Cerebral aneurysms treated with flow-diverting stents: computational models with intravascular blood flow measurements

Safety and efficacy of coated flow diverters in the treatment of ruptured intracranial aneurysms: a retrospective multicenter study

BACKGROUND

This multicenter study evaluated the safety and efficacy of coated flow diverters (cFDs) for the treatment of ruptured intracranial aneurysms.

METHODS

Consecutive patients treated with different cFDs for ruptured aneurysms under tirofiban at eight neurovascular centers between 2016 and 2023 were retrospectively analyzed. The majority of patients were loaded with dual antiplatelet therapy after the treatment. Aneurysm occlusion was determined using the O'Kelly-Marotta (OKM) grading scale. Primary outcome measures were major procedural complications and aneurysmal rebleeding during hospitalization.

RESULTS

The study included 60 aneurysms (posterior circulation: 28 (47%)) with a mean size of 5.8±4.7 mm. Aneurysm morphology was saccular in 28 (47%), blister-like in 12 (20%), dissecting in 13 (22%), and fusiform in 7 (12%). Technical success was 100% with a mean of 1.1 cFDs implanted per aneurysm. Adjunctive coiling was performed in 11 (18%) aneurysms. Immediate contrast retention was observed in 45 (75%) aneurysms. There was 1 (2%) major procedural complication (a major stroke, eventually leading to death) and no aneurysmal rebleeding. A good outcome (modified Rankin Scale 0-2) was achieved in 40 (67%) patients. At a mean follow-up of 6 months, 27/34 (79%) aneurysms were completely occluded (OKM D), 3/34 (9%) had an entry remnant (OKM C), and 4/34 (12%) had residual filling (OKM A or B). There was 1 (3%) severe in-stent stenosis during follow-up that was treated with balloon angioplasty.

CONCLUSIONS

Treatment of ruptured aneurysms with cFDs was reasonably safe and efficient and thus represents a valid treatment option, especially for complex cases.

Standardized viscosity as a source of error in computational fluid dynamic simulations of cerebral aneurysms

BACKGROUND

Computational fluid dynamics (CFD) simulations are a powerful tool for studying cerebral aneurysms, capable of evaluating hemodynamics in a way that is infeasible with imaging alone. However, the difficulty of incorporating patient-specific information and inherent obstacles of in vivo validation have limited the clinical usefulness of CFD of cerebral aneurysms. In this work we investigate the effect of using standardized blood viscosity values in CFD simulations of cerebral aneurysms when compared to simulations of the same aneurysms using patient-specific viscosity values derived from hematocrit measurements.

PURPOSE

The objective of this work is to determine the level of error, on average, that is, caused by using standardized values of viscosity in CFD simulations of cerebral aneurysms. By quantifying this error, we demonstrate the need for incorporating patient-specific viscosity in future CFD investigations of cerebral aneurysms.

METHODS

CFD simulations of forty-one cerebral aneurysms were conducted using patient-specific boundary conditions. For each aneurysm two simulations were conducted, one utilizing patient-specific blood viscosity derived from hematocrit measurements and another using a standardized value for blood viscosity. Hemodynamic parameters such as wall shear stress (WSS), wall shear stress gradient (WSSG), and the oscillatory shear index (OSI) were calculated for each of the simulations for each aneurysm. Paired t-tests for differences in the time-averaged maps of these hemodynamic parameters between standardized and patient-specific viscosity simulations were conducted for each aneurysm. Bland-Altman analysis was used to examine the cohort-wide changes in the hemodynamic parameters. Subjects were broken into two groups, those with higher than standard viscosity and those with lower than standard viscosity. An unpaired t-test was used to compare the percent change in WSS, WSSG, and OSI between patient-specific and standardized viscosity simulations for the two cohorts. The percent changes in hemodynamic parameters were correlated against the direction and magnitude of percent change in viscosity, aneurysm size, and aneurysm location. For all t-tests, a Bonferroni-corrected significance level of 0.0167 was used.

RESULTS

63.2%, 41.5%, and 48.7% of aneurysms showed statistically significant differences between patient-specific and standardized viscosity simulations for WSS, WSSG, and OSI respectively. No statistically significant difference was found in the percent changes in WSS, WSSG, and OSI between the group with higher than standard viscosity and those with lower than standard viscosity, indicating an increase in viscosity can cause either an increase or decrease in each of the hemodynamic parameters. On a study-wide level no significant bias was found in either direction for WSS, WSSG, or OSI between the simulation groups due to the bidirectional effect of changing viscosity. No correlation was found between percent change of viscosity and percent change of WSS, WSSG, or OSI, meaning an after-the-fact correction for patient-specific viscosity is not feasible.

CONCLUSION

Standardizing viscosity values in CFD of cerebral aneurysms has a large and unpredictable impact on the calculated WSS, WSSG, and OSI when compared to CFD simulations of the same aneurysms using a patient-specific viscosity. We recommend implementing hematocrit-based patient-specific blood viscosity values for all CFD simulations of cerebral aneurysms.

Improving the accuracy of computational fluid dynamics simulations of coiled cerebral aneurysms using finite element modeling

Cerebral aneurysms are a serious clinical challenge, with ∼half resulting in death or disability. Treatment via endovascular coiling significantly reduces the chances of rupture, but the techniquehas failure rates of ∼20 %. This presents a pressing need to develop a method fordetermining optimal coildeploymentstrategies. Quantification of the hemodynamics of coiled aneurysms using computational fluid dynamics (CFD) has the potential to predict post-treatment outcomes, but representing the coil mass in CFD simulations remains a challenge. We use the Finite Element Method (FEM) for simulating patient-specific coil deployment for n = 4 ICA aneurysms for which 3D printed in vitro models were also generated, coiled, and scanned using ultra-high resolution synchrotron micro-CT. The physical and virtual coil geometries were voxelized onto a binary structured grid and porosity maps were generated for geometric comparison. The average binary accuracy score is 0.8623 and the average error in porosity map is 4.94 %. We then conduct patient-specific CFD simulations of the aneurysm hemodynamics using virtual coils geometries, micro-CT generated oil geometries, and using the porous medium method to represent the coil mass. Hemodynamic parameters including Neck Inflow Rate (Qneck) and Wall Shear Stress (WSS) were calculated for each of the CFD simulations. The average relative error in Qneck and WSS from CFD using FEM geometry were 6.6 % and 21.8 % respectively, while the error from CFD using a porous media approximation resulted in errors of 55.1 % and 36.3 % respectively; demonstrating a marked improvement in the accuracy of CFD simulations using FEM generated coil geometries.

Assessment of the flow-diverter efficacy for intracranial aneurysm treatment considering pre- and post-interventional hemodynamics

Endovascular treatment of intracranial aneurysms with flow diverters (FD) has become one of the most promising interventions. Due to its woven high-density structure they are particularly applicable for challenging lesions. Although several studies have already conducted realistic hemodynamic quantification of the FD efficacy, a comparison with morphologic post-interventional data is still missing. This study analyses the hemodynamics of ten intracranial aneurysm patients treated with a novel FD device. Based on pre- and post-interventional 3D digital subtraction angiography image data, patient-specific 3D models of both treatment states are generated applying open source threshold-based segmentation methods. Using a fast virtual stenting approach, the real stent positions available in the post-interventional data are virtually replicated and both treatment scenarios were characterized using image-based blood flow simulations. The results show FD-induced flow reductions at the ostium by a decrease in mean neck flow rate (51%), inflow concentration index (56%) and mean inflow velocity (53%). Intraluminal reductions in flow activity for time-averaged wall shear stress (47%) and kinetic energy (71%) are present as well. However, an intra-aneurysmal increase in flow pulsatility (16%) for the post-interventional cases can be observed. Patient-specific FD simulations demonstrate the desired flow redirection and activity reduction inside the aneurysm beneficial for thrombosis formation. Differences in the magnitude of hemodynamic reduction exist over the cardiac cycle which may be addressed in a clinical setting by anti-hypertensive treatment in selected cases.

