Physical factors effecting cerebral aneurysm pathophysiology

Determination of Significant Three-Dimensional Hemodynamic Features for Postembolization Recanalization in Cerebral Aneurysms Through Explainable Artificial Intelligence

BACKGROUND

Recanalization poses challenges after coil embolization in cerebral aneurysms. Establishing predictive models for postembolization recanalization is important for clinical decision making. However, conventional statistical and machine learning (ML) models may overlook critical parameters during the initial selection process.

METHODS

In this study, we automated the identification of significant hemodynamic parameters using a PointNet-based deep neural network (DNN), leveraging their three-dimensional spatial features. Further feature analysis was conducted using saliency mapping, an explainable artificial intelligence (XAI) technique. The study encompassed the analysis of velocity, pressure, and wall shear stress in both precoiling and postcoiling models derived from computational fluid dynamics simulations for 58 aneurysms.

RESULTS

Velocity was identified as the most pivotal parameter, supported by the lowest P value from statistical analysis and the highest area under the receiver operating characteristic curves/precision-recall curves values from the DNN model. Moreover, visual XAI analysis showed that robust injection flow zones, with notable impingement points in precoiling models, as well as pronounced interplay between flow dynamics and the coiling plane, were important three-dimensional features in identifying the recanalized aneurysms.

CONCLUSIONS

The combination of DNN and XAI was found to be an accurate and explainable approach not only at predicting postembolization recanalization but also at discovering unknown features in the future.

The Management of Intracranial Aneurysms: Current Trends and Future Directions

Intracranial aneurysms represent a major global health burden. Rupture of an intracranial aneurysm is a catastrophic event. Without access to treatment, the fatality rate is 50% in the first 30 days. Over the last three decades, treatment approaches for intracranial aneurysms have changed dramatically. There have been improvements in the medical management of aneurysmal subarachnoid haemorrhage, and there has been an evolution of treatment strategies. Endovascular therapy is now the mainstay of the treatment of ruptured intracranial aneurysms based on robust randomised controlled trial data. There is now an expansion of treatment indications for unruptured intracranial aneurysms to prevent rupture with both microsurgical clipping and endovascular treatment. Both microsurgical and endovascular treatment modalities have evolved, in particular with the introduction of innovative endovascular treatment options including flow diversion and intra-saccular flow disruption. These novel therapies allow clinicians to treat more complex and previously untreatable aneurysms. We aim to review the evolution of treatment strategies for intracranial aneurysms over time, and discuss emerging technologies that could further improve treatment safety and functional outcomes for patients with an intracranial aneurysm.

A matched-pair case control study identifying hemodynamic predictors of cerebral aneurysm growth using computational fluid dynamics

Introduction: Initiation and progression of cerebral aneurysms is known to be driven by complex interactions between biological and hemodynamic factors, but the hemodynamic mechanism which drives aneurysm growth is unclear. We employed robust modeling and computational methods, including temporal and spatial convergence studies, to study hemodynamic characteristics of cerebral aneurysms and identify differences in these characteristics between growing and stable aneurysms. Methods: Eleven pairs of growing and non-growing cerebral aneurysms, matched in both size and location, were modeled from MRA and CTA images, then simulated using computational fluid dynamics (CFD). Key hemodynamic characteristics, including wall shear stress (WSS), oscillatory shear index (OSI), and portion of the aneurysm under low shear, were evaluated. Statistical analysis was then performed using paired Wilcoxon rank sum tests. Results: The portion of the aneurysm dome under 70% of the parent artery mean wall shear stress was higher in growing aneurysms than in stable aneurysms and had the highest significance among the tested metrics (p = 0.08). Other metrics of area under low shear had similar levels of significance. Discussion: These results align with previously observed hemodynamic trends in cerebral aneurysms, indicating a promising direction for future study of low shear area and aneurysm growth. We also found that mesh resolution significantly affected simulated WSS in cerebral aneurysms. This establishes that robust computational modeling methods are necessary for high fidelity results. Together, this work demonstrates that complex hemodynamics are at play within cerebral aneurysms, and robust modeling and simulation methods are needed to further study this topic.

Pathophysiology of intracranial aneurysms in monozygotic twins: A rare case study from hemodynamic perspectives

Hemodynamic mechanisms of the formation and growth of intracranial aneurysms (IA) in monozygotic twins (MTs) are still under-reported. To partially fill such knowledge gap, this study employed an experimentally validated numerical model to compare hemodynamics in 3 anatomical and 5 ablation study neurovascular models from a rare pair of MTs in terms of 7 critical hemodynamic parameters. Numerical results showed significant differences in hemodynamics between the MTs, although they share the same genes, indicating that genetic mutation and environmental factors might affect neurovascular morphologies and cause hemodynamic changes. After virtual removals of IAs in the ablation study, the locations where the aneurysmal sac/bleb generated in bifurcated anterior cerebral arteries (ACAs) register a locally high instantaneous wall shear stress (IWSS) of 52.9 and 70.1 Pa at the systolic peak in twin A and twin B, respectively. Same scenario can be observed in the distribution of instantaneous wall shear stress gradient (IWSSG), with 571.1 Pa/mm for twin A and 301.3 Pa/mm for twin B due to aggressive blood impingements, leading to IA generation. The fenestrated complex approaching ACA bifurcations in twin A may assist IA growth and rupture, via. Causing abnormal IWSS of 116.3 Pa, IWSSG of 832.5 Pa/mm, and oscillatory shear index (OSI) of 0.49. The bleb in twin B has high risks of progression and possible rupture as the IA suffers relatively low IWSS and high OSI. Additionally, IA generation can change blood flow rates in each connected artery, then affecting blood supplies to associated tissues and organs.

Morphology and Hemodynamics of Cerebral Arteries and Aneurysms in a Rare Pair of Monozygotic Twins

In this preliminary study, the underlying pathophysiology mechanisms of cerebral aneurysms (CAs) in monozygotic twins (MTs) were investigated via a rare pair of MTs (twin A and twin B) involving four reconstructed arterial models using preclinical information. First, dimensions and configurated outlines of three-perspective geometries were compared. Adopting an in-vitro validated numerical CA model, hemodynamic characteristics were investigated in the MTs, respectively. Despite expected genetic similarities, morphological comparisons show that configurations of cerebral arteries exhibit significant differences between the twins. The ICA size of twin A is larger than that in twin B (2.23~25.86%), varying with specific locations, attributing to variations during embryological developments and environmental influences. Numerical modeling indicates the MTs have some hemodynamic similarities such as pressure distributions (~13,400 Pa) and their oscillatory shear index (OSI) (0~0.49), but present significant differences in local regions. Specifically, the difference in blood flow rate in the MTs is from 16% to 221%, varying with specifically compared arteries. The maximum time-averaged wall shear stress (53.6 Pa vs. 37.8 Pa) and different local OSI distributions were also observed between the MTs. The findings revealed that morphological variations in MTs could be generated by embryological and environmental factors, further influencing hemodynamic characteristics on CA pathophysiology.

Real-Time Intravital Imaging of Acoustic Cluster Therapy-Induced Vascular Effects in the Murine Brain

OBJECTIVE

The blood-brain barrier (BBB) is an obstacle for cerebral drug delivery. Controlled permeabilization of the barrier by external stimuli can facilitate the delivery of drugs to the brain. Acoustic Cluster Therapy (ACT®) is a promising strategy for transiently and locally increasing the permeability of the BBB to macromolecules and nanoparticles. However, the mechanism underlying the induced permeability change and subsequent enhanced accumulation of co-injected molecules requires further elucidation.

METHODS

In this study, the behavior of ACT® bubbles in microcapillaries in the murine brain was observed using real-time intravital multiphoton microscopy. For this purpose, cranial windows aligned with a ring transducer centered around an objective were mounted to the skull of mice. Dextrans labeled with 2 MDa fluorescein isothiocyanate (FITC) were injected to delineate the blood vessels and to visualize extravasation.

DISCUSSION

Activated ACT® bubbles were observed to alter the blood flow, inducing transient and local increases in the fluorescence intensity of 2 MDa FITC-dextran and subsequent extravasation in the form of vascular outpouchings. The observations indicate that ACT® induced a transient vascular leakage without causing substantial damage to the vessels in the brain.

