Hemodynamics of Focal Versus Global Growth of Small Cerebral Aneurysms

Modeling and evaluation of biomechanics and hemodynamic based on patient-specific small intracranial aneurysm using fluid-structure interaction

BACKGROUND AND OBJECTIVE

Rupture of small intracranial aneurysm (IA) often leads to the development of highly fatal clinical syndromes such as subarachnoid hemorrhage. Due to the patient specificity of small IA, there are many difficulties in evaluating the rupture risk of small IA such as multiple influencing factors, high clinical experience requirements and poor reusability.

METHODS

In this study, clinical methods such as transcranial doppler (TCD) and magnetic resonance imaging (MRI) are used to obtain patient-specific parameters, and the fluid-structure interaction method (FSI) is used to model and evaluate the biomechanics and hemodynamics of patient-specific small IA.

RESULTS

The results show that a spiral vortex stably exists in the patient-specific small IA. Due to the small size of the patient-specific small IA, the blood flow velocity still maintains a high value with maximum reaching 3 m/s. The inertial impact of blood flow and vortex convection have certain influence on hemodynamic and biomechanics parameters. They cause three high value areas of WSSM on the patient-specific small IA with maximum of 180 Pa, 130 Pa and 110 Pa, respectively. They also cause two types of WSS concentration points, positive normal stress peak value areas and negative normal stress peak value areas to appear.

CONCLUSION

This paper found that the factors affecting hemodynamic parameters and biomechanical parameters are different. Unlike hemodynamic parameters, biomechanical parameters are also affected by blood pressure in addition to blood flow velocity. This study reveals the relationship between the flow field distribution and changes of patient-specific small IA, biomechanics and hemodynamics.

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

OBJECTIVE

Configuration changes of the parent artery (PA) after flow-diverter (FD) stent reconstruction, caused by the bending force of the device, may have an additional role in aneurysm occlusion as a result of the secondary alteration of intra-aneurysmal hemodynamics related to the geometry alteration of the vessel. To determine the degree of PA deformation and aneurysm occlusion rates after deployment of 2 different types of FD.

METHODS

Patients treated with 2 different designs of cobalt-chromium braid (48 and 64 wire braid) structure FD were subject to analysis. Vascular angle changes at the level of the reconstructed segment immediately after FD deployment and at 1 year follow-up were measured and the potential relationship with aneurysmal occlusion rate was analyzed.

RESULTS

Forty-two patients harboring 48 aneurysms were included in the present study. The aneurysms were divided into side wall (85.4%) and bifurcation types (14.6%). Twenty-six aneurysms were treated using the Pipeline FD (48 wire braid; 54.2%) and 22 using the Evolve FD (64 wire braid; 45.8%). Of the 48 aneurysms, 42 (87.5%) met the primary end point of complete occlusion at 12 months. The median postdeployment angle change was 7.04°± 4.59° for the Pipeline and 5.05°± 2.49° for the Evolve, whereas the median 12 months follow-up angle change was 15.49°± 10.99° and 10.01°± 8.83°, respectively. PA angle changes were significantly higher in the bifurcation group compared with the side wall group both during procedure and at 12 months follow-up. Angle change had a statistically nonsignificant association with complete aneurysm occlusion.

CONCLUSIONS

PA deformation starts immediately after deployment and remodeling continues for 1 year after. Aneurysms located in the vessel bifurcation were more prone to PA straightening after FD deployment than were side wall aneurysms. Furthermore, Pipeline seemed to be more prone to inducing vascular deformation, compared with Evolve.

Numerical investigation of unruptured middle cerebral artery bifurcation aneurysms: influence of aspect ratio

An aneurysm is a disease condition, which is due to the pathological weakening of an arterial wall. These aneurysms are often found in various branch points and bifurcations of an artery in the cerebral circulation. Most aneurysms come to medical attention, either due to brain hemorrhages caused by rupture or found unruptured. To consider surgically invasive treatment modalities, clinicians need scientific methods such as, hemodynamic analysis to assess rupture risk. The arterial wall loses its structural integrity when wall shear stress (WSS) and other hemodynamic parameters exceed a certain threshold. In the present study, numerical simulations are carried out for unruptured middle cerebral artery (MCA) aneurysms. Three distinct representative sizes are chosen from a larger patient pool of 26 MCA aneurysms. Logically, these aneurysms represent three growth stages of any patient with similar anatomical structure. Simulations are performed to compare the three growth phases (with different aspect ratios) of an aneurysm and correlate their hemodynamic parameters. Simulations with patient specific boundary conditions reveal that, aneurysms with a higher aspect ratio (AR) correspond to an attendant decrease in both time-averaged wall shear stress (TAWSS) and spatial wall shear stress gradients (WSSG). Smaller MCAs were observed to have higher positive wall shear stress divergence (WSSD), exemplifying the tensile nature of arterial wall stretching. Present study identifies positive wall shear stress divergence (PWSSD) to be a potential biomarker for evaluating the growth of an aneurysm.