Modeling the Mechanical Microenvironment of Coiled Cerebral Aneurysms

Successful occlusion of cerebral aneurysms using coil embolization is contingent upon stable thrombus formation, and the quality of the thrombus depends upon the biomechanical environment. The goal of this study was to investigate how coil embolization alters the mechanical micro-environment within the aneurysm dome. Inertialess particles were injected in three-dimensional, computational simulations of flow inside patient aneurysms using patient-specific boundary conditions. Coil embolization was simulated as a homogenous porous medium of known permeability and inertial constant. Lagrangian particle tracking was used to calculate the residence time and shear stress history for particles in the flow before and after treatment. The percentage of particles entering the aneurysm dome correlated with the neck surface area before and after treatment (pretreatment: R2 = 0.831, P < 0.001; post-treatment: R2 = 0.638, P < 0.001). There was an inverse relationship between the change in particles entering the dome and coil packing density (R2 = 0.600, P < 0.001). Following treatment, the particles with the longest residence times tended to remain within the dome even longer while accumulating lower shear stress. A significant correlation was observed between the treatment effect on residence time and the ratio of the neck surface area to porosity (R2 = 0.390, P = 0.007). The results of this study suggest that coil embolization triggers clot formation within the aneurysm dome via a low shear stress-mediated pathway. This hypothesis links independently observed findings from several benchtop and clinical studies, furthering our understanding of this treatment strategy.

Modeling Flow in Cerebral Aneurysm After Coils Embolization Treatment: A Realistic Patient-Specific Porous Model Approach

PURPOSE

Computational fluid dynamics (CFD) has been used to evaluate the efficiency of endovascular treatment in coiled cerebral aneurysms. The explicit geometry of the coil mass cannot typically be incorporated into CFD simulations since the coil mass cannot be reconstructed from clinical images due to its small size and beam hardening artifacts. The existing methods use imprecise porous medium representations. We propose a new porous model taking into account the porosity heterogeneity of the coils deployed in the aneurysm.

METHODS

The porosity heterogeneity of the coil mass deployed inside two patients' cerebral aneurysm phantoms is first quantified based on 3D X-ray synchrotron images. These images are also used to compute the permeability and the inertial factor arising in porous models. A new homogeneous porous model (porous crowns model), considering the coil's heterogeneity, is proposed to recreate the flow within the coiled aneurysm. Finally, the validity of the model is assessed through comparisons with coil-resolved simulations.

RESULTS

The strong porosity gradient of the coil measured close to the aneurysmal wall is well captured by the porous crowns model. The permeability and the inertial factor values involved in this model are closed to the ideal homogeneous porous model leading to a mean velocity in the aneurysmal sac similar as in the coil-resolved model.

CONCLUSION

The porous crowns model allows for an accurate description of the mean flow within the coiled cerebral aneurysm.

Challenges in Modeling Hemodynamics in Cerebral Aneurysms Related to Arteriovenous Malformations

PURPOSE

The significantly higher incidence of aneurysms in patients with arteriovenous malformations (AVMs) suggests a strong hemodynamic relationship between these lesions. The presence of an AVM alters hemodynamics in proximal vessels by drastically changing the distal resistance, thus affecting intra-aneurysmal flow. This study discusses the challenges associated with patient-specific modeling of aneurysms in the presence of AVMs.

METHODS

We explore how the presence of a generic distal AVM affects upstream aneurysms by examining the relationship between distal resistance and aneurysmal wall shear stress using physiologically realistic estimates for the influence of the AVM on hemodynamics. Using image-based computational models of aneurysms and surrounding vasculature, aneurysmal wall-shear stress is calculated for a range of distal resistances corresponding to the presence of AVMs of various sizes and compared with a control case representing the absence of an AVM.

RESULTS

In the patient cases considered, the alteration in aneurysmal wall shear stress due to the presence of an AVM is considerable, as much as 19 times the base case wall shear stress. Furthermore, the relationship between aneurysmal wall shear stress and distal resistance is shown to be highly geometry-dependent and nonlinear. In most cases, the range of physiologically realistic possibilities for AVM-related distal resistance are so large that patient-specific flow measurements are necessary for meaningful predictions of wall shear stress.

CONCLUSIONS

The presented work offers insight on the impact of distal AVMs on aneurysmal wall shear stress using physiologically realistic computational models. Patient-specific modeling of hemodynamics in aneurysms and associated AVMs has great potential for understanding lesion pathogenesis, surgical planning, and assessing the effect of treatment of one lesion relative to another. However, we show that modeling approaches cannot usually meaningfully quantify the impact of AVMs if based solely on imaging data from CT and X-ray angiography, currently used in clinical practice. Based on recent studies, it appears that 4D flow MRI is one promising approach to obtaining meaningful patient-specific flow boundary conditions that improve modeling fidelity.

A proposed framework for cerebral venous congestion

BACKGROUND

While venous congestion in the peripheral vasculature has been described and accepted, intracranial venous congestion remains poorly understood. The characteristics, pathophysiology, and management of cerebral venous stasis, venous hypertension and venous congestion remain controversial, and a unifying conceptual schema is absent. The cerebral venous and lymphatic systems are part of a complex and dynamic interaction between the intracranial compartments, with interplay between the parenchyma, veins, arteries, cerebrospinal fluid, and recently characterized lymphatic-like systems in the brain. Each component contributes towards intracranial pressure, occupying space within the fixed calvarial volume. This article proposes a framework to consider conditions resulting in brain and neck venous congestion, and seeks to expedite further study of cerebral venous diagnoses, mechanisms, symptomatology, and treatments.

METHODS

A multi-institution retrospective review was performed to identify unique patient cases, complemented with a published case series to assess a spectrum of disease states with components of venous congestion affecting the brain. These diseases were organized according to anatomical location and purported mechanisms. Outcomes of treatments were also analyzed. Illustrative cases were identified in the venous treatment databases of the authors.

CONCLUSION

This framework is the first clinically structured description of venous pathologies resulting in intracranial venous and cerebrospinal fluid hypertension. Our proposed system highlights unique clinical symptoms and features critical for appropriate diagnostic work-up and potential treatment. This novel schema allows clinicians effectively to approach cases of intracranial hypertension secondary to venous etiologies, and furthermore provides a framework by which researchers can better understand this developing area of cerebrovascular disease.

Hemodynamic analysis for endovascular treatment in small unruptured intracranial aneurysms: a matched comparison study of flow diverter versus LVIS

BACKGROUND

We compared the treatment of small unruptured intracranial aneurysms (UIAs) with flow diverter and LVIS-assisted coiling to determine the effects of hemodynamic changes caused by different stent and coil packing in endovascular treatment.