CONCLUSION

The study gave novel insights into the mechanism underlying ACT®-induced enhanced BBB permeability which will be important considering treatment optimization for a safe and efficient clinical translation of ACT®.

Hemodynamics of anterior circulation intracranial aneurysms with daughter blebs: investigating the multidirectionality of blood flow fields

Recent advances in diagnostic neuroradiological imaging, allowed the detection of unruptured intracranial aneurysms (IAs). The shape - irregular or multilobular - of the aneurysmal dome, is considered as a possible rupture risk factor, independently of the size, the location and patient medical background. Disturbed blood flow fields in particular is thought to play a key role in IAs progression. However, there is an absence of widely-used hemodynamic indices to quantify the extent of a multi-directional disturbed flow. We simulated blood flow in twelve patient-specific anterior circulation unruptured intracranial aneurysms with daughter blebs utilizing the spectral/hp element framework Nektar++. We simulated three cardiac cycles using a volumetric flow rate waveform while we considered blood as a Newtonian fluid. To investigate the multidirectionality of the blood flow fields, besides the time-averaged wall shear stress (TAWSS), we calculated the oscillatory shear index (OSI), the relative residence time (RRT) and the time-averaged cross flow index (TACFI). Our CFD simulations suggest that in the majority of our vascular models there is a formation of complex intrasaccular flow patterns, resulting to low and highly oscillating WSS, especially in the area of the daughter blebs. The existence of disturbed multi-directional blood flow fields is also evident by the distributions of the RRT and the TACFI. These findings further support the theory that IAs with daughter blebs are linked to a potentially increased rupture risk.

Developing an in vitro validated 3D in silico internal carotid artery sidewall aneurysm model

Introduction: Direct quantification of hemodynamic factors applied to a cerebral aneurysm (CA) remains inaccessible due to the lack of technologies to measure the flow field within an aneurysm precisely. This study aimed to develop an in vitro validated 3D in silico patient-specific internal carotid artery sidewall aneurysm (ICASA) model which can be used to investigate hemodynamic factors on the CA pathophysiology. Methods: The validated ICASA model was developed by quantifying and comparing the flow field using particle image velocimetry (PIV) measurements and computational fluid dynamics (CFD) simulations. Specifically, the flow field characteristics, i.e., blood flowrates, normalized velocity profiles, flow streamlines, and vortex locations, have been compared at representative time instants in a cardiac pulsatile period in two designated regions of the ICASA model, respectively. One region is in the internal carotid artery (ICA) inlet close to the aneurysm sac, the other is across the middle of the aneurysmal sac. Results and Discussion: The results indicated that the developed computational fluid dynamics model presents good agreements with the results from the parallel particle image velocimetry and flowrate measurements, with relative differences smaller than 0.33% in volumetric flow rate in the ICA and relative errors smaller than 9.52% in averaged velocities in the complex aneurysmal sac. However, small differences between CFD and PIV in the near wall regions were observed due to the factors of slight differences in the 3D printed model, light reflection and refraction near arterial walls, and flow waveform uncertainties. The validated model not only can be further employed to investigate hemodynamic factors on the cerebral aneurysm pathophysiology statistically, but also provides a typical model and guidance for other professionals to evaluate the hemodynamic effects on cerebral aneurysms.

Reliability of using generic flow conditions to quantify aneurysmal haemodynamics: A comparison against simulations incorporating boundary conditions measured in vivo

BACKGROUND AND OBJECTIVES

Initiation, growth, and rupture of intracranial aneurysms are believed to be closely related to their local haemodynamic environment. While haemodynamics can be characterised by use of computational fluid dynamics (CFD), its reliability depends heavily upon accurate assumption of the boundary conditions. Herein, we compared the simulated aneurysmal haemodynamics obtained by use of generic boundary conditions against those obtained under flow conditions measured in vivo.

METHODS

We prospectively recruited 19 patients with intracranial aneurysms requiring 3-dimensional rotational angiography, during which blood pressure at the internal carotid artery was probed by catheter and flowrate measured by a dedicated software tool. Using these flow conditions measured in vivo, we quantified the aneurysmal haemodynamics for each patient by CFD, and then compared the results with those derived from a generic condition reported in the literature, in terms of the time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), relative residence time (RRT), and percentage of the intra-aneurysmal flow (PIAF). In addition, the effects on aneurysmal haemodynamics of different outflow strategies (splitting method vs. Murray's Law) and simulation schemes (transient vs. steady-state) relative to each flow condition were also assessed.

RESULTS

Differences in the simulated TAWSS (-6.08 ± 10.64 Pa, p = 0.001), OSI (0.06 ± 0.13, p = 0.001), and PIAF (-0.05 ± 0.20, p = 0.012) between the patient-specific and generic boundary conditions were found to be statistically significant, in contrast to that in the RRT (49 ± 307 Pa^-1, p = 0.062). Outflow strategies did not yield statistically significant differences in any of the investigated parameters (all p > 0.05); rather, the resulting parameters were found to be in good correlations (all r > 0.71, p < 0.001). Difference between the aneurysmal TAWSS and the WSS derived from cycle-averaged flowrate condition was found to be minor (0.66 ± 1.36 Pa, p = 0.000), so was that between PIAFs obtained respectively from the transient and steady-state simulations (0.02 ± 0.05, p = 0.000).

CONCLUSIONS

Incorporating into simulation the patient-specific boundary conditions is critical for CFD to characterise aneurysmal haemodynamics, while outflow strategies may not introduce significant uncertainties. Steady-state simulation incorporating the cycle-averaged flow condition may produce unbiased WSS and PIAF compared to the transient analysis.

Single Center Retrospective Analysis of Cerebral Aneurysms from a Patient Sample Data Collection at a Comprehensive Stroke Center

INTRODUCTION

Institutional self-monitoring of cerebral aneurysm data should occur regularly. The objective of this retrospective single center study was to examine the reproducibility of a data collection and analytic method to examine cerebral aneurysm characteristics and trends.

METHODS

A single center retrospective analysis was performed from 2018 to 2021 of the most recent 100 patient presentations with a newly diagnosed cerebral aneurysm. Data collection included patient demographics, radiographic features, ruptured or unruptured status, location, grading scale, treatment strategy, survival, and length of stay, which were extracted and presented in tabular form and analyzed for overall trends.

RESULTS

Of the collected 100 patients meeting ICD-10 criteria, 10 (10%) patients were excluded due to having been previously diagnosed at the institution and not meeting the criteria of a new discovery of cerebral aneurysm for inclusion. The remaining 90 sample patients presented with newly diagnosed aneurysms to the authors' Emergency Department between 2018 and 2021. Most patients were between the ages of 25 and 65 with 55 (61%) patients identifying themselves as female sex. Of the 90 eligible sample patients, 59 (66%) had aneurysms that were not ruptured. Eighty-eight (97.7%) patients had cerebral aneurysms that were < 7mm in size. The most common location for aneurysms was in the anterior cerebral circulation, with identification of 27 middle cerebral artery aneurysms. Length of stay (LOS) ranged from 0-171 days with a mean of 11.97 days (SD = 19.9). Of the seven (7.7%) patients who expired, four (57%) experienced spontaneous subarachnoid hemorrhages, with two (29%) occurring in the anterior communicating artery and one (14%) in the left middle cerebral artery and basilar artery respectively.

CONCLUSIONS

The typical presentation of a cerebral aneurysm is unruptured with a pre-dominance in middle-aged females. Our findings are congruent with the literature regarding the location of the aneurysm originating in the anterior circulation. However, most aneurysms in our clinical cohort were located on the MCA/ICA in contrast to the literature reported (i.e., most anterior communicating artery). Of those patients who presented unruptured, outpatient follow-up and routine monitoring were appropriate with medical management in the setting of small aneurysms. The risk of progression and subsequent rupture was relatively small in this patient cohort. Multi-year examinations of single institution comprehensive stroke centers regarding cerebral aneurysms would enable researchers to conduct regional analyses and comparisons to national and international trends.

