methods to biomechanically characterize the thoracic aorta. However, any singular approach typically focuses on only material properties of the aortic wall, intra-aneurysmal hemodynamics, or in vivo vessel dynamics, neglecting combinatorial factors that influence aneurysm development and progression. In this review, we briefly summarize the current understanding of TAA causes, treatment, and progression, before discussing recent advances in biomechanical studies of TAAs and possible future directions. We identify the need for comprehensive approaches that combine multiple characterization methods to study the mechanisms contributing to focal weakening and rupture. We hope this summary and analysis will inspire future studies leading to improved prediction of thoracic aneurysm progression and rupture, improving patient diagnoses and outcomes.

Stent-induced vessel deformation after intracranial aneurysm treatment – A hemodynamic pilot study

BackgroundStent-assisted coiling has become an important treatment option for intracranial aneurysms. However, studies have shown that this procedure can lead to the deformation of the local vasculature. Its effect on intra-aneurysmal hemodynamics still remains unclear. MethodsPre- and post-interventional image data of three representative middle cerebral artery aneurysms are considered in this study. This includes virtually deployed stents and coils. To evaluate the proportional effect of a) vessel deformation, b) stent deployment, and c) coil placement, 24 unsteady blood flow simulations were carried out focusing on the separated effects related to intra-aneurysmal hemodynamics. Four flow parameters (velocity within the aneurysm sac, aneurysm neck inflow rate, inflow concentration index, and ostium inflow area) and four shear parameters (wall shear stress, normalized wall shear stress, shear concentration index, and high shear area) were quantified. ResultsAll of the considered flow and shear parameters, except for the shear concentration index, were clearly reduced due to treatment. Coiling and stenting caused a distinct and smaller neck inflow rate, respectively, while the impact of deformation was inconsistent among the aneurysms. Overall, coiling appears to have the strongest impact on local hemodynamics. ConclusionStent-induced vessel deformation has a clear impact on intra-aneurysmal hemodynamics. This effect is neglected by the majority of previous studies, which consider the pre-interventional state for investigating the relation of stents and hemodynamics. The findings of this pilot study suggest that while stent-assisted coiling can lead to an improved hemodynamic situation, undesired flow conditions may occur in response to treatment.

Hemodynamic investigation of a patient-specific abdominal aortic aneurysm with iliac artery tortuosity

This paper describes a systematic investigation on the hemodynamic environment in a patient-specific AAA with tortuous common iliac artery(CIA) and external iliac artery (EIA). 3D reconstructions from CT scans and subsequent computational simulation are carried out. It is found out that the Newtonian and non-Newtonian models have very similar flow field and WSS distribution. More importantly, it is revealed that the torturous CIA maintained its helical flow. It is concluded that the assumption of Newtonian blood is adequate in capturing the intra-aneurysmal hemodynamics. Moreover, it is speculated that the physiological spiral flow protects the twisted CIA from the thrombosis formation.

Stretchable, Implantable, Nanostructured Flow-Diverter System for Quantification of Intra-aneurysmal Hemodynamics.

Random weakening of an intracranial blood vessel results in abnormal blood flow into an aneurysmal sac. Recent advancements show that an implantable flow diverter, integrated with a medical stent, enables a highly effective treatment of cerebral aneurysms by guiding blood flow into the normal vessel path. None of such treatment systems, however, offers post-treatment monitoring to assess the progress of sac occlusion. Therefore, physicians rely heavily on either angiography or magnetic resonance imaging. Both methods require a dedicated facility with sophisticated equipment settings and time-consuming, cumbersome procedures. In this paper, we introduce an implantable, stretchable, nanostructured flow-sensor system for quantification of intra-aneurysmal hemodynamics. The open-mesh membrane device is capable of effective implantation in complex neurovascular vessels with extreme stretchability (500% radial stretching) and bendability (180° with 0.75 mm radius of curvature) for monitoring of the treatment progress. A collection of quantitative mechanics, fluid dynamics, and experimental studies establish the fundamental aspects of design criteria for a highly compliant, implantable device. Hemocompatibility study using fresh ovine blood captures the device feasibility for long-term insertion in a blood vessel, showing less platelet deposition compared to that in existing implantable materials. In vitro demonstrations of three types of flow sensors show quantification of intra-aneurysmal blood flow in a pig aorta and the capability of observation of aneurysm treatment with a great sensitivity (detection limit as small as 0.032 m/s). Overall, this work describes a mechanically soft flow-diverter system that offers an effective treatment of aneurysms with an active monitoring of intra-aneurysmal hemodynamics.