METHODS

Fifty-one UIAs in 51 patients treated with pipeline embolization device (PED) were included in this study and defined as the PED group. We matched controls 1:1 and enrolled 51 UIAs who were treated with LVIS stent, which were defined as the LVIS group. Computational fluid dynamics were performed to assess hemodynamic alterations between PED and LVIS. Clinical analysis was also performed between these two groups after the match.

RESULTS

There was no difference in procedural complications between the two groups (P = 0.558). At the first angiographic follow-up, the complete occlusion rate was significantly higher in the LVIS group compared with that in the PED group (98.0% vs. 82.4%, P = 0.027). However, during the further angiographic follow-up, the complete occlusion rate in the PED group achieved 100%, which was higher than that in the LVIS group (98.0%). Compared with the LVIS group after treatment, cases in the PED group showed a higher value of velocity in the aneurysm (0.03 ± 0.09 vs. 0.01 ± 0.01, P = 0.037) and WSS on the aneurysm (2.32 ± 5.40 vs. 0.33 ± 0.47, P = 0.011). Consequently, the reduction ratios of these two parameters also showed statistical differences. These parameters in the LVIS group showed much higher reduction ratios. However, the reduction ratio of the velocity on the neck plane was comparable between two groups.

CONCLUSIONS

Both LVIS and PED were safe and effective for the treatment of small UIAs. However, LVIS-assisted coiling produced greater hemodynamic alterations in the aneurysm sac compared with PED. The hemodynamics in the aneurysm neck may be a key factor for aneurysm outcome.

Midterm Outcomes After Low-Profile Visualization Endoluminal Support or Atlas Stent-Assisted Coiling of Intracranial Aneurysms: A Propensity Score Matching Analysis

BACKGROUND

The interplay of various commercially available stents during coil embolization of intracranial aneurysms and their ultimate impact are subject to debate.

OBJECTIVE

To compare midterm outcomes of Low-Profile Visualization Endoluminal Support (LVIS) (MicroVention Inc) and Atlas (Stryker) stent-assisted coiling procedures.

METHODS

A total of 459 intracranial aneurysms subjected to coil embolization using LVIS (n = 318) or Atlas stents (n = 141) between April 2015 and December 2019 were eligible for study. To assess occlusive status postembolization, magnetic resonance angiography and/or conventional angiography were used. The Raymond classification was applied to categorize recanalization. Our analysis was propensity score matched according to probability of stent type deployed.

RESULTS

Eventually, 41 aneurysms (8.9%) displayed recanalization (minor, 28; major, 13) 6 mo after coiling. Patient age (P = .018), sex (P = .015), aneurysmal location (P < .001), and type of aneurysm (P < .001) differed significantly by group. Overall and major recanalization rates at midterm were similar in both groups (9.1% and 8.5% vs 3.1% and 2.1%, respectively), and there was no significant difference even after 1:1 propensity score matching (odds ratio [OR] = 0.75 [P = .514] and OR = 0.75 [P = .706], respectively).

CONCLUSION

In stent-assisted coil embolization of intracranial aneurysms, midterm outcomes of LVIS and Atlas device groups were similar, despite theoretic LVIS superiority. Further randomized comparative studies are needed to confirm our findings.

Retrospective analysis of intracranial aneurysms after flow diverter treatment including color-coded imaging (syngo iFlow) as a predictor of aneurysm occlusion

PURPOSE

Flow Diverters (FD) have immensely extended the treatment of cerebral aneurysms in the past years. Complete aneurysm occlusion is a process that often takes a certain amount of time and is usually difficult to predict. Our aim was to investigate different syngo iFlow parameters in order to predict aneurysm occlusion.

METHODS

Between 2014 and 2018 patients with unruptured cerebral aneurysms treated with a FD were reviewed. Aneurysm occlusion and complication rates have been assessed.In addition, various quantitative criteria were assessed using syngo iFlow before, after the intervention, and after short and long-term digital subtraction angiography (DSA).

RESULTS

A total of 66 patients hosting 66 cerebral aneurysms were included in this study. 87.9% (n = 58) aneurysms in the anterior and 12.1% (n = 8) in the posterior circulation were treated. Adequate aneurysm occlusion at long-term follow-up (19.05 ± 15.1 months) was achieved in 90.9% (n = 60). Adequately occluded aneurysm revealed a significantly greater peak intensity delay (PI-D, p = 0.008) and intensity decrease ratio (ID-R, p < 0.001) compared to insufficiently occluded aneurysms. Increased intra-aneurysmal contrast agent intensity (>100%) after FD implantation resulted in an ID-R < 1, which was associated with aneurysm growth during follow-up DSA. Retreatment with another FD due to foreshortening and/or aneurysm growth was performed in 10.6% (n = 7). Overall morbidity and mortality rates were 1.5% (n = 1) and 0%.

CONCLUSION

The applied syngo iFlow parameters were found to be useful in predicting adequate aneurysm occlusion and foresee aneurysm growth, which might indicate the implantation of another FD.

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.

Lagrangian Trajectory Simulation of Platelets and Synchrotron Microtomography Augment Hemodynamic Analysis of Intracranial Aneurysms Treated With Embolic Coils

As frequency of endovascular treatments for intracranial aneurysms increases, there is a growing need to understand the mechanisms for coil embolization failure. Computational fluid dynamics (CFD) modeling often simplifies modeling the endovascular coils as a homogeneous porous medium (PM), and focuses on the vascular wall endothelium, not considering the biomechanical environment of platelets. These assumptions limit the accuracy of computations for treatment predictions. We present a rigorous analysis using X-ray microtomographic imaging of the coils and a combination of Lagrangian (platelet) and Eulerian (endothelium) metrics. Four patient-specific, anatomically accurate in vitro flow phantoms of aneurysms are treated with the same patient-specific endovascular coils. Synchrotron tomography scans of the coil mass morphology are obtained. Aneurysmal hemodynamics are computationally simulated before and after coiling, using patient-specific velocity/pressure measurements. For each patient, we analyze the trajectories of thousands of platelets during several cardiac cycles, and calculate residence times (RTs) and shear exposure, relevant to thrombus formation. We quantify the inconsistencies of the PM approach, comparing them with coil-resolved (CR) simulations, showing the under- or overestimation of key hemodynamic metrics used to predict treatment outcomes. We fully characterize aneurysmal hemodynamics with converged statistics of platelet RT and shear stress history (SH), to augment the traditional wall shear stress (WSS) on the vascular endothelium. Incorporating microtomographic scans of coil morphology into hemodynamic analysis of coiled intracranial aneurysms, and augmenting traditional analysis with Lagrangian platelet metrics improves CFD predictions, and raises the potential for understanding and clinical translation of computational hemodynamics for intracranial aneurysm treatment outcomes.