Minimum wall shear stress points and their underlying intra-aneurysmal flow structures of unruptured cerebral aneurysms on 4D flow MRI

BACKGROUND AND PURPOSE

Minimum wall shear stress (Min-WSS) points may be associated with wall instability of unruptured cerebral aneurysms. We aimed to investigate the relationship between the locations of Min-WSS points and their underlying intra-aneurysmal flow structure patterns in unruptured cerebral aneurysms using four-dimensional (4D) flow magnetic resonance imaging (MRI).

MATERIALS AND METHODS

Min-WSS points and the intra-aneurysmal flow structure patterns were identified in 50 unruptured aneurysms by 4D flow MRI.

RESULTS

The Min-WSS points were located around a vortex core tip in 31 (62.0%) aneurysms and on an intra-bleb vortex center in 7 (14.0%). Sixteen (32.0%) aneurysms had the Min-WSS points on the aneurysmal apex, and in 24 (48.0%) were on the neck. The Min-WSS values of aneurysms with the Min-WSS points on an intra-bleb flow were significantly lower than those of the other groups (P = 0.030). Aneurysms with the Min-WSS points on the neck had significantly higher Min-WSS values than the other aneurysms (P = 0.008).

CONCLUSIONS

The location of the Min-WSS point was corresponding to the vortex core or center in 76% of all aneurysms. The underlying intra-aneurysmal flow structure and location of the Min-WSS point affect the Min-WSS value. Further studies are needed to characterize Min-WSS points to identify aneurysms with a higher risk of wall instability.

Association of bleb formation with peri-aneurysmal contact in unruptured intracranial aneurysms

The mechanism of bleb formation in unruptured intracranial aneurysms (UIAs) remains unclear. This study aimed to investigate the association between peri-aneurysmal contact (PAC) and bleb formation. Forty-five aneurysms were classified depending on the presence of blebs and PAC using computed tomographic angiography and magnetic resonance imaging. Aneurysmal hemodynamics were assessed using computational fluid dynamics. The independent variables associated with bleb formation were statistically assessed. Fourteen aneurysms (31.1%) had blebs, all of which were located at the site of PAC (group A). Thirty-one aneurysms (68.9%) had no bleb, of which 13 had a PAC (group B) and 18 had no PAC (group C). PAC was the only independent variable associated with bleb formation (p < 0.05). Aneurysmal volumes were significantly higher in group A, followed by groups B and C in series. Aneurysmal wall shear stress (WSS) tended to be lowest in group A, followed by groups B and C in series. The maximum WSS at the blebs was only 17% of the maximum WSS at the aneurysmal domes. This study demonstrated that bleb formation in UIAs was associated with the establishment of PAC during their growth, which may have more detrimental effects on bleb formation than hemodynamics.

Underlying mechanism of hemodynamics and intracranial aneurysm

In modern society, subarachnoid hemorrhage, mostly caused by intracranial aneurysm rupture, is accompanied by high disability and mortality rate, which has become a major threat to human health. Till now, the etiology of intracranial aneurysm has not been entirely clarified. In recent years, more and more studies focus on the relationship between hemodynamics and intracranial aneurysm. Under the physiological condition, the mechanical force produced by the stable blood flow in the blood vessels keeps balance with the structure of the blood vessels. When the blood vessels are stimulated by the continuous abnormal blood flow, the functional structure of the blood vessels changes, which becomes the pathophysiological basis of the inflammation and atherosclerosis of the blood vessels and further promotes the occurrence and development of the intracranial aneurysm. This review will focus on the relationship between hemodynamics and intracranial aneurysms, will discuss the mechanism of occurrence and development of intracranial aneurysms, and will provide a new perspective for the research and treatment of intracranial aneurysms.

Moments of Intra-Dome Velocity Distribution as Robust Predictors of Rupture Status in Cerebral Aneurysms

BACKGROUND

Wall shear stress (WSS), the spatial gradient of flow velocity at luminal surface, has been employed for aneurysmal hemodynamic analysis, but it is sensitive to surface irregularities and noise. We devised a volumetric approach to evaluate discriminant power of intra-dome flow velocity distribution and modal analysis in rupture status determination compared with previously described WSS analysis.