The Effect of Stents in Cerebral Aneurysms: A Review.

The etiology of up to 95% of cerebral aneurysms may be accounted for by hemodynamically-induced factors that create vascular injury. The purpose of this review is to describe key physical properties that stents have and how they affect cerebral aneurysms. We performed a two-step screening process. First, a structured search was performed using the PubMed database. The following search terms and keywords were used: “Hemodynamics,” “wall shear stress (WSS),” “velocity,” “viscosity,” “cerebral aneurysm,” “intracranial aneurysm,” “stent,” “flow diverter,” “stent porosity,” “stent geometry,” “stent configuration,” and “stent design.” Reports were considered if they included original data, discussed hemodynamic changes after stent-based treatment of cerebral aneurysms, examined the hemodynamic effects of stent deployment, and/or described the geometric characteristics of both stents and the aneurysms they were used to treat. The search strategy yielded a total of 122 articles, 61 were excluded after screening the titles and abstracts. Additional articles were then identified by cross-checking reference lists. The final collection of 97 articles demonstrates that the geometric characteristics and configurations of deployed stents influenced hemodynamic parameters such as aneurysmal WSS, inflow, and pressure. The geometric characteristics of the aneurysm and its position also had significant influences on intra-aneurysmal hemodynamics after treatment. In conclusion, changes in specific aneurysmal hemodynamic parameters that result from stenting relate to a number of factors including the geometric properties and configurations of deployed stents, the geometric properties of the aneurysm, and the pretreatment hemodynamics.

Aneurysmal Parent Artery-Specific Inflow Conditions for Complete and Incomplete Circle of Willis Configurations.

Hemodynamics are thought to play a role in intracranial aneurysm growth and rupture. Computational fluid dynamics is frequently performed to assess intra-aneurysmal hemodynamics, using generalized flow waveforms of healthy volunteers as inflow boundary conditions. The purpose of this study was to assess differences in inflow conditions for different aneurysmal parent artery locations and variations of circle of Willis configurations. In a series of 96 patients with 103 aneurysms, velocity measurements were acquired using 2D phase-contrast MR imaging perpendicular to the aneurysmal parent arteries in the circle of Willis. Circle of Willis configurations were inspected for variations using multiple overlapping thin-slab-acquisition MRAs. Flow rates, velocity magnitudes, and pulsatility indices were calculated for each parent artery location in subgroups of complete and incomplete circle of Willis configurations. Flow rates, velocity magnitudes, and pulsatility indices were significantly different among aneurysmal parent arteries. Incomplete circle of Willis configurations were observed in 24% of the cases. Significantly lower basilar artery flow rates were observed in configurations with hypoplastic P1 segments. Significantly higher A1 flow rates were observed in configurations with a hypoplastic contralateral A1 segment. Inflow conditions vary substantially between aneurysmal parent arteries and circle of Willis configurations. We have created a collection of parent artery-specific inflow conditions tailored to the patient-specific circle of Willis configuration that can be used in future computational fluid dynamics studies analyzing intra-aneurysmal hemodynamics.

Variability of intra-aneurysmal hemodynamics caused by stent-induced vessel deformation

Abstract Clinical observation revealed deformations of the local vasculature due to implanted devices related to treatment of intracranial aneurysms. Pre- and post-interventional image data is reviewed and segmented. Based on Computational Fluid Dynamics (CFD) the effect of the vessel deformation on the intra-aneurysmal hemodynamics is investigated. The simulations incorporate the vessel trees with aneurysm excluding the device itself in order to consider the isolated effect of geometric modifications. As a result, the aneurysm inflow jet is redirected, changing the local flow quantities. Finally, a neck inflow rate reduction of 52.5 % is archived. Thus, a targeted vessel straightening as a novel tool for treatment planning appear to be an idea worth to discuss in the community.

Collar Sign in Incompletely Occluded Aneurysms after Pipeline Embolization: Evaluation with Angiography and Optical Coherence Tomography.