A pilot study on biaxial mechanical, collagen microstructural, and morphological characterizations of a resected human intracranial aneurysm tissue

Intracranial aneurysms (ICAs) are focal dilatations that imply a weakening of the brain artery. Incidental rupture of an ICA is increasingly responsible for significant mortality and morbidity in the American’s aging population. Previous studies have quantified the pressure-volume characteristics, uniaxial mechanical properties, and morphological features of human aneurysms. In this pilot study, for the first time, we comprehensively quantified the mechanical, collagen fiber microstructural, and morphological properties of one resected human posterior inferior cerebellar artery aneurysm. The tissue from the dome of a right posterior inferior cerebral aneurysm was first mechanically characterized using biaxial tension and stress relaxation tests. Then, the load-dependent collagen fiber architecture of the aneurysm tissue was quantified using an in-house polarized spatial frequency domain imaging system. Finally, optical coherence tomography and histological procedures were used to quantify the tissue’s microstructural morphology. Mechanically, the tissue was shown to exhibit hysteresis, a nonlinear stress-strain response, and material anisotropy. Moreover, the unloaded collagen fiber architecture of the tissue was predominantly aligned with the testing Y-direction and rotated towards the X-direction under increasing equibiaxial loading. Furthermore, our histological analysis showed a considerable damage to the morphological integrity of the tissue, including lack of elastin, intimal thickening, and calcium deposition. This new unified characterization framework can be extended to better understand the mechanics-microstructure interrelationship of aneurysm tissues at different time points of the formation or growth. Such specimen-specific information is anticipated to provide valuable insight that may improve our current understanding of aneurysm growth and rupture potential.

Platelet Dynamics and Hemodynamics of Cerebral Aneurysms Treated with Flow-Diverting Stents

Flow-diverting stents (FDS) are used to treat cerebral aneurysms. They promote the formation of a stable thrombus within the aneurysmal sac and, if successful, isolate the aneurysmal dome from mechanical stresses to prevent rupture. Platelet activation, a mechanism necessary for thrombus formation, is known to respond to biomechanical stimuli, particularly to the platelets' residence time and shear stress exposure. Currently, there is no reliable method for predicting FDS treatment outcomes, either a priori or after the procedure. Eulerian computational fluid dynamic (CFD) studies of aneurysmal flow have searched for predictors of endovascular treatment outcome; however, the hemodynamics of thrombus formation cannot be fully understood without considering the platelets' trajectories and their mechanics-triggered activation. Lagrangian analysis of the fluid mechanics in the aneurysmal vasculature provides novel metrics by tracking the platelets' residence time (RT) and shear history (SH). Eulerian and Lagrangian parameters are compared for 19 patient-specific cases, both pre- and post-treatment, to assess the degree of change caused by the FDS and subsequent treatment efficacy.

Assessment of blood flow velocities and venous pressures using a dual-sensor guidewire in symptomatic dural sinus stenoses

Lateral venous sinus stenoses have been associated with idiopathic intracranial hypertension and venous pulsatile tinnitus. Venous pressure measurement is traditionally performed to assess the indications for stenting in patients with idiopathic intracranial hypertension. However, its reliability has recently been questioned by many authors. The dual-sensor guidewire was first developed for advanced physiological assessment of fractional and coronary flow reserves in coronary artery stenoses. It allows measurement of both venous pressure and blood flow velocities. The authors used this device in 14 consecutively treated patients to explore for symptomatic lateral sinus stenosis. They found that venous blood flow was significantly accelerated inside the stenotic lesion. This acceleration, as well as the pulsatile tinnitus, resolved in all patients following stent placement. According to the authors’ results, this guidewire can be helpful for establishing an indication for stenting in patients with pulsatile tinnitus and idiopathic intracranial hypertension.

Efficacy of LVIS vs. Enterprise Stent for Endovascular Treatment of Medium-Sized Intracranial Aneurysms: A Hemodynamic Comparison Study

Background: We conducted a computational fluid dynamics (CFD) study and compared the treatment of medium-sized intracranial aneurysms with LVIS and Enterprise stent-assisted coil embolization (SACE) to determine the effects of hemodynamic changes caused by different stent and coil packing densities (PDs) in endovascular treatment.

Methods: We enrolled 87 consecutive patients, with 87 medium-sized intracranial aneurysms (≥7, ≤ 12 mm), who underwent LVIS or Enterprise SACE. Aneurysms treated with LVIS SACE were allocated to the LVIS group, and the remainder were allocated to the Enterprise group. CFD were performed to assess hemodynamic alterations between before treatment, after stent deployment, and after SACE.

Results: One aneurysm recanalized in the LVIS group (n = 42), and five recanalized in the Enterprise group (n = 45) (recanalization rate: 2.4 vs. 11.1%, respectively; P = 0.108). Higher complete obliteration rate (P = 0.069) was found in the LVIS group. Velocity at the neck plane showed a greater reduction ratio than velocity and WSS of the aneurysm in both groups after stent deployment, while velocity and WSS of the aneurysm showed a greater reduction ratio after coil placement. Further, there was a greater reduction in velocity at the neck plane (59.52 vs. 39.81%), aneurysmal velocity (88.46 vs. 69.45%), and wall shear stress (WSS) (85.45 vs. 69.49%) on the aneurysm in the LVIS group (P < 0.001 for all). Specifically, the reduction ratio of velocity at the neck plane showed significant difference between the groups in the multivariate analysis (P = 0.013).

Conclusions : LVIS SACE showed a lower recanalization for endovascular treatment of medium-sized intracranial aneurysms, and the greater hemodynamic alterations might be the key factors.

Predictors of Impaired Cerebral Perfusion After Flow Diversion Therapy

BACKGROUND

Flow-diverting stents (FDS) are relatively safe and highly efficacious in treating cerebral aneurysms; however, a small subset of patients experience devastating hemorrhagic complications owing to presumed alterations in local aneurysm and distal cerebral blood flow. The downstream effects of FDS on distal cerebral blood flow is not well understood, but isolated reports of hyperperfusion injury have been described in the literature. We investigate the incidence and clinical factors contributing to abnormal cerebral blood flow after FDS placement.

METHODS

A retrospective analysis of patients undergoing FDS for elective aneurysm treatment between 2014-2017 was performed. Patients who underwent perfusion imaging within 24-hours posttreatment were included for further analysis. Univariate and multivariate analyses were performed to assess the impact of multiple variables on the postoperative perfusion changes.

RESULTS

A total of 69 patients underwent FDS therapy to treat unruptured intracranial aneurysms. Thirteen patients (18.8%) developed abnormal perfusion changes. A significant difference of the median was found in aneurysm projection width, depth, neck width, calculated approximate volume, and size ratio between the hypoperfused, normal, and hyperperfusion cases. On multivariate analysis, history of smoking (P = 0.0117), and approximate calculated volume (P = 0.0145) were significant predictors of hyperperfusion identified on posttreatment imaging.

CONCLUSIONS

This study yielded several novel findings. We demonstrate that cerebral blood flow alterations will occur in a significant subset of patients undergoing FDS treatment. We also provide new evidence that aneurysm volume and history of smoking may predict the developing of postoperative perfusion anomalies. Future studies are needed to evaluate the clinical ramifications of cerebral blood flow disruption in large prospective studies.