METHODS

Catheter three-dimensional rotational angiographic datasets matched for volume were segmented in 20 sidewall aneurysms (10 ruptured), computational fluid dynamics simulations were performed, and velocity distributions were extracted from mesh-independent isometric sampling followed by moment analysis (mean, variance, skewness, and kurtosis). Univariate and multivariate analysis was used to evaluate discriminant performance of velocity moments. Sensitivity of velocity moments and WSS was evaluated to bleb presence and surface irregularity using digital bleb removal and surface noise addition.

RESULTS

Velocity moments of ruptured aneurysms showed higher skewness (2.45 ± 0.57 vs. 1.36 ± 0.82, P = 0.003) and kurtosis (11.83 ± 4.77 vs. 6.05 ± 4.65, P = 0.01) with lower mean (0.019 ± 0.01 vs. 0.038 ± 0.02, P = 0.03) compared with unruptured lesions; in multivariate modeling, skewness alone emerged as best predictor (area under the curve = 0.88). Bleb removal increased low WSS by 548%, and surface noise decreased it by 85.8% while having a smaller (<7%) effect on velocity skewness and kurtosis.

CONCLUSIONS

High aneurysm dome flow velocity skewness and kurtosis suggest an exponential distribution in ruptured lesions, with high peaks at low velocities, consistent with areas of slow flow. In contrast to WSS-based techniques, this approach is robust against surface variations, with promising improved rupture status discriminant performance that requires further validation in expanded future studies.

Hemodynamic changes after intracranial aneurysm growth

OBJECTIVE

For accurate risk assessment of unruptured intracranial aneurysms, it is important to understand the underlying mechanisms that lead to rupture. It is known that hemodynamic anomalies contribute to aneurysm growth and rupture, and that growing aneurysms carry higher rupture risks. However, it is unknown how growth affects hemodynamic characteristics. In this study, the authors assessed how hemodynamic characteristics change over the course of aneurysm growth.

METHODS

The authors included patients with observed aneurysm growth on longitudinal MRA in the period between 2012 and 2016. Patient-specific vascular models were created from baseline and follow-up images. Subsequently, intraaneurysmal hemodynamic characteristics were computed using computational fluid dynamics. The authors computed the normalized wall shear stress, oscillatory shear index, and low shear area to quantify hemodynamic characteristics. Differences between baseline and follow-up measurements were analyzed using paired t-tests.

RESULTS

Twenty-five patients with a total of 31 aneurysms were included. The aneurysm volume increased by a median (IQR) of 26 (9-39) mm3 after a mean follow-up period of 4 (range 0.4-10.9) years. The median wall shear stress decreased significantly after growth. Other hemodynamic parameters did not change significantly, although large individual changes with large variability were observed.

CONCLUSIONS

Hemodynamic characteristics change considerably after aneurysm growth. On average, wall shear stress values decrease after growth, but there is a large variability in hemodynamic changes between aneurysms.

Prediction of bleb formation in intracranial aneurysms using machine learning models based on aneurysm hemodynamics, geometry, location, and patient population

BACKGROUND

Bleb presence in intracranial aneurysms (IAs) is a known indication of instability and vulnerability.

OBJECTIVE

To develop and evaluate predictive models of bleb development in IAs based on hemodynamics, geometry, anatomical location, and patient population.

METHODS

Cross-sectional data (one time point) of 2395 IAs were used for training bleb formation models using machine learning (random forest, support vector machine, logistic regression, k-nearest neighbor, and bagging). Aneurysm hemodynamics and geometry were characterized using image-based computational fluid dynamics. A separate dataset with 266 aneurysms was used for model evaluation. Model performance was quantified by the area under the receiving operating characteristic curve (AUC), true positive rate (TPR), false positive rate (FPR), precision, and balanced accuracy.

RESULTS

The final model retained 18 variables, including hemodynamic, geometrical, location, multiplicity, and morphology parameters, and patient population. Generally, strong and concentrated inflow jets, high speed, complex and unstable flow patterns, and concentrated, oscillatory, and heterogeneous wall shear stress patterns together with larger, more elongated, and more distorted shapes were associated with bleb formation. The best performance on the validation set was achieved by the random forest model (AUC=0.82, TPR=91%, FPR=36%, misclassification error=27%).