Flow diversion with the Pipeline Embolization Device has emerged as an attractive treatment for cerebral aneurysms. Processes involved in aneurysm occlusion include changes in intra-aneurysmal hemodynamics and endothelialization of the device. Here, we call attention to a radiographic sign not previously reported that is detected in incompletely occluded aneurysms after treatment with the Pipeline Embolization Device at angiographic follow-up and referred to as the "collar sign." A retrospective review of all patients who underwent placement of a Pipeline Embolization Device for cerebral aneurysms between January 2014 and May 2016 was performed. All aneurysms found to show the collar sign at follow-up were included. Optical coherence tomography was performed in 1 case. One hundred thirty-five aneurysms were treated in 115 patients. At angiographic follow-up, 17 (10.7%) aneurysms were found to be incompletely occluded. Ten (58.8%) of these aneurysms (average diameter, 7.9 ± 5.0 mm) were found to have the collar sign at angiographic follow-up (average, 5.5 ± 1.0 months). Four (40.0%) of the aneurysms underwent a second angiographic follow-up (average, 11.0 ± 0.9 months) after treatment, and again were incompletely occluded and showing the collar sign. Two patients underwent retreatment with a second Pipeline Embolization Device. Optical coherence tomography showed great variability of endothelialization at the proximal end of the Pipeline Embolization Device. The collar sign appears to be indicative of endothelialization, but continued blood flow into the aneurysm. This is unusual given the processes involved in aneurysm occlusion after placement of the Pipeline Embolization Device and has not been previously reported.

Does Arterial Flow Rate Affect the Assessment of Flow-Diverter Stent Performance?

Our aim was to assess the performance of flow-diverter stents. The pre- and end-of-treatment angiographies are commonly compared. However, the arterial flow rate may change between acquisitions; therefore, a better understanding of its influence on the local intra-aneurysmal hemodynamics before and after flow-diverter stent use is required. Twenty-five image-based aneurysm models extracted from 3D rotational angiograms were conditioned for computational fluid dynamics simulations. Pulsatile simulations were performed at different arterial flow rates, covering a wide possible range of physiologic flows among 1-5 mL/s. The effect of flow-diverter stents on intra-aneurysmal hemodynamics was numerically simulated with a porous medium model. Spatiotemporal-averaged intra-aneurysmal flow velocity and flow rate were calculated for each case to quantify the hemodynamics after treatment. The short-term flow-diverter stent performance was characterized by the relative velocity reduction inside the aneurysm. Spatiotemporal-averaged intra-aneurysmal flow velocity before and after flow-diverter stent use is linearly proportional to the mean arterial flow rate (minimum R2 > 0.983 of the linear regression models for untreated and stented models). Relative velocity reduction asymptotically decreases with increasing mean arterial flow rate. When the most probable range of arterial flow rate was considered (3-5 mL/s), instead of the wide possible flow range, the mean SD of relative velocity reduction was reduced from 3.6% to 0.48%. Both intra-aneurysmal aneurysm velocity and flow-diverter stent performance depend on the arterial flow rate. The performance could be considered independent of the arterial flow rates within the most probable range of physiologic flows.

Hemodynamic differences between unstable and stable unruptured aneurysms independent of size and location: a pilot study

BackgroundWhile clinical and angiographic risk factors for intracranial aneurysm instability are well established, it is reasonable to postulate that intra-aneurysmal hemodynamics also have a role in aneurysm instability.ObjectiveTo identify hemodynamic...

Effects of Variations of Flow and Heart Rate on Intra-Aneurysmal Hemodynamics in a Ruptured Internal Carotid Artery Aneurysm During Exercise.

Background:Hemodynamics is thought to play an important role in the mechanisms responsible for initiation, growth, and rupture of intracranial aneurysms. Computational fluid dynamic (CFD) analysis is used to assess intra-aneurysmal hemodynamics.Objectives:This study aimed to investigate the effects of variations in heart rate and internal carotid artery (ICA) flow rate on intra-aneurysmal hemodynamics, in an ICA aneurysm, by using computational fluid dynamics.Patients and Methods:Computed tomography angiography (CTA) was performed in a 55 years old female case, with a saccular ICA aneurysm, to create a patient-specific geometrical anatomic model of the aneurysm. The intra-aneurysmal hemodynamic environments for three states with different flow and heart rates were analyzed using patient-specific image-based CFD modeling.Results:Results showed significant changes for the three simulated states. For a proportion of the states examined, results were counterintuitive. Systolic and time-averaged wall shear stress and pressure on the aneurysm wall showed a proportional evolution with the mainstream flow rate.Conclusion:Results reinforced the pivotal role of vascular geometry, with respect to hemodynamics, together with the importance of performing patient-specific CFD analyses, through which the effect of different blood flow conditions on the aneurysm hemodynamics could be evaluated.