CFD-Based Comparison Study of a New Flow Diverting Stent and Commercially-Available Ones for the Treatment of Cerebral Aneurysms

Flow-diverting stents (FDSs) show considerable promise for the treatment of cerebral aneurysms by diverting blood flow away from the aneurysmal sacs, however, post-treatment complications such as failure of occlusion and subarachnoid haemorrhaging remain and vary with the FDS used. Based on computational fluid dynamics (CFD), this study aimed to investigate the performance of a new biodegradable stent as compared to two metallic commercially available FDSs. CFD models were developed for an idealized cerebral artery with a sidewall aneurysmal sac treated by deploying the aforementioned stents of different porosities (90, 80, and 70 % ) respectively. By using these models, the simulation and analysis were performed, with a focus on comparing the local hemodynamics or the blood flow in the stented arteries as compared to the one without the stent deployment. For the comparison, we computed and compared the flow velocity, wall shear stress (WSS) and pressure distributions, as well as the WSS related indices, all of which are of important parameters for studying the occlusion and potential rupture of the aneurysm. Our results illustrate that the WSS decreases within the aneurysmal sac on the treated arteries, which is more significant for the stents with lower porosity or finer mesh. Our results also show that the maximum WSS near the aneurysmal neck increases regardless of the stents used. In addition, the WSS related indices including the time-average WSS, oscillatory shear index and relative residence time show different distributions, depending on the FDSs. Together, we found that the finer mesh stents provide more flow reduction and smaller region characterized by high oscillatory shear index, while the new stent has a higher relative residence time.

Hemodynamic differences between Pipeline and coil-adjunctive intracranial stents

Introduction

Modern coil-adjunctive intracranial stent designs have increased metal surface coverage to construct putative ‘flow diverter lights.’ This is rooted in the assumption that flow diversion is linearly correlated with metal surface coverage rather than being a threshold to be reached by device porosity and design.

Objective

To evaluate this assumption, by performing computational flow analysis on three aneurysm models treated with low metal surface coverage stents (ATLAS and Enterprise), a Pipeline flow diverter, and the LVIS Blue stent.

Methods

Computational flow analysis was performed on virtual deployment models entailing deployment of an ATLAS, Enterprise, LVIS Blue, or Pipeline. The impact of device deployment on velocity vectors at the neck, maximum wall shear stress, inflow rate into the aneurysm, and turnover time was determined.

Results

Velocity vector plots demonstrated low magnitude, localized inflow jets for Pipeline only; asymmetric, selectively high inflow jets were seen for LVIS Blue, and broader velocity vector clusters were seen for Atlas and Enterprise. Reduction in wall shear stress as compared with baseline was significant for all devices and greatest for the Pipeline. Mean peak wall shear stress was significantly lower for LVIS Blue in comparison with ATLAS or Enterprise but significantly lower for Pipeline than for LVIS Blue. Reduction of inflow rate into the aneurysm was significant for LVIS Blue and Pipeline but significantly lower for Pipeline than for LVIS Blue. Turnover time was statistically similar for ATLAS, Enterprise, and LVIS Blue, but significantly increased for Pipeline.

Conclusion

Considerable differences in peak wall shear stress, inflow rates, and turnover time between flow diverters, moderate- and low-porosity stents reinforce the assumption that effective flow diversion represents a threshold in device design, encompassing metal surface coverage only in part.

Quantification of near-wall hemodynamic risk factors in large-scale cerebral arterial trees

Detailed hemodynamic analysis of blood flow in pathological segments close to aneurysm and stenosis has provided physicians with invaluable information about the local flow patterns leading to vascular disease. However, these diseases have both local and global effects on the circulation of the blood within the cerebral tree. The aim of this paper is to demonstrate the importance of extending subject-specific hemodynamic simulations to the entire cerebral arterial tree with hundreds of bifurcations and vessels, as well as evaluate hemodynamic risk factors and waveform shape characteristics throughout the cerebral arterial trees. Angioarchitecture and in vivo blood flow measurement were acquired from healthy subjects and in cases with symptomatic intracranial aneurysm and stenosis. A global map of cerebral arterial blood flow distribution revealed regions of low to high hemodynamic risk that may significantly contribute to the development of intracranial aneurysms or atherosclerosis. Comparison of pre-intervention and post-intervention of pathological cases further shows large angular phase shift (~33.8°), and an augmentation of the peak-diastolic velocity. Hemodynamic indexes of waveform analysis revealed on average a 16.35% reduction in the pulsatility index after treatment from lesion site to downstream distal vessels. The lesion regions not only affect blood flow streamlines of the proximal sites but also generate pulse wave shift and disturbed flow in downstream vessels. This network effect necessitates the use of large-scale simulation to visualize both local and global effects of pathological lesions.

Assessment of blood flow velocities and venous pressures using a dual-sensor guidewire in symptomatic dural sinus stenoses

Lateral venous sinus stenoses have been associated with idiopathic intracranial hypertension and venous pulsatile tinnitus. Venous pressure measurement is traditionally performed to assess the indications for stenting in patients with idiopathic intracranial hypertension. However, its reliability has recently been questioned by many authors. The dual-sensor guidewire was first developed for advanced physiological assessment of fractional and coronary flow reserves in coronary artery stenoses. It allows measurement of both venous pressure and blood flow velocities. The authors used this device in 14 consecutively treated patients to explore for symptomatic lateral sinus stenosis. They found that venous blood flow was significantly accelerated inside the stenotic lesion. This acceleration, as well as the pulsatile tinnitus, resolved in all patients following stent placement. According to the authors' results, this guidewire can be helpful for establishing an indication for stenting in patients with pulsatile tinnitus and idiopathic intracranial hypertension.

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.

A Patient-Specific Three-Dimensional Hemodynamic Model of the Circle of Willis

Circle of Willis (CoW) is one of the most important cerebral arteries in the human body and various attempts have been made to study the hemodynamic of blood flow in this vital part of the brain. In the present study, blood flow in a patient specific CoW is numerically modeled to predict disease-prone regions of the CoW. Medical images and computer aided design software are used to construct a realistic three-dimensional model of the CoW for this particular case. The arteries are considered as elastic conduits and the interactions between arterial walls and the blood flow are taken into account. Mooney-Rivlin hyperelastic model is used to describe the behavior of arterial walls and blood is considered as a non-Newtonian fluid obeying the Carreau model. An available experimental-based pulsatile velocity profile is used at the entrance of the CoW. The finite element-based commercial software, ADINA, is used to solve the governing equations. Blood pressure and velocity and arterial wall shear stress are calculated in different regions of the CoW. A simplified form of the model is also compared with the available published data. Results affirmed that the proposed computational model has the potential to capture the hemodynamic characteristics of the CoW. The computational results can be used to determine disease-prone locations for a given CoW.

MR Coagulation: A Novel Minimally Invasive Approach to Aneurysm Repair

PURPOSE

To demonstrate a proof of concept of magnetic resonance (MR) coagulation, in which MR imaging scanner-induced radiofrequency (RF) heating at the end of an intracatheter long wire heats and coagulates a protein solution to effect a vascular repair by embolization.

MATERIALS AND METHODS

MR coagulation was simulated by finite-element modeling of electromagnetic fields and specific absorption rate (SAR) in a phantom. A glass phantom consisting of a spherical cavity joined to the side of a tube was incorporated into a flow system to simulate an aneurysm and flowing blood with velocities of 0-1.7 mL/s. A double-lumen catheter containing the wire and fiberoptic temperature sensor in 1 lumen was passed through the flow system into the aneurysm, and 9 cm³ of protein solution was injected into the aneurysm through the second lumen. The distal end of the wire was laid on the patient table as an antenna to couple RF from the body coil or was connected to a separate tuned RF pickup coil. A high RF duty-cycle turbo spin-echo pulse sequence excited the wire such that RF energy deposited at the tip of the wire coagulated the protein solution, embolizing the aneurysm.