Effects of Pulsatile Flow Rate and Shunt Ratio in Bifurcated Distal Arteries on Hemodynamic Characteristics Involved in Two Patient-Specific Internal Carotid Artery Sidewall Aneurysms: A Numerical Study

The pulsatile flow rate (PFR) in the cerebral artery system and shunt ratios in bifurcated arteries are two patient-specific parameters that may affect the hemodynamic characteristics in the pathobiology of cerebral aneurysms, which needs to be identified comprehensively. Accordingly, a systematic study was employed to study the effects of pulsatile flow rate (i.e., PFR−I, PFR−II, and PFR−III) and shunt ratio (i.e., 75:25 and 64:36) in bifurcated distal arteries, and transient cardiac pulsatile waveform on hemodynamic patterns in two internal carotid artery sidewall aneurysm models using computational fluid dynamics (CFD) modeling. Numerical results indicate that larger PFRs can cause higher wall shear stress (WSS) in some local regions of the aneurysmal dome that may increase the probability of small/secondary aneurysm generation than under smaller PFRs. The low WSS and relatively high oscillatory shear index (OSI) could appear under a smaller PFR, increasing the potential risk of aneurysmal sac growth and rupture. However, the variances in PFRs and bifurcated shunt ratios have rare impacts on the time-average pressure (TAP) distributions on the aneurysmal sac, although a higher PFR can contribute more to the pressure increase in the ICASA−1 dome due to the relatively stronger impingement by the redirected bloodstream than in ICASA−2. CFD simulations also show that the variances of shunt ratios in bifurcated distal arteries have rare impacts on the hemodynamic characteristics in the sacs, mainly because the bifurcated location is not close enough to the sac in present models. Furthermore, it has been found that the vortex location plays a major role in the temporal and spatial distribution of the WSS on the luminal wall, varying significantly with the cardiac period.

Hemodynamic impact of proximal anterior cerebral artery aneurysm: Mind the posteriorly projecting ones!

Intracranial aneurysm projected posteriorly is associated with high risk of aneurysm rupture. In order to investigate the biomechanical mechanisms for the adverse event, three-dimension intracranial cerebral aneurysms were constructed based on clinical data, and we numerically compared effect of location, position, size, and shape of aneurysm on hemodynamic conditions including velocity, pressure, and wall shear stress (WSS). The numerical results showed that the aneurysm projected posteriorly even at small sizes led to abnormal hemodynamic environment, which was featured by a local high pressure and stress concentration near aneurysm neck area. Moreover, the one located at the proximal A1 segment and ellipsoidal aneurysm would further worse local hemodynamic environment, causing high local stresses. These findings indicated the potential mechanical mechanism for high rupture rate of the aneurysms projected posteriorly, underscoring importance of early and accurate diagnosis and promptly treatment for improved the clinical outcome, even if these aneurysms are of small sizes.

Application of Dynamic Mode Decomposition to Study Temporal Flow Behavior in a Saccular Aneurysm

Aneurysms are abnormal expansion of weakened blood vessels which can cause mortality or long-term disability upon rupture. Several studies have shown that inflow conditions spatially and temporally influence aneurysm flow behavior. The objective of this investigation is to identify impact of inflow conditions on spatio-temporal flow behavior in an aneurysm using dynamic mode decomposition (DMD). For this purpose, low-frame rate velocity field measurements are performed in an idealized aneurysm model using particle image velocimetry (PIV). The inflow conditions are precisely controlled using a ViVitro SuperPump system where nondimensional fluid parameters such as peak Reynolds number (Rep) and Womersely number (α) are varied from 50-270 and 2-5, respectively. The results show the ability of DMD to identify the spatial flow structures and their frequency content. Furthermore, DMD captured the impact of inflow conditions, and change in mode shapes, amplitudes, frequency, and growth rate information is observed. The DMD low-order flow reconstruction also showed the complex interplay of flow features for each inflow scenario. Furthermore, the low-order reconstruction results provided a mathematical description of the flow behavior in the aneurysm which captured the vortex formation, evolution, and convection in detail. These results indicated that the vortical structure behavior varied with the change in α while its strength and presence of secondary structures are influenced by the change in Rep.

Characterizing Intracranial Hemodynamics in Sickle Cell Anemia: Impact of Patient-Specific Viscosity

PURPOSE

Pediatric and adult patients with sickle cell anemia (SCA) are at increased risk of stroke and cerebrovascular accident. In the general adult population, there is a relationship between arterial hemodynamics and pathology; however, this relationship in SCA patients remains to be elucidated. The aim of this work was to characterize circle of Willis hemodynamics in patients with SCA and quantify the impact of viscosity choice on pathophysiologically-relevant hemodynamics measures.

METHODS

Based on measured vascular geometries, time-varying flow rates, and blood parameters, detailed patient-specific simulations of the circle of Willis were conducted for SCA patients (n = 6). Simulations quantified the impact of patient-specific and standard blood viscosities on wall shear stress (WSS).

RESULTS

These results demonstrated that use of a standard blood viscosity introduces large errors into the estimation of pathophysiologically-relevant hemodynamic parameters. Standard viscosity models overpredicted peak WSS by 55% and 49% for steady and pulsatile flow, respectively. Moreover, these results demonstrated non-uniform, spatial patterns of positive and negative WSS errors related to viscosity, and standard viscosity simulations overpredicted the time-averaged WSS by 32% (standard deviation = 7.1%). Finally, differences in shear rate demonstrated that the viscosity choice alters the simulated near-wall flow field, impacting hemodynamics measures.

CONCLUSIONS

This work presents simulations of circle of Willis arterial flow in SCA patients and demonstrates the importance and feasibility of using a patient-specific viscosity in these simulations. Accurately characterizing cerebrovascular hemodynamics in SCA populations has potential for elucidating the pathophysiology of large-vessel occlusion, aneurysms, and tissue damage in these patients.

Biophysical targeting of high-risk cerebral aneurysms

Localized delivery of diagnostic/therapeutic agents to cerebral aneurysms, lesions in brain arteries, may offer a new treatment paradigm. Since aneurysm rupture leading to subarachnoid hemorrhage is a devastating medical emergency with high mortality, the ability to noninvasively diagnose high-risk aneurysms is of paramount importance. Moreover, treatment of unruptured aneurysms with invasive surgery or minimally invasive neurointerventional surgery poses relatively high risk and there is presently no medical treatment of aneurysms. Here, leveraging the endogenous biophysical properties of brain aneurysms, we develop particulate carriers designed to localize in aneurysm low-shear flows as well as to adhere to a diseased vessel wall, a known characteristic of high-risk aneurysms. We first show, in an in vitro model, flow guided targeting to aneurysms using micron-sized (2 μm) particles, that exhibited enhanced targeting (>7 folds) to the aneurysm cavity while smaller nanoparticles (200 nm) showed no preferable accumulation. We then functionalize the microparticles with glycoprotein VI (GPVI), the main platelet receptor for collagen under low-medium shear, and study their targeting in an in vitro reconstructed patient-specific aneurysm that contained a disrupted endothelium at the cavity. Results in this model showed that GPVI microparticles localize at the injured aneurysm an order of magnitude (>9 folds) more than control particles. Finally, effective targeting to aneurysm sites was also demonstrated in an in vivo rabbit aneurysm model with a disrupted endothelium. Altogether, the presented biophysical strategy for targeted delivery may offer new treatment opportunities for cerebral aneurysms.