CONCLUSIONS

Based on the premise that aneurysm characteristics prior to bleb formation resemble those derived from vascular reconstructions with their blebs virtually removed, machine learning models can identify aneurysms prone to bleb development with good accuracy. Pending further validation with longitudinal data, these models may prove valuable for assessing the propensity of IAs to progress to vulnerable states and potentially rupturing.

Hemodynamic factor evaluation using computational fluid dynamics analysis for de novo bleb formation in unruptured intracranial aneurysms

Background

Although bleb formation increases the risk of rupture of intracranial aneurysms, previous computational fluid dynamic (CFD) studies have been unable to identify robust causative hemodynamic factors, due to the morphological differences of prebleb aneurysm models and a small number of aneurysms with de novo bleb formation. This study investigated the influences of differences in the aneurysm-models and identify causative hemodynamic factors for de novo bleb formation.

Materials and methods

CFD analysis was conducted on three aneurysm models, actual prebleb, postbleb, and virtual prebleb models of two unruptured aneurysms with de novo bleb formation. A new multipoint method was introduced in this study. We evenly distributed points with a 0.5-mm distance on the aneurysm surface of the actual prebleb models (146 and 152 points in the individual aneurysm, respectively), and we statistically compared hemodynamics at the points in the areas with and without bleb formation (19 and 279 points, respectively).

Results

Visually, blebs formed on an aneurysm surface area with similar hemodynamic characteristics in the actual and virtual prebleb models. Statistical analysis using the multipoint method revealed that the de novo bleb formation area was significantly correlated with high pressure (p < 0.001), low wall shear stress (WSS) (p < 0.001), and the center of divergent WSS vectors (p = 0.025).

Conclusions

De novo bleb formation in intracranial aneurysms may occur in areas associated with the combination of high pressure, low WSS, and the center of divergent WSS vectors. The multipoint method is useful for statistical analysis of hemodynamics in a limited number of aneurysms.

Cerebral arterial architectonics and CFD simulation in mice with type 1 diabetes mellitus of different duration

Type 1 diabetes is a chronic autoimmune disease that affects tens of millions of people. Diabetes mellitus is one of the strongest factors in the development of cerebrovascular diseases. In this study we used NOD.CB17 Prkdcscid mice and the pharmacological model of type 1 diabetes mellitus of different duration to study changes in the cerebral vasculature. We used two combined approaches using magnetic resonance angiography both steady and transient CFD blood flow modeling. We identified the influence of type 1 diabetes on the architectonics and hemodynamics of the large blood vessels of the brain as the disease progresses. For the first time, we detected a statistically significant change in angioarchitectonics (the angles between the vessels of the circle of Willis, cross-sections areas of vessels) and hemodynamic (maximum blood flow rate, hydraulic resistance) in animals with diabetes duration of 2 months, that is manifested by the development of asymmetry of cerebral blood flow. The result shows the negative effect of diabetes on cerebral circulation as well as the practicability of CFD modeling. This may be of extensive interest, in pharmacological and preclinical studies.

A retrospective longitudinal study of age-related shifts and deformations in the basilar artery bifurcation

PURPOSE

Previous studies have indicated that cerebral arterial morphology is linked to aging and some cerebrovascular diseases. However, the mechanisms of morphological changes remain unclear. This study evaluated age-related positional changes in the basilar artery (BA) bifurcation based on longitudinal computed tomography angiography (CTA) data.

METHODS

This retrospective study evaluated clinical and imaging data from 72 subjects who underwent two CTA scans between July 2011 and August 2019. Three-dimensional (3D) models were reconstructed for each subject based on the two CTA scans with the longest separating interval. Skull landmarks were used to fuse the two models, and the fused model was used to evaluate positional changes in the BA bifurcation. Univariable and multivariable analyses were used to identify variables that were correlated to BA bifurcation shifting. Pearson's correlation test was used to analyze the correlation between the shifting distance and change in the BA bifurcation angle.

RESULTS

Significant differences between aneurysm and non-aneurysm cases were observed in terms of sex (p = 0.004), CTA scan interval (p = 0.023), and BA bifurcation shifting distance (p = 0.007). Multivariable linear regression analysis revealed that the BA bifurcation shifting distance was significantly correlated with the CTA scan interval (p = 0.038) and the presence of aneurysms (p < 0.001). Furthermore, the shifting distance was positively correlated with widening of the BA bifurcation angle (p = 0.002).

CONCLUSIONS

Aging-related widening of the BA bifurcation angle may be related to distal shifting of the BA bifurcation's position, and larger distal shifting of the BA bifurcation may be associated with the risk of aneurysm formation.