RESULTS

The protein coagulation temperature of 60°C was reached in the aneurysm in ∼12 seconds, yielding a coagulated mass that largely filled the aneurysm. The heating rate was controlled by adjusting pulse-sequence parameters.

CONCLUSIONS

MR coagulation has the potential to embolize vascular defects by coagulating a protein solution delivered by catheter using MR imaging scanner-induced RF heating of an intracatheter wire.

Treatment of ruptured blood blister aneurysms using primary flow-diverter stenting with considerations for adjunctive coiling: A single-centre experience and literature review

Objective The objective of this article is to conduct a single-centre evaluation and quick literature review of the effectiveness of primary flow-diverter (FD) treatment of ruptured blood blister aneurysms (BBAs), with additional relevance of adjunctive coiling. Methods Patients presenting with subarachnoid haemorrhage (SAH) due to ruptured BBAs and subsequently treated with FDs were retrospectively selected from June 2010 to January 2017. Treatment techniques, angiographic data on occlusion rates and procedural success as well as clinical outcomes using the modified Rankin Scale (mRS) were collated. Cross-reference of results were made with available literature. Results Thirteen patients harbouring 14 BBAs were recruited. Of the 14 aneurysms, five (35.7%) showed immediate complete occlusion after the procedure (four of these five patients had adjunctive coiling). All of the aneurysms showed complete occlusion by the six- to nine-month control diagnostic angiogram. No rebleed or retreatment was experienced. Twelve of 13 (92%) patients had an mRS score of 0-1 at the last clinical follow-up. From the pooled data of the literature review, eventual aneurysm occlusion was achieved in 48/56 patients, with five patients requiring further endovascular treatment. In the clinical follow-up period, an mRS of 0-2 was recorded for 83.3% (45/54) of patients. Conclusion Endovascular reconstruction of BBAs using FD treatment is an effective method with good final clinical outcomes. Adjunctive use of coiling achieves higher incidence of immediate complete occlusion of BBAs.

Cerebral Revascularization for Aneurysms in the Flow-Diverter Era

BACKGROUND: Cerebral bypass has been an important tool in the treatment of complex intracranial aneurysms. The recent advent of flow-diverting stents (FDS) has expanded the capacity for endovascular arterial reconstruction.

OBJECTIVE: We investigated how the advent of FDS has impacted the application and outcomes of cerebral bypass in the treatment of intracranial aneurysms.

METHODS: We reviewed a consecutive series of cerebral bypasses during aneurysm surgery over the course of 10 years. FDS were in active use during the last 5 years of this series. We compared the clinical characteristics, surgical technique, and outcomes of patients who required cerebral bypass for aneurysm treatment during the preflow diversion era (PreFD) with those of the postflow diversion era (PostFD).

RESULTS: We treated 1061 aneurysms in the PreFD era (from July 2005 through June 2010) and 1348 in the PostFD era (from July 2010 through June 2015). Eighty-five PreFD patients (8%) and 45 PostFD patients (3%) were treated with cerebral bypass. PreFD patients had better baseline functional status compared to PostFD patients with average preoperative modified Rankin Scale score of 0.55 in PreFD and 1.18 in PostFD.

CONCLUSION: After the introduction of FDS, cerebral bypass was performed in a lower proportion of patients with aneurysms. Patients selected for bypass in the flow-diverter era had worse preoperative modified Rankin Scale scores indicating a greater complexity of the patients. Cerebral bypass in well-selected patients and revascularization remains an important technique in vascular neurosurgery. It is also useful as a rescue technique after failed FDS treatment of aneurysms.

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

BACKGROUND AND PURPOSE

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.

MATERIALS AND METHODS

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.

RESULTS

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%.

CONCLUSIONS

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.

Computational fluid dynamics of cerebral aneurysm coiling using high-resolution and high-energy synchrotron X-ray microtomography: comparison with the homogeneous porous medium approach

BACKGROUND

Computational modeling of intracranial aneurysms provides insights into the influence of hemodynamics on aneurysm growth, rupture, and treatment outcome. Standard modeling of coiled aneurysms simplifies the complex geometry of the coil mass into a homogeneous porous medium that fills the aneurysmal sac. We compare hemodynamics of coiled aneurysms modeled from high-resolution imaging with those from the same aneurysms modeled following the standard technique, in an effort to characterize sources of error from the simplified model.

MATERIALS

Physical models of two unruptured aneurysms were created using three-dimensional printing. The models were treated with coil embolization using the same coils as those used in actual patient treatment and then scanned by synchrotron X-ray microtomography to obtain high-resolution imaging of the coil mass. Computational modeling of each aneurysm was performed using patient-specific boundary conditions. The coils were modeled using the simplified porous medium or by incorporating the X-ray imaged coil surface, and the differences in hemodynamic variables were assessed.

RESULTS

X-ray microtomographic imaging of coils and incorporation into computational models were successful for both aneurysms. Porous medium calculations of coiled aneurysm hemodynamics overestimated intra-aneurysmal flow, underestimated oscillatory shear index and viscous dissipation, and over- or underpredicted wall shear stress (WSS) and WSS gradient compared with X-ray-based coiled computational fluid dynamics models.

CONCLUSIONS

Computational modeling of coiled intracranial aneurysms using the porous medium approach may inaccurately estimate key hemodynamic variables compared with models incorporating high-resolution synchrotron X-ray microtomographic imaging of complex aneurysm coil geometry.

Flow diverter effect of LVIS stent on cerebral aneurysm hemodynamics: a comparison with Enterprise stents and the Pipeline device

Background

The aim of this study was to quantify the effect of the new Low-profile Visualized Intraluminal Support (LVIS®D) device and the difference of fluid diverting effect compared with the Pipeline device and the Enterprise stent using computational fluid dynamics (CFD).

Methods

In this research, we simulated three aneurysms constructed from 3D digital subtraction angiography (DSA). The Enterprise, LVIS and the Pipeline device were virtually conformed to fit into the vessel lumen and placed across the aneurysm orifice. Computational fluid dynamics analysis was performed to compare the hemodynamic differences such as WSS, Velocity and Pressure among these stents.

Results

Control referred to the unstented model, the percentage of hemodynamic changes were all compared to Control. A single LVIS stent caused more wall shear stress reduction than double Enterprise stents (39.96 vs. 30.51 %) and velocity (23.13 vs. 18.64 %). Significant reduction in wall shear stress (63.88 %) and velocity (46.05 %) was observed in the double-LVIS stents. A single Pipeline showed less reduction in WSS (51.08 %) and velocity (37.87 %) compared with double-LVIS stent. The double-Pipeline stents resulted in the most reduction in WSS (72.37 %) and velocity (54.26 %). Moreover, the pressure increased with minuscule extent after stenting, compared with the unstented model.

Conclusions

This is the first study analyzing flow modifications associated with LVIS stents. We found that the LVIS stent has certain hemodynamic effects on cerebral aneurysms: a single LVIS stent caused more flow reductions than the double-Enterprise stent but less than a Pipeline device. Nevertheless, the double-LVIS stent resulted in a better flow diverting effect than a Pipeline device.