MicroRNA-124-5p delays the progression of cerebral aneurysm by regulating FoxO1

Cerebral aneurysm (CA) is a common brain disease, and the development of cerebral aneurysm is driven by inflammation and hemodynamic stress. MicroRNA (miR)-124-5p is reported to be associated with inflammatory response in brain disease such as cerebral ischemia-reperfusion injury. However, the function and molecular mechanism of miR-124-5p in CA are not clear, thus, the effects of miR-124-5p on inflammatory response in CA were explored. Firstly, the expression of miR-124-5p in the peripheral blood of patients with CA and the control group was detected by reverse transcription-quantitative PCR. Then, the human umbilical vein endothelial cells (HUVECs) were used as an in vitro model system and stimulated with interleukin (IL)-1β to simulate the inflammatory environment of CA, and the expression of miR-124-5p was detected. Next, the effect of miR-124-5p on the migration and invasion of HUVECs was detected using Transwell assays. Meanwhile, the function of miR-124-5p on various inflammatory factors was determined by western blotting and enzyme-linked immunosorbent assay (ELISA). Next, the TargetScan website was used to predict FoxO1 as a target gene of miR-124-5p, and this target association was validated by double luciferase reporter assay and western blotting. Finally, the interaction of miR-124-5p with FoxO1 in CA was measured by Transwell western blotting and ELISA assays. The results showed that the expression level of miR-124-5p in the peripheral blood of patients with CA was lower compared with that of control group, and the miR-124-5p in HUVECs stimulated by IL-1β was less compared with that in normal HUVECs. Besides, miR-124-5p could inhibit the migration and invasion abilities of HUVECs and the release of inflammatory factors. Additionally, the overexpression of miR-124-5p was able to inhibit the expression of FoxO1. miR-124-5p-inhibitor promoted the migration and invasion of HUVECs, as well as inflammatory response, which was weakened following the introduction of FoxO1 small interfering RNA. Overall, the present study demonstrated that miR-124-5p could prevent the occurrence and development of cerebral aneurysm by downregulating the expression of FoxO1.

Quality of life scores in patients with unruptured cerebral aneurysm: Prospective cohort study

BACKGROUND

Living with the diagnosis of an unruptured cerebral aneurysm can understandably cause distress to a patient. The goal of preventive treatment is to increase the number of years with good quality of life (QoL).

OBJECTIVE

This study aimed to measure the effect of unruptured intracranial aneurysm treatment on change in QoL scores measured by the SF36 and EQ-5D-5L.

METHODS

We prospectively collected SF36 and EQ-5D-5L survey data for patients with unruptured intracranial aneurysms at two time-points over 1 year between 2 treatment groups: observation and intervention (microsurgical and endovascular). Multivariable linear regression was used to examine treatment group differences in the mean change in scores from baseline to 1 year when adjusted for covariates.

RESULTS

92 patients were included in the observation group and 68 patients were included in the intervention group, for a total of 160 patients. The intervention group had lower SF36 total scores at baseline (p = 0.001). With multivariate linear regression models, the effect of treatment on mean change in SF36 total score from baseline to 1 year was not statistically significant (p = 0.4); similarly, there was no difference in mean change in EQ-5D-5L.

CONCLUSION

In this large prospective study, preventive aneurysm treatment was not associated with a significant change in QoL score at 1 year compared to observation as measured by the SF36 and EQ-5D-5L. Further studies are needed to explore the lower QoL scores in patients seeking treatment and its impact on management decision making.

Application of Proper Orthogonal Decomposition to Study Coherent Flow Structures in a Saccular Aneurysm

Aneurysms are localized expansions of weakened blood vessels that can be debilitating or fatal upon rupture. Previous studies have shown that flow in an aneurysm exhibits complex flow structures that are correlated with its inflow conditions. Therefore, the objective of this study was to demonstrate the application of proper orthogonal decomposition (POD) to study the impact of different inflow conditions on energetic flow structures and their temporal behavior in an aneurysm. To achieve this objective, experiments were performed on an idealized rigid sidewall aneurysm model. A piston pump system was used for precise inflow control, i.e., peak Reynolds number (Rep) and Womersley number (α) were varied from 50 to 270 and 2 to 5, respectively. The velocity flow field measurements at the midplane location of the idealized aneurysm model were performed using particle image velocimetry (PIV). The results demonstrate the efficacy of POD in decomposing complex data, and POD was able to capture the energetic flow structures unique to each studied inflow condition. Furthermore, the time-varying coefficient results highlighted the interplay between the coefficients and their corresponding POD modes, which in turn helped explain how POD modes impact certain flow features. The low-order reconstruction results were able to capture the flow evolution and provide information on complex flow in an aneurysm. The POD and low-order reconstruction results also indicated that vortex formation, evolution, and convection varied with an increase in α, while vortex strength and formation of secondary structures were correlated with an increase in Rep.

A converging artery-sized model for shear adhesion mapping of particles

Drug carriers for targeting cardiovascular diseases have been gaining a respectable attention, however, designing such carriers is challenging due to the biophysical complexity of the vascular system. Wall shear stress (WSS), exerted by blood flow on the endothelium surface, is a crucial factor in the circulatory system. WSS affects the adhesion and preferential accumulation of drug carriers. Here, we propose, an innovative approach to investigate particle adhesion in a converging artery-sized model, lined with human endothelial cells. Unlike widely used microfluidic and in vivo setups, our model enables to investigate particle accumulation in a continuous WSS range, performed in a single experiment, and at the right scale relevant for human arteries. First, we characterized the flow and the WSS map along the designed model, which can span along the entire arterial WSS range. We then used the model to examine the effect of particle size and the suspension buffer on particle adhesion distribution. The results demonstrated the role of particle size, where the same particles with a diameter of 2 µm exhibit shear-decreased adhesion while 500 nm particles exhibit shear-enhanced adhesion. Furthermore, under the same WSS, particles show a similar behavior when suspended in a Dextran buffer, having a viscosity analogous to blood, compared to a phosphate buffer solution without Dextran. Moreover, experiments with RBCs in the phosphate buffer, at a 40% physiological hematocrit, decreased particle adhesion and affected their deposition pattern. Altogether, our study suggests an original platform for investigating and optimizing intravascular drug carriers and their targeting properties.

Can biomechanical analysis shed some light on aneurysmal pathophysiology? Preliminary study on ex vivo cerebral arterial walls

BACKGROUND

The pathophysiology of cerebral aneurysm is complex and poorly understood, and it can have the most catastrophic clinical presentation. Flow dynamics is a key player in the initiation and progression of aneurysm. Better understanding the interaction between hemodynamic loading and biomechanical wall responses can help to add the missing piece on aneurysmal pathophysiology. In this laboratory study we aimed to analyze the effect of the application of a mechanical force to cerebral arterial walls.

METHODS

Displacement control tests were performed on five porcine cerebral arteries. The test machine was the T150 Nanotensile. The stiffness variation with the increment of the strain level is modeled as the outcome of an isotropic hyperelastic material model.

FINDINGS

Through the application of an axial force we obtained Stress/Strain curves that showed a marked isotropic hyperelastic behavior, characterized by an increasing of stiffness with the level of strain. This behavior of the cerebral arterial wall is different from the well-established behavior of other arterial vessel (as the aortic vessel) characterized by a marked anisotropic behavior. Additionally, the data scattering observed for higher values of the applied stress are related to different individual packing of collagen fibers that represent the load-bearing mechanics at higher level of the strain.

INTERPRETATION

The data obtained by test in this paper represent a first step in our ongoing research about the mechanics of multi-axial loads on cerebral arterial walls, and in producing more comprehensive patient-specific calculations for potential applications on cerebral aneurysm management.

Cardiovascular Problems in the Fragile X Premutation

There is a dearth of information about cardiovascular problems in fragile X premutation carriers who have 55–200 CGG repeats in fragile X mental retardation 1 (FMR1) gene. The FMR1 expansion in the premutation range leads to toxic RNA gain-of-function resulting in cellular dysregulation. The mechanism of RNA toxicity underlies all of the premutation disorders including fragile X-associated tremor/ataxia syndrome, fragile X-associated primary ovarian insufficiency, and fragile X-associated neuropsychiatric disorder. Cardiovascular problems particularly autonomic dysfunction, hypertension, and cardiac arrhythmias are not uncommon in premutation carriers. Some arterial problems and valvular heart diseases have also been reported. This article reviews cardiovascular problems in premutation carriers and discusses possible contributing mechanisms including RNA toxicity and mild fragile X mental retardation protein deficiency. Further research studies are needed in order to prove a direct association of the cardiovascular problems in fragile X premutation carriers because such knowledge will lead to better preventative treatment.

Aneurysms of the Lenticulostriate Artery: A Systematic Review

BACKGROUND

Lenticulostriate artery aneurysms (LSAs) are rare vascular aberrations. Despite the potentially catastrophic sequelae of aneurysmal rupture, the optimal management strategy for LSA aneurysms has not been determined. The aim of the present review is to provide an overview of the clinical presentation and treatment strategies for LSA aneurysms.