Symptomatic Very Delayed Parent Artery Occlusion After Flow Diversion Stent Embolization

Flow diversion stents (FDSs) are constructed from high-density braided mesh, which alters intra-aneurysmal hemodynamics and leads to aneurysm occlusion by inducing thrombus formation. Although there are potential complications associated with FDS embolization, one of the serious complications is the parent artery occlusion due to the in-stent thrombosis. A 72-year-old woman with a symptomatic giant fusiform aneurysm in the cavernous segment of ICA underwent single-layer pipeline embolization device (PED) embolization. Six-month and 1-year follow-up conventional angiographies showed the residual blood flow in the aneurysm. Two-year follow-up MRI showed the aneurysm sac shrinkage and the antiplatelet therapy was discontinued. The patient suffered from symptomatic parent artery occlusion due to the in-stent thrombosis, 4 months after antiplatelet therapy discontinuation. The patient with the incompletely occluded aneurysm after PED embolization should be given long-term antiplatelet therapy because of the risk of delayed parent artery occlusion.

Additional Value of Intra-Aneurysmal Hemodynamics in Discriminating Ruptured versus Unruptured Intracranial Aneurysms.

Hemodynamics are thought to play an important role in the rupture of intracranial aneurysms. We tested whether hemodynamics, determined from computational fluid dynamics models, have additional value in discriminating ruptured and unruptured aneurysms. Such discriminative power could provide better prediction models for rupture. A cross-sectional study was performed on patients eligible for endovascular treatment, including 55 ruptured and 62 unruptured aneurysms. Association with rupture status was tested for location, aneurysm type, and 4 geometric and 10 hemodynamic parameters. Patient-specific spatiotemporal velocities measured with phase-contrast MR imaging were used as inflow conditions for computational fluid dynamics. To assess the additional value of hemodynamic parameters, we performed 1 univariate and 2 multivariate analyses: 1 traditional model including only location and geometry and 1 advanced model that included patient-specific hemodynamic parameters. In the univariate analysis, high-risk locations (anterior cerebral arteries, posterior communicating artery, and posterior circulation), daughter sacs, unstable inflow jets, impingements at the aneurysm body, and unstable complex flow patterns were significantly present more often in ruptured aneurysms. In both multivariate analyses, only the high-risk location (OR, 3.92; 95% CI, 1.77-8.68) and the presence of daughter sacs (OR, 2.79; 95% CI, 1.25-6.25) remained as significant independent determinants. In this study population of patients eligible for endovascular treatment, we found no independent additional value of aneurysmal hemodynamics in discriminating rupture status, despite high univariate associations. Only traditional parameters (high-risk location and the presence of daughter sacs) were independently associated with ruptured aneurysms.

A novel low-profile flow sensor for monitoring of hemodynamics in cerebral aneurysm

A low-profile flow sensor has been designed, fabricated, and characterized to demonstrate the feasibility for monitoring hemodynamics in cerebral aneurysm. The prototype device is composed of three micro-membranes (500-µm-thick polyurethane film with 6-µm-thick layers of nitinol above and below). A novel super-hydrophilic surface treatment offers excellent hemocompatibility for the thin nitinol electrode. A computational study of the deformable mechanics optimizes the design of the flow sensor and the analysis of computational fluid dynamics estimates the flow and pressure profiles within the simulated aneurysm sac. Experimental studies demonstrate the feasibility of the device to monitor intra-aneurysmal hemodynamics in a blood vessel. The mechanical compression test shows the linear relationship between the applied force and the measured capacitance change. Analytical calculation of the resonant frequency shift due to the compression force agrees well with the experimental results. The results have the potential to address important unmet needs in wireless monitoring of intra-aneurysm hemodynamic quiescence.