MR derived volumetric flow rate waveforms of internal carotid artery in patients treated for unruptured intracranial aneurysms by flow diversion technique

Little is known about the hemodynamic disturbances induced by the cerebral aneurysms in the parent artery and the effect of flow diverter stents (FDS) on these latter. A better understanding of the aneurysm-parent vessel complex relationship may aid our understanding of this disease and to optimize its treatment. The ability of volumetric flow rate (VFR) waveform to reflect the arterial compliance modifications is well known. By analyzing the VFR waveform and the pulsatility in the parent vessel, this study aimed to test the hypotheses that (1) intracranial aneurysms might disrupt the blood flow of the parent vessel and (2) the treatment by FDS might have measurable corrective effect on these changes. Ten patients followed for unruptured intracranial aneurysms treated by FDS and ten healthy volunteers as control group were included in this study. Two-dimensional quantitative phase-contrast magnetic resonance imaging (MRI) was performed on each patient on the ICA artery upstream and downstream to the aneurysm, and on each volunteer at similar locations. The aneurysms altered significantly the parent vessel pulsatility and this effect was correlated to their volume. The aneurysms treatment by FDS allowed for the restoration of a normally modulated flow and pulsatility correction in the parent vessel.

In vitro validation of endovascular Doppler-derived flow rates in models of the cerebral circulation

This study presents validation of endovascular Doppler velocimetry-based volumetric flow rate measurements conducted in a pulsatile flow loop simulating conditions in both the internal carotid and basilar artery. In vitro models of cerebral vessels, each containing an aneurysm, were fabricated from patient anatomies extracted from 3D rotational angiography. Flow velocity measurements were collected with three different experimental techniques: an endovascular Doppler wire, Particle Image Velocimetry, and a time-resolved ultrasonic flow meter. Womersley's theory of pulsatile flow in a cylindrical vessel was used to compute time-resolved volumetric flow rates from the endovascular Doppler velocity. The volumetric flow rates computed from the Doppler measurements were compared to those from the Particle Image Velocimetry profile measurements, and the direct measurements from the ultrasonic flow meter. The study establishes confidence intervals for any systematic or random errors associated with the wire-derived flow rates as benchmarked to the other two modalities. There is an approximately 10% random error in the Doppler-derived peak and time-averaged flow rates. There is a measurable uniform bias, about 15% too low, in the time-averaged Doppler-derived flow rates. There is also a small proportional bias in the peak systolic Doppler-derived flow rates. Potential sources of error are also discussed.

Endovascular treatment of ophthalmic artery aneurysms: ophthalmic artery patency following flow diversion versus coil embolization

BACKGROUND AND PURPOSE

The Pipeline Embolization Device (PED) has been shown to effectively treat complex internal carotid artery aneurysms while maintaining patency of covered side branches. The purpose of this retrospective matched cohort study is to evaluate the effect of flow diversion on the patency of the ophthalmic artery when treating ophthalmic artery aneurysms.

METHODS

A retrospective review of our prospectively collected institutional database identified 19 ophthalmic artery aneurysms treated with a PED. These were matched according to aneurysm diameter in a 1:2 fashion to ophthalmic artery aneurysms treated via coil embolization, although it is important to note that there was a statistically significance difference in the neck diameter between the two groups (p=0.045). Clinical and angiographic outcomes were recorded and analyzed.

RESULTS

On follow-up angiography, decreased flow through the ophthalmic artery was observed in 26% of the PED cohort and 0% of the coil embolization cohort (p=0.003). No ophthalmologic complications were noted in either cohort. Complete occlusion at 12 months was more common following PED treatment than coil embolization (74% vs 47%; p=0.089), although lower than reported in previous trials. This may be due to inflow into the ophthalmic artery keeping the aneurysm patent. Retreatments were more common following coil embolization than PED (24% vs 11%), but this was not significant (p=0.304). Permanent morbidity rates were not significantly different between the PED (11%) and coil embolization (3%) cohorts (p=0.255).

CONCLUSIONS

Our results suggest that ophthalmic artery aneurysms may be adequately and safely treated with either the PED or coil embolization. However, treatment with the PED carries a higher risk of impeding flow to the ophthalmic artery, although this did not result in clinical sequelae in the current study.

Determination of a shear rate threshold for thrombus formation in intracranial aneurysms

Background

Particular intra-aneurysmal blood flow conditions, created naturally by the growth of an aneurysm or induced artificially by implantation of a flow diverter stent (FDS), can potentiate intra-aneurysmal thrombosis. The aim of this study was to identify hemodynamic indicators, relevant to this process, which could be used as a prediction of the success of a preventive endovascular treatment.

Method

A cross sectional study on 21 patients was carried out to investigate the possible association between intra-aneurysmal spontaneous thrombus volume and the dome to neck aspect ratio (AR) of the aneurysm. The mechanistic link between these two parameters was further investigated through a Fourier analysis of the intra-aneurysmal shear rate (SR) obtained by computational fluid dynamics (CFD). This analysis was first applied to 10 additional patients (4 with and 6 without spontaneous thrombosis) and later to 3 patients whose intracranial aneurysms only thrombosed after FDS implantation.

Results

The cross sectional study revealed an association between intra-aneurysmal spontaneous thrombus volume and the AR of the aneurysm (R2=0.67, p<0.001). Fourier analysis revealed that in cases where thrombosis occurred, the SR harmonics 0, 1, and 2 were always less than 25/s, 10/s, and 5/s, respectively, and always greater than these values where spontaneous thrombosis was not observed.

Conclusions

Our study suggests the existence of an SR threshold below which thrombosis will occur. Therefore, by analyzing the SR on patient specific data with CFD techniques, it may be potentially possible to predict whether or the intra-aneurysmal flow conditions, after FDS implantation, will become prothrombotic.

Computation of the change in length of a braided device when deployed in realistic vessel models

PURPOSE

An important issue in the deployment of braided stents, such as flow diverters, is the change in length, also known as foreshortening, underwent by the device when is released from the catheter into a blood vessel. The position of the distal end is controlled by the interventionist, but knowing a priori the position of the proximal end of the device is not trivial. In this work, we assess and validate a novel computer method to predict the length that a braided stent will adopt inside a silicon model of an anatomically accurate vessel.

METHODS

Three-dimensional rotational angiography images of aneurysmatic patients were used to generate surface models of the vessels (3D meshes) and then create accurate silicon models from them. A braided stent was deployed into each silicon model to measure its length. The same stents deployed on the silicon models were virtually deployed on the 3D meshes using the method being evaluated.

RESULTS

The method was applied to five stent placements on three different silicon models. The length adopted by the real braided device in the silicon models varies between 15 and 30% from the stent length specified by the manufacturer. The final length predicted by the method was within the estimated error of the measured real stent length.

CONCLUSIONS

The method provides, in a few seconds, the length of a braided stent deployed inside a vessel, showing an accurate estimation of the final length for the cases studied. This technique could provide useful information for planning the intervention and improve endovascular treatment of intracranial aneurysms in the future.

Three-dimensional printing of anatomically accurate, patient specific intracranial aneurysm models

OBJECTIVE

To develop and validate a method for creating realistic, patient specific replicas of cerebral aneurysms by means of fused deposition modeling.