METHODS

Using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, a literature search was conducted in the PubMed, Cochrane, EBSCOhost, Scopus, Web of Science, and ProQuest search engines to identify reported studies of LSA aneurysms until July 1, 2020. A descriptive analysis was performed.

RESULTS

A total of 71 studies with 112 cases of LSA aneurysms were included. Patient age ranged from 2 months to 83 years (median, 44.5 years). Male and female patients were affected similarly (49% and 51%, respectively). The most common presentation was aneurysmal rupture (78%), and headache was the most frequently reported symptom (36%). Overall, 48% of the patients had undergone underwent surgical treatment, 30% conservative management, 21% endovascular treatment, and 1% radiosurgery. Four patients died, all of whom had presented with aneurysmal rupture.

CONCLUSION

We have summarized the reported cases of LSA aneurysms, with their clinical presentation, management, and outcomes, for physicians who may be confronted with this diagnosis. Future studies that use available classification systems and include as much detail as possible should be encouraged to fully elucidate the optimal management strategy for these patients.

Hemodynamic and morphological characteristics of a growing cerebral aneurysm

The growth of cerebral aneurysms is linked to local hemodynamic conditions, but the driving mechanisms of the growth are poorly understood. The goal of this study was to examine the association between intraaneurysmal hemodynamic features and areas of aneurysm growth, to present the key hemodynamic parameters essential for an accurate prediction of the growth, and to gain a deeper understanding of the underlying mechanisms. Patient-specific images of a growing cerebral aneurysm in 3 different growth stages acquired over a period of 40 months were segmented and reconstructed. A unique aspect of this patient-specific case study was that while one side of the aneurysm stayed stable, the other side continued to grow. This unique case enabled the authors to examine their aims in the same patient with parent and daughter arteries under the same inlet flow conditions. Pulsatile flow in the aneurysm models was simulated using computational fluid dynamics and was validated with in vitro experiments using particle image velocimetry measurements. The authors' detailed analysis of intrasaccular hemodynamics linked the growing regions of aneurysms to flow instabilities and complex vortex structures. Extremely low velocities were observed at or around the center of the unstable vortex structure, which matched well with the growing regions of the studied cerebral aneurysm. Furthermore, the authors observed that the aneurysm wall regions with a growth greater than 0.5 mm coincided with wall regions of lower (< 0.5 Pa) time-averaged wall shear stress (TAWSS), lower instantaneous (< 0.5 Pa) wall shear stress (WSS), and high (> 0.1) oscillatory shear index (OSI). To determine which set of parameters can best identify growing and nongrowing aneurysms, the authors performed statistical analysis for consecutive stages of the growing CA. The results demonstrated that the combination of TAWSS and the distance from the center of the vortical structure has the highest sensitivity and positive predictive value, and relatively high specificity and negative predictive value. These findings suggest that an unstable, recirculating flow structure within the aneurysm sac created in the region adjacent to the aneurysm wall with low TAWSS may be introduced as an accurate criterion to explain the hemodynamic conditions predisposing the aneurysm to growth. The authors' findings are based on one patient's data set, but the study lays out the justification for future large-scale verification. The authors' findings can assist clinicians in differentiating stable and growing aneurysms during preinterventional planning.

Computational and experimental investigation of particulate matter deposition in cerebral side aneurysms

Intracranial aneurysms frequently develop blood clots, plaque and inflammations, which are linked to enhanced particulate mass deposition. In this work, we propose a computational model for particulate deposition, that accounts for the influence of field forces, such as gravity and electrostatics, which produce an additional flux of particles perpendicular to the fluid motion and towards the wall. This field-mediated flux can significantly enhance particle deposition in low-shear environments, such as in aneurysm cavities. Experimental investigation of particle deposition patterns in in vitro models of side aneurysms, demonstrated the ability of the model to predict enhanced particle adhesion at these sites. Our results showed a significant influence of gravity and electrostatic forces (greater than 10%), indicating that the additional terms presented in our models may be necessary for modelling a wide range of physiological flow conditions and not only for ultra-low shear regions. Spatial differences between the computational model and the experimental results suggested that additional transport and fluidic mechanisms affect the deposition pattern within aneurysms. Taken together, the presented findings may enhance our understanding of pathological deposition processes at cardiovascular disease sites, and facilitate rational design and optimization of cardiovascular particulate drug carriers.

Effect of bifurcation in the hemodynamic changes and rupture risk of small intracranial aneurysm

The role of bifurcations is prominent in the intracranial aneurysm (IA) evaluation, and there are many contradictions and complexities in the rupture risk of small IA. Therefore, in the present study, the effect of bifurcation on the manner of hemodynamic changes and the rupture risk of the small middle cerebral artery (MCA) aneurysm is investigated. 3D anatomical models of the MCAs of 21 healthy subjects, 19 patients/IA/bifurcation, and 19 patients/IA were generated, and the models were analyzed by the computational fluid dynamic (CFD) analysis. The presence of bifurcation in the pathway of the blood flow in the parent artery of healthy subjects has reduced the maximum velocity, flow rate, and wall shear stress (WSS) by 25.8%, 38.6%, and 11.1%, respectively. The bifurcation decreased the maximum velocity and flow rate in the neck and sac of the aneurysm by 1.65~2.1 times, respectively. It increased the maximum WSS, and phase lag between the WSS graph of healthy subjects and patients by 12.8%~13.9% and 10.2%~40.4%, respectively. The effect of bifurcation on the Womersley number change in the aneurysm was insignificant, and the blood flow was in the laminar flow condition in all samples. The results also showed bifurcation increased the phase lag between the flow rate and pressure gradient graphs up to approximately 1.5 times. The rupture prediction index for patients/IA/bifurcation and patients/IA was 62.1%(CV = 4.1) and 51.8%(CV = 4.4), respectively. Thus, in equal conditions, the presence of bifurcation increased the probability of the rupture of the aneurysm by 19.9%.

Aspirin associated with decreased rate of intracranial aneurysm growth

OBJECTIVE

Aspirin has emerged as a potential agent in the prevention of rupture of intracranial aneurysms (IAs). In this study, the authors' goal was to test if aspirin is protective against aneurysm growth in patients harboring multiple IAs ≤ 5 mm.

METHODS

The authors performed a retrospective review of a prospectively maintained database covering the period July 2009 through January 2019. Patients' data were included if the following criteria were met: 1) the patient harbored multiple IAs; 2) designated primary aneurysms were treated by surgical/endovascular means; 3) the remaining aneurysms were observed for growth; and 4) a follow-up period of at least 5 years after the initial treatment was available. Demographics, earlier medical history, the rupture status of designated primary aneurysms, aneurysms' angiographic features, and treatment modalities were gathered.

RESULTS

The authors identified 146 patients harboring a total of 375 IAs. At the initial encounter, 146 aneurysms were treated and the remaining 229 aneurysms (2-5 mm) were observed. During the follow-up period, 24 (10.48%) of 229 aneurysms grew. All aneurysms observed to grow later underwent treatment. None of the observed aneurysms ruptured. Multivariate analysis showed that aspirin was significantly associated with a decreased rate of growth (odds ratio [OR] 0.19, 95% confidence interval [CI] 0.05-0.63). Variables associated with an increased rate of growth included hypertension (OR 14.38, 95% CI 3.83-53.94), drug abuse (OR 11.26, 95% CI 1.21-104.65), history of polycystic kidney disease (OR 9.48, 95% CI 1.51-59.35), and subarachnoid hemorrhage at presentation (OR 5.91, 95% CI 1.83-19.09).

CONCLUSIONS

In patients with multiple IAs, aspirin significantly decreased the rate of aneurysm growth over time. Additional prospective interventional studies are needed to validate these findings.

Risk of rupture of the cerebral aneurysm in relation to traumatic brain injury using a patient-specific fluid-structure interaction model

BACKGROUND AND OBJECTIVE

Cerebral aneurysm, which is defined as one of the weakened area in the wall of an artery in the brain, ruptures when wall tension exceeds its mechanical strength. Traumatic brain injury (TBI) by exerting a sudden impact load to the brain can lead to mechanical failure of the cerebral blood vessels followed by an alteration in not only the structure but also the function of the cerebrovascular. TBI also alters the hemodynamics of the blood flow in the cerebrovascular, while it has been shown that hemodynamics has a key asset in the progression and rupture of the cerebral aneurysms. So far, there is a lack of knowledge on the risk of rupture of the cerebral aneurysm in relation to TBI. Therefore, this study aimed to calculate the mechanical stresses and deformations in the arterial wall as well as the pressure and velocity of the blood using a fluid-structure interaction (FSI) model of the cerebral aneurysm located in the anterior circulation region of the circle of Willis.