METHODS

The luminal boundaries of 10 cerebral aneurysms, together with adjacent proximal and distal sections of the parent artery, were segmented based on DSA images, and corresponding virtual three-dimensional (3D) surface reconstructions were created. From these, polylactic acid and MakerBot Flexible Filament replicas of each aneurysm were created by means of fused deposition modeling. The accuracy of the replicas was assessed by quantifying statistical significance in the variations of their inner dimensions relative to 3D DSA images. Feasibility for using these replicas as flow phantoms in combination with phase contrast MRI was demonstrated.

RESULTS

3D printed aneurysm models were created for all 10 subjects. Good agreement was seen between the models and the source anatomy. Aneurysm diameter measurements of the printed models and source images correlated well (r=0.999; p<0.001), with no statistically significant group difference (p=0.4) or observed bias. The SDs of the measurements were 0.5 mm and 0.2 mm for source images and 3D models, respectively. 3D printed models could be imaged with flow via MRI.

CONCLUSIONS

The 3D printed aneurysm models presented were accurate and were able to be produced inhouse. These models can be used for previously cited applications, but their anatomical accuracy also enables their use as MRI flow phantoms for comparison with ongoing studies of computational fluid dynamics. Proof of principle imaging experiments confirm MRI flow phantom utility.

Blood flow in intracranial aneurysms treated with Pipeline embolization devices: computational simulation and verification with Doppler ultrasonography on phantom models

Purpose:

The aim of this study was to validate a computational fluid dynamics (CFD) simulation of flow-diverter treatment through Doppler ultrasonography measurements in patient-specific models of intracranial bifurcation and side-wall aneurysms.

Methods:

Computational and physical models of patient-specific bifurcation and sidewall aneurysms were constructed from computed tomography angiography with use of stereolithography, a three-dimensional printing technology. Flow dynamics parameters before and after flow-diverter treatment were measured with pulse-wave and color Doppler ultrasonography, and then compared with CFD simulations.

Results:

CFD simulations showed drastic flow reduction after flow-diverter treatment in both aneurysms. The mean volume flow rate decreased by 90% and 85% for the bifurcation aneurysm and the side-wall aneurysm, respectively. Velocity contour plots from computer simulations before and after flow diversion closely resembled the patterns obtained by color Doppler ultrasonography.

Conclusion:

The CFD estimation of flow reduction in aneurysms treated with a flow-diverting stent was verified by Doppler ultrasonography in patient-specific phantom models of bifurcation and side-wall aneurysms. The combination of CFD and ultrasonography may constitute a feasible and reliable technique in studying the treatment of intracranial aneurysms with flow-diverting stents.

Extra-aneurysmal flow modification following pipeline embolization device implantation: focus on regional branches, perforators, and the parent vessel

BACKGROUND AND PURPOSE

Flow-diverter technology has proved to be a safe and effective treatment for intracranial aneurysm based on the concept of flow diversion allowing parent artery and collateral preservation and aneurysm healing. We investigated the patency of covered side branches and flow modification within the parent artery following placement of the Pipeline Embolization Device in the treatment of intracranial aneurysms.

MATERIALS AND METHODS

Sixty-six aneurysms in 59 patients were treated with 96 Pipeline Embolization Devices. We retrospectively reviewed imaging and clinical results during the postoperative period at 6 and 12 months to assess flow modification through the parent artery and side branches. Reperfusion syndrome was assessed by MR imaging and clinical evaluation.

RESULTS

Slow flow was observed in 13 of 68 (19.1%) side branches covered by the Pipeline Embolization Device. It was reported in all cases of anterior cerebral artery coverage, in 3/5 cases of M2-MCA coverage, and in 5/34 (14.7%) cases of ophthalmic artery coverage. One territorial infarction was observed in a case of M2-MCA coverage, without arterial occlusion. One case of deep Sylvian infarct was reported in a case of coverage of MCA perforators. Two ophthalmic arteries (5.9%) were occluded, and 11 side branches (16.2%) were narrowed at 12 months' follow-up; patients remained asymptomatic. Parent vessel flow modification was responsible for 2 cases (3.4%) of reperfusion syndrome. Overall permanent morbidity and mortality rates were 5.2% and 6.9%, respectively. We did not report any permanent deficit or death in case of slow flow observed within side branches.

CONCLUSIONS

After Pipeline Embolization Device placement, reperfusion syndrome was observed in 3.4%, and territorial infarction, in 3.4%. Delayed occlusion of ophthalmic arteries and delayed narrowing of arteries covered by the Pipeline Embolization Device were observed in 5.9% and 16.2%, respectively. No permanent morbidity or death was related to side branch coverage at midterm follow-up.

Pipeline Embolization Device as primary treatment for blister aneurysms and iatrogenic pseudoaneurysms of the internal carotid artery

BACKGROUND

Blood blister type aneurysms (BBAs) and pseudoaneurysms create a unique treatment challenge. Despite many advances in open surgical and endovascular techniques, this subset of patients retains relatively high rates of morbidity and mortality. Recently, BBAs have been treated with flow-diverting stents such as the Pipeline Embolization Device (PED) with overall positive results.

METHODS

Four patients presented with dissecting internal carotid artery (ICA) aneurysms treated with the PED (two BBAs presenting with subarachnoid hemorrhage (SAH), two pseudoaneurysms after injury during endoscopic trans-sphenoidal tumor surgery).

RESULTS

Three patients had a successful angiographic and neurological outcome. One patient with a BBA re-ruptured during initial PED placement, again in the postoperative period, and later died. Primary PED treatment involved telescoping stents in two patients and coil embolization supplementation in one patient.

CONCLUSIONS

The PED should be used selectively in the setting of acute SAH. Dual antiplatelet therapy can complicate hydrocephalus management, and the lack of immediate aneurysm occlusion creates the risk of short-term re-rupture. PED treatment for iatrogenic ICA pseudoaneurysms can provide a good angiographic and neurological outcome.

Accuracy of computational cerebral aneurysm hemodynamics using patient-specific endovascular measurements

Computational hemodynamic simulations of cerebral aneurysms have traditionally relied on stereotypical boundary conditions (such as blood flow velocity and blood pressure) derived from published values as patient-specific measurements are unavailable or difficult to collect. However, controversy persists over the necessity of incorporating such patient-specific conditions into computational analyses. We perform simulations using both endovascularly-derived patient-specific and typical literature-derived inflow and outflow boundary conditions. Detailed three-dimensional anatomical models of the cerebral vasculature are developed from rotational angiography data, and blood flow velocity and pressure are measured in situ by a dual-sensor pressure and velocity endovascular guidewire at multiple peri-aneurysmal locations in 10 unruptured cerebral aneurysms. These measurements are used to define inflow and outflow boundary conditions for computational hemodynamic models of the aneurysms. The additional in situ measurements which are not prescribed in the simulation are then used to assess the accuracy of the simulated flow velocity and pressure drop. Simulated velocities using patient-specific boundary conditions show good agreement with the guidewire measurements at measurement locations inside the domain, with no bias in the agreement and a random scatter of ≈25%. Simulated velocities using the simplified, literature-derived values show a systematic bias and over-predicted velocity by ≈30% with a random scatter of ≈40%. Computational hemodynamics using endovascularly measured patient-specific boundary conditions have the potential to improve treatment predictions as they provide more accurate and precise results of the aneurysmal hemodynamics than those based on commonly accepted reference values for boundary conditions.