METHOD

A patient-specific FSI model of the human skull, brain, and cerebral aneurysm, was established using human computed tomography (CT)/ magnetic resonance imaging (MRI) data and subjected to a frontal TBI.

RESULTS

The results revealed considerable increasing of ∼ 8 kPa (60 mmHg) and 0.40 m/s in the pressure and velocity of the blood in the intraluminal of the cerebral artery after TBI. The von Mises stress, shear stress, and deformation of the cerebral aneurysm wall also showed the increasing of 56.03 kPa, 15.66 Pa, and 0.072 mm after TBI, respectively.

CONCLUSIONS

Although the injury to the aneurysm wall after TBI is lower than that of the aneurysm wall mechanical strength, it still can alter the stress pattern in the wall and disrupt the hemodynamics of the blood. These results have implications in understanding the rupture risk of the cerebral aneurysm due to TBI, which may contribute in establishing preventive and/or treatment methods.

High-frequency wall vibrations in a cerebral patient-specific aneurysm model

The presence of high-frequency velocity fluctuations in aneurysms have been confirmed by in-vivo measurements and by several numerical simulation studies. Only a few studies have located and recorded wall vibrations in in-vitro experiments using physiological patient models. In this study, we investigated the wall fluctuations produced by a flowing perfusion fluid in a true-to-scale elastic model of a cerebral fusiform aneurysm using a laser Doppler vibrometer (LDV). The model was obtained from patient data. The experimental setup reproduced physiologically relevant conditions using a compliant perfusion system, physiological flow parameters, unsteady flow and a non-Newtonian fluid. Three geometrically identical models with different wall elasticities were used for measurements. The influence of five different flow rates was considered. Wall vibrations were predominantly found at frequencies in the range 40-60 Hz and 255-265 Hz. Their amplitude increased with increasing elasticity of the model, but the spectral peaks remained at about the same frequency. Varying the flow rate produced almost no changes in the frequency domain of the models. The frequency of the spectral peaks varied slightly between points at the lateral wall and at the bottom of the aneurysm. Indeed, embedding the model in a fluid during measurements produced higher and smoother amplitude fluctuations.

Mapping the Transport Kinetics of Molecules and Particles in Idealized Intracranial Side Aneurysms

Intracranial side aneurysms (IA) are pathological blood-filled bulges in cerebral blood vessels. Unlike healthy blood vessels where mass transport is dominated by convection, both diffusion and convection can play an active role in aneurysm sites. Here, we study via dye washout experiments and numerical simulations, the transport characteristics of particles (1 micron) and small molecules (300 Da) into simplified side aneurysms models following bolus injection. Time-lapse fluorescent microscopy imaging performed in our idealized aneurysm models showed that the parent artery geometry (located on the inner vs. outer curvature) as well as the aneurysm aspect ratio (AR) affect the washout kinetics while the pulsatile nature of the flow, maintained within the physiological range, carries only a minor effect. Importantly, in the absence of effective diffusion, particles that are located on slow streamlines linger within the aneurysm cavity, a phenomenon that could be of importance in deposition of cells and nano/micro-particles within aneurysms. Altogether, mass transport studies may provide valuable insights for better understanding of aneurysm pathophysiology as well as for the design of new diagnostic and theranostic nano-medicines.

Flow instability detected in ruptured versus unruptured cerebral aneurysms at the internal carotid artery

Flow instability has emerged as a new hemodynamic metric hypothesized to have potential value in assessing the rupture risk of cerebral aneurysms. However, diverse findings have been reported in the literature. In the present study, high-resolution hemodynamic simulations were performed retrospectively on 35 aneurysms (10 ruptured & 25 unruptured) located at the internal carotid artery (ICA). Simulated hemodynamic parameters were statistically compared between the ruptured and unruptured aneurysms, with emphasis on examining the correlation of flow instability with the status of aneurysm rupture. Pronounced flow instability was detected in 20% (2 out of 10) of the ruptured aneurysms, whereas in 44% (11 out of 25) of the unruptured aneurysms. Statistically, the flow instability metric (quantified by the temporally and spatially averaged fluctuating kinetic energy over the aneurysm sac) did not differ significantly between the ruptured and unruptured aneurysms. In contrast, low wall shear stress area (LSA) and pressure loss coefficient (PLC) exhibited significant correlations with the status of aneurysm rupture. In conclusion, the present study suggests that the presence of flow instability may not correlate closely with the status of aneurysm rupture, at least for ICA aneurysms. On the other hand, the retrospective nature of the study and the small sample size may have to some extent compromised the reliability of the conclusion, and therefore large-scale prospective studies would be needed to further address the issue.

Effect of Local Coil Density on Blood Flow Stagnation in Densely Coiled Cerebral Aneurysms: A Computational Study Using a Cartesian Grid Method

Aneurysm recurrence is the most critical concern following coil embolization of a cerebral aneurysm. Adequate packing density (PD) and coil uniformity are believed necessary to achieve sufficient flow stagnation, which decreases the risk of aneurysm recurrence. The effect of coil distribution on the extent of flow stagnation, however, especially in cases of dense packing (high PD), has received less attention. Thus, the cause of aneurysm recurrence despite dense packing is still an open question. The primary aim of this study is to evaluate the effect of local coil density on the extent of blood flow stagnation in densely coiled aneurysms. For this purpose, we developed a robust computational framework to determine blood flow using a Cartesian grid method, by which the complex fluid pathways in coiled aneurysms could be flexibly treated using an implicit function. This tool allowed us to conduct blood flow analyses in two patient-specific geometries with 50 coil distribution patterns in each aneurysm at clinically adequate PD. The results demonstrated that dense packing in the aneurysm may not necessarily block completely the inflow into the aneurysm and local flow that formed in the neck region, whose strength was inversely related to this local PD. This finding suggests that local coil density in the neck region still plays an important role in disturbing the remaining local flow, which possibly prevents thrombus formation in a whole aneurysm sac, increasing the risk of aneurysm regrowth and subsequent recurrence.

Screening of Critical Genes and MicroRNAs in Blood Samples of Patients with Ruptured Intracranial Aneurysms by Bioinformatic Analysis of Gene Expression Data

Background

This study aimed to identify more potential genes and miRNAs associated with the pathogenesis of intracranial aneurysms (IAs).

Material/Methods

The dataset of GSE36791 (accession number) was downloaded from the Gene Expression Omnibus database. Differentially expressed genes (DEGs) were screened for in the blood samples from patients with ruptured IAs and controls, followed by functional and pathway enrichment analyses. In addition, gene co-expression network was constructed and significant modules were extracted from the network by WGCNA R package. Screening for miRNAs that could regulate DEGs in the modules was performed and an analysis of regulatory relationships was conducted.

Results

A total of 304 DEGs (167 up-regulated and 137 down-regulated genes) were screened for in blood samples from patients with ruptured IAs compared with those from controls. Functional enrichment analysis showed that the up-regulated genes were mainly associated with immune response and the down-regulated DEGs were mainly concerned with the structure of ribosome and translation. Besides, six functional modules were significantly identified, including four modules enriched by up-regulated genes and two modules enriched by down-regulated genes. Thereinto, the blue, yellow, and turquoise modules of up-regulated genes were all linked with immune response. Additionally, 16 miRNAs were predicted to regulate DEGs in the three modules associated with immune response, such as hsa-miR-1304, hsa-miR-33b, hsa-miR-125b, and hsa-miR-125a-5p.

Conclusions

Several genes and miRNAs (such as miR-1304, miR-33b, IRS2 and KCNJ2) may take part in the pathogenesis of IAs.

Verification of a research prototype for hemodynamic analysis of cerebral aneurysms

Owing to its clinical importance, there has been a growing body of research on understanding the hemodynamics of cerebral aneurysms. Traditionally, this work has been performed using general-purpose, state-of-the-art commercial solvers. This has meant requiring engineering expertise for making appropriate choices on the geometric discretization, time-step selection, choice of boundary conditions etc. Recently, a CFD research prototype has been developed (Siemens Healthcare GmbH, Prototype - not for diagnostic use) for end-to-end analysis of aneurysm hemodynamics. This prototype enables anatomical model preparation, hemodynamic computations, advanced visualizations and quantitative analysis capabilities. In this study, we investigate the accuracy of the hemodynamic solver in the prototype against a commercially available CFD solver ANSYS CFX 16.0 (ANSYS Inc., Canonsburg, PA, www.ansys.com) retrospectively on a sample of twenty patient-derived aneurysm models, and show good agreement of hemodynamic parameters of interest.

Hemodynamics of Flow Diverters

Cerebral aneurysms are pathological focal evaginations of the arterial wall at and around the junctions of the circle of Willis. Their tenuous walls predispose aneurysms to leak or rupture leading to hemorrhagic strokes with high morbidity and mortality rates. The endovascular treatment of cerebral aneurysms currently includes the implantation of fine-mesh stents, called flow diverters, within the parent artery bearing the aneurysm. By mitigating flow velocities within the aneurysmal sac, the devices preferentially induce thrombus formation in the aneurysm within hours to days. In response to the foreign implant, an endothelialized arterial layer covers the luminal surface of the device over a period of days to months. Organization of the intraneurysmal thrombus leads to resorption and shrinkage of the aneurysm wall and contents, eventually leading to beneficial remodeling of the pathological site to a near-physiological state. The devices' primary function of reducing flow activity within aneurysms is corollary to their mesh structure. Complete specification of the device mesh structure, or alternately device permeability, necessarily involves the quantification of two variables commonly used to characterize porous media—mesh porosity and mesh pore density. We evaluated the flow alteration induced by five commercial neurovascular devices of varying porosity and pore density (stents: Neuroform, Enterprise, and LVIS; flow diverters: Pipeline and FRED) in an idealized sidewall aneurysm model. As can be expected in such a model, all devices substantially reduced intraneurysmal kinetic energy as compared to the nonstented case with the coarse-mesh stents inducing a 65–80% reduction whereas the fine-mesh flow diverters induced a near-complete flow stagnation (∼98% reduction). We also note a trend toward greater device efficacy (lower intraneurysmal flow) with decreasing device porosity and increasing device pore density. Several such flow studies have been and are being conducted in idealized as well as patient-derived geometries with the overarching goals of improving device design, facilitating treatment planning (what is the optimal device for a specific aneurysm), and predicting treatment outcome (will a specific aneurysm treated with a specific device successfully occlude over the long term). While the results are generally encouraging, there is poor standardization of study variables between different research groups, and any consensus will only be reached after standardized studies are conducted on collectively large datasets. Biochemical variables may have to be incorporated into these studies to maximize predictive values.

Protocol for Isolating the Mouse Circle of Willis

The cerebral arterial circle (circulus arteriosus cerebri) or circle of Willis (CoW) is a circulatory anastomosis surrounding the optic chiasma and hypothalamus that supplies blood to the brain and surrounding structures. It has been implicated in several cerebrovascular disorders, including cerebral amyloid angiopathy (CAA)-associated vasculopathies, intracranial atherosclerosis and intracranial aneurysms. Studies of the molecular mechanisms underlying these diseases for the identification of novel drug targets for their prevention require animal models. Some of these models may be transgenic, whereas others will involve isolation of the cerebro-vasculature, including the CoW.The method described here is suitable for CoW isolation in any mouse lineage and has considerable potential for screening (expression of genes, protein production, posttranslational protein modifications, secretome analysis, etc.) studies on the large vessels of the mouse cerebro-vasculature. It can also be used for ex vivo studies, by adapting the organ bath system developed for isolated mouse olfactory arteries.

Fractional modeling of viscoelasticity in 3D cerebral arteries and aneurysms

We develop efficient numerical methods for fractional order PDEs, and employ them to investigate viscoelastic constitutive laws for arterial wall mechanics. Recent simulations using one-dimensional models [1] have indicated that fractional order models may offer a more powerful alternative for modeling the arterial wall response, exhibiting reduced sensitivity to parametric uncertainties compared with the integer-calculus-based models. Here, we study three-dimensional (3D) fractional PDEs that naturally model the continuous relaxation properties of soft tissue, and for the first time employ them to simulate flow structure interactions for patient-specific brain aneurysms. To deal with the high memory requirements and in order to accelerate the numerical evaluation of hereditary integrals, we employ a fast convolution method [2] that reduces the memory cost to O(log(N)) and the computational complexity to O(N log(N)). Furthermore, we combine the fast convolution with high-order backward differentiation to achieve third-order time integration accuracy. We confirm that in 3D viscoelastic simulations, the integer order models strongly depends on the relaxation parameters, while the fractional order models are less sensitive. As an application to long-time simulations in complex geometries, we also apply the method to modeling fluid-structure interaction of a 3D patient-specific compliant cerebral artery with an aneurysm. Taken together, our findings demonstrate that fractional calculus can be employed effectively in modeling complex behavior of materials in realistic 3D time-dependent problems if properly designed efficient algorithms are employed to overcome the extra memory requirements and computational complexity associated with the non-local character of fractional derivatives.

Correlation Between Aneurysm Size and Hemodynamics in One Individual with Multiple Small Intracranial Aneurysms

Objective

A large number of cases are needed in the patient-specific modeling of intracranial aneurysms to establish the statistical significance due to individual variation of risk factors that are difficult to account for. However, these risk factors are critical in hemorrhage risk as demonstrated in large clinical studies. Rupture risks for aneurysms in an individual are easier to compare because these aneurysms are under the same physiological environment, and their only differences are the local hemodynamic factors associated with their anatomic locations.

Methods

Eight small aneurysms (< 7 mm) from one individual were analyzed using patient-specific hemodynamic modeling. Four scenarios with different perfusion assumptions were performed to account for the flow rate at two smaller communicating arteries. Wall shear stresses (WSS) at these aneurysms were compared to determine their relationship with the aneurysm size.

Results

Each of the three largest aneurysms is either the most proximal or distal aneurysm in a given artery so that blood pressure does not have a direct influence on aneurysm size. No wall shear stress-derived hemodynamic variables are found to be related to aneurysm size.

Discussion

A study of multiple aneurysms from one individual offers a unique opportunity to examine various hemodynamic factors without selection biases. Aneurysms greater than 4 mm (Group 1) have a higher product of maximum WSS and area of low WSS; aneurysms smaller than 4 mm (Group 2) have a lower product of maximum WSS and area of low WSS. In addition, aneurysm size is linearly correlated with the flow rate at the parent artery in each group.

In vitro investigation of contrast flow jet timing in patient-specific intracranial aneurysms

BACKGROUND

The direction and magnitude of intra-aneurysmal flow jet are significant risk factors of subarachnoid hemorrhage, and the change of flow jet during an endovascular procedure has been used for prediction of aneurysm occlusion or whether an additional flow diverter (FD) is warranted. However, evaluation of flow jets is often unreliable due to a large variation of flow jet on the digital subtraction angiograms, and this flow pattern variation may result in incorrect clinical diagnosis Therefore, factors contributing to the variation in flow jet are examined at an in vitro setting, and the findings can help us to understand the nature of flow jet and devise a better plan to quantify the aneurysmal hemodynamics accurately.

METHODS

Intra-aneurysmal flows in three patient-specific aneurysms between 11 and 25 mm were investigated in vitro, and a FD was deployed in each aneurysm model. X-ray imaging of these models were performed at injection rates between 0.2 and 2 mL/s. Pulsatile blood pump and aneurysm model were imaged together to determine the timing of flow jet.

RESULTS

The contrast bolus arrives at the aneurysm early at high contrast injection rates. The flow patterns with slow injection rates exhibit strong inertia that is associated with the systole flow. Flow jets arrive at the aneurysms at the peak systole when the bolus is injected at 0.2 mL/s. The contrast-to-signal ratio is the highest at the injection rate of 0.5 mL/s. Effect of flow diversion can only be assessed at an injection rate greater than 0.5 mL/s.

CONCLUSIONS

Intra-aneurysmal flow jet is highly dependent on the injection rate of the contrast agent. For the internal carotid artery (ICA) aneurysms, the systolic flows can be visualized at slow injection rates (<0.5 mL/s), while the diastolic flow jets are visible at higher injection rates (>1 mL/s). Dependence of flow jet on the contrast injection rate has serious clinical implications and needs to be considered during diagnostic procedures; a protocol with a consistent injection rate is highly recommended.

The Computational Fluid Dynamics Rupture Challenge 2013—Phase II: Variability of Hemodynamic Simulations in Two Intracranial Aneurysms

With the increased availability of computational resources, the past decade has seen a rise in the use of computational fluid dynamics (CFD) for medical applications. There has been an increase in the application of CFD to attempt to predict the rupture of intracranial aneurysms, however, while many hemodynamic parameters can be obtained from these computations, to date, no consistent methodology for the prediction of the rupture has been identified. One particular challenge to CFD is that many factors contribute to its accuracy; the mesh resolution and spatial/temporal discretization can alone contribute to a variation in accuracy. This failure to identify the importance of these factors and identify a methodology for the prediction of ruptures has limited the acceptance of CFD among physicians for rupture prediction. The International CFD Rupture Challenge 2013 seeks to comment on the sensitivity of these various CFD assumptions to predict the rupture by undertaking a comparison of the rupture and blood-flow predictions from a wide range of independent participants utilizing a range of CFD approaches. Twenty-six groups from 15 countries took part in the challenge. Participants were provided with surface models of two intracranial aneurysms and asked to carry out the corresponding hemodynamics simulations, free to choose their own mesh, solver, and temporal discretization. They were requested to submit velocity and pressure predictions along the centerline and on specified planes. The first phase of the challenge, described in a separate paper, was aimed at predicting which of the two aneurysms had previously ruptured and where the rupture site was located. The second phase, described in this paper, aims to assess the variability of the solutions and the sensitivity to the modeling assumptions. Participants were free to choose boundary conditions in the first phase, whereas they were prescribed in the second phase but all other CFD modeling parameters were not prescribed. In order to compare the computational results of one representative group with experimental results, steady-flow measurements using particle image velocimetry (PIV) were carried out in a silicone model of one of the provided aneurysms. Approximately 80% of the participating groups generated similar results. Both velocity and pressure computations were in good agreement with each other for cycle-averaged and peak-systolic predictions. Most apparent “outliers” (results that stand out of the collective) were observed to have underestimated velocity levels compared to the majority of solutions, but nevertheless identified comparable flow structures. In only two cases, the results deviate by over 35% from the mean solution of all the participants. Results of steady CFD simulations of the representative group and PIV experiments were in good agreement. The study demonstrated that while a range of numerical schemes, mesh resolution, and solvers was used, similar flow predictions were observed in the majority of cases. To further validate the computational results, it is suggested that time-dependent measurements should be conducted in the future. However, it is recognized that this study does not include the biological aspects of the aneurysm, which needs to be considered to be able to more precisely identify the specific rupture risk of an intracranial aneurysm.

Imaging of small animal peripheral artery disease models: recent advancements and translational potential

Peripheral artery disease (PAD) is a broad disorder encompassing multiple forms of arterial disease outside of the heart. As such, PAD development is a multifactorial process with a variety of manifestations. For example, aneurysms are pathological expansions of an artery that can lead to rupture, while ischemic atherosclerosis reduces blood flow, increasing the risk of claudication, poor wound healing, limb amputation, and stroke. Current PAD treatment is often ineffective or associated with serious risks, largely because these disorders are commonly undiagnosed or misdiagnosed. Active areas of research are focused on detecting and characterizing deleterious arterial changes at early stages using non-invasive imaging strategies, such as ultrasound, as well as emerging technologies like photoacoustic imaging. Earlier disease detection and characterization could improve interventional strategies, leading to better prognosis in PAD patients. While rodents are being used to investigate PAD pathophysiology, imaging of these animal models has been underutilized. This review focuses on structural and molecular information and disease progression revealed by recent imaging efforts of aortic, cerebral, and peripheral vascular disease models in mice, rats, and rabbits. Effective translation to humans involves better understanding of underlying PAD pathophysiology to develop novel therapeutics and apply non-invasive imaging techniques in the clinic.

Hemodynamics in growing and stable cerebral aneurysms

Objective

The detailed mechanisms of cerebral aneurysm evolution are poorly understood but are important for objective aneurysm evaluation and improved patient management. The purpose of this study was to identify hemodynamic conditions that may predispose aneurysms to growth.

Methods

A total of 33 intracranial unruptured aneurysms longitudinally followed with three-dimensional imaging were studied. Patient-specific computational fluid dynamics models were constructed and used to quantitatively characterize the hemodynamic environments of these aneurysms. Hemodynamic characteristics of growing (n=16) and stable (n=17) aneurysms were compared. Logistic regression statistical models were constructed to test the predictability of aneurysm growth by hemodynamic features.

Results

Growing aneurysms had significantly smaller shear rate ratios (p=0.01), higher concentration of wall shear stress (p=0.03), smaller vorticity ratios (p=0.01), and smaller viscous dissipation ratios (p=0.01) than stable aneurysms. They also tended to have larger areas under low wall shear stress (p=0.06) and larger aspect ratios (p=0.18), but these trends were not significant. Mean wall shear stress was not significantly different between growing and stable aneurysms. Logistic regression models based on hemodynamic variables were able to discriminate between growing and stable aneurysms with a high degree of accuracy (94–100%).

Conclusions

Growing aneurysms tend to have complex intrasaccular flow patterns that induce non-uniform wall shear stress distributions with areas of concentrated high wall shear stress and large areas of low wall shear stress. Statistical models based on hemodynamic features seem capable of discriminating between growing and stable aneurysms.

Prognostic value of premorbid hypertension and neurological status in aneurysmal subarachnoid hemorrhage: pooled analyses of individual patient data in the SAHIT repository

OBJECT

The literature has conflicting reports about the prognostic value of premorbid hypertension and neurological status in aneurysmal subarachnoid hemorrhage (SAH). The aim of this study was to investigate the prognostic value of premorbid hypertension and neurological status in the SAH International Trialists repository.

METHODS

Patient-level meta-analyses were conducted to investigate univariate associations between premorbid hypertension (6 studies; n = 7249), admission neurological status measured on the World Federation of Neurosurgical Societies (WFNS) scale (10 studies; n = 10,869), and 3-month Glasgow Outcome Scale (GOS) score. Multivariable analyses were performed to sequentially adjust for the effects of age, CT clot burden, aneurysm location, aneurysm size, and modality of aneurysm repair. Prognostic associations were estimated across the ordered categories of the GOS using proportional odds models. Nagelkerke's R(2) statistic was used to quantify the added prognostic value of hypertension and neurological status beyond those of the adjustment factors.

RESULTS

Premorbid hypertension was independently associated with poor outcome, with an unadjusted pooled odds ratio (OR) of 1.73 (95% confidence interval [CI] 1.50-2.00) and an adjusted OR of 1.38 (95% CI 1.25-1.53). Patients with a premorbid history of hypertension had higher rates of cardiovascular and renal comorbidities, poorer neurological status (p ≤ 0.001), and higher odds of neurological complications including cerebral infarctions, hydrocephalus, rebleeding, and delayed ischemic neurological deficits. Worsening neurological status was strongly independently associated with poor outcome, including WFNS Grades II (OR 1.85, 95% CI 1.68-2.03), III (OR 3.85, 95% CI 3.32-4.47), IV (OR 5.58, 95% CI 4.91-6.35), and V (OR 14.18, 95% CI 12.20-16.49). Neurological status had substantial added predictive value greater than the combined value of other prognostic factors (R(2) increase > 10%), while the added predictive value of hypertension was marginal (R(2) increase < 0.5%).

CONCLUSIONS

This study confirmed the strong prognostic effect of neurological status as measured on the WFNS scale and the independent but weak prognostic effect of premorbid hypertension. The effect of premorbid hypertension could involve multifactorial mechanisms, including an increase in the severity of initial bleeding, the rate of comorbid events, and neurological complications.