Modeling and Reducing the Effect of Geometric Uncertainties in Intracranial Aneurysms with Polynomial Chaos Expansion, Data Decomposition, and 4D-Flow MRI

PURPOSE

Variations in the vessel radius of segmented surfaces of intracranial aneurysms significantly influence the fluid velocities given by computer simulations. It is important to generate models that capture the effect of these variations in order to have a better interpretation of the numerically predicted hemodynamics. Also, it is highly relevant to develop methods that combine experimental observations with uncertainty modeling to get a closer approximation to the blood flow behavior.

METHODS

This work applies polynomial chaos expansion to model the effect of geometric uncertainties on the simulated fluid velocities of intracranial aneurysms. The radius of the vessel is defined as the uncertainty variable. Proper orthogonal decomposition is applied to characterize the solution space of fluid velocities. Next, a process of projecting the 4D-Flow MRI velocities on the basis vectors followed by coefficient mapping using generalized dynamic mode decomposition enables the merging of 4D-Flow MRI with the uncertainty propagated fluid velocities.

RESULTS

Polynomial chaos expansion propagates the fluid velocities with an error of 2% in velocity magnitude relative to computer simulations. Also, the bifurcation region (or impingement location) shows a standard deviation of 0.17 m/s (since an available reported variance in the vessel radius is adopted to model the uncertainty, the expected standard deviation may be different). Numerical phantom experiments indicate that the proposed approach reconstructs the fluid velocities with 0.3% relative error in presence of geometric uncertainties.

CONCLUSION

Polynomial chaos expansion is an effective approach to propagate the effect of the uncertainty variable in the blood flow velocities of intracranial aneurysms. Merging 4D-Flow MRI and uncertainty propagated fluid velocities leads to more realistic flow trends relative to ignoring the uncertainty in the vessel radius.

Analysis of hemodynamic changes from aneurysm inception to large sizes

While previous studies have identified many risk factors for the progression and rupture of cerebral aneurysms, the changes in aneurysm flow characteristics during its evolution are not fully understood. This work analyzes the changes in the aneurysm hemodynamic environment from its initial development to later stages when the aneurysm has substantially enlarged. A total of 88 aneurysms at four locations were studied with image based computational fluid dynamics (CFD). Two synthetic sequences representing the aneurysm geometry at three earlier stages were generated by shrinking the aneurysm sac while keeping the neck fixed or shrinking the neck simultaneously. The flow conditions were then quantitatively compared between these two modes of evolution. As aneurysms enlarged, the inflow rate increased in growing neck sequences, but decreased in fixed neck sequences. The inflow jet became more concentrated in both sequences. The mean aneurysm flow velocity and wall shear stress decreased in both sequences, but they decreased faster in enlarging aneurysms if the neck was fixed. Additionally, the intra-aneurysmal flows became more complex and more unstable, wall shear stress distribution became more oscillatory, and the area under low wall shear stress increased for both sequences. The evolution of flow characteristics of aneurysms with fixed and growing necks are different. The observed trends suggest that fixed neck aneurysms may evolve towards a flow environment characteristic of stable aneurysms faster than aneurysms with growing necks, which could also evolve towards a more disfavorable environment.

Hemodynamic conditions that favor bleb formation in cerebral aneurysms

BACKGROUND

Although it is generally believed that blebs represent weaker spots in the walls of intracranial aneurysms (IAs), it is largely unknown which aneurysm characteristics favor their development.

OBJECTIVE

To investigate possible associations between aneurysm hemodynamic and geometric characteristics and the development of blebs in intracranial aneurysms.

METHODS

A total of 270 IAs in 199 patients selected for surgical clipping were studied. Blebs were visually identified and interactively marked on patient-specific vascular models constructed from presurgical images. Blebs were then deleted from the vascular reconstruction to approximate the aneurysm before bleb formation. Computational fluid dynamics studies were performed in these models and in cases without blebs. Hemodynamic and geometric characteristics of aneurysms with and without blebs were compared.

RESULTS

A total of 173 aneurysms had no blebs, while 97 aneurysms had a total of 122 blebs. Aneurysms favoring bleb formation had stronger (p<0.0001) and more concentrated inflow jets (p<0.0001), higher flow velocity (p=0.0061), more complex (p<0.0001) and unstable (p=0.0157) flow patterns, larger maximum wall shear stress (WSS; p<0.0001), more concentrated (p=0.0005) and oscillatory (p=0.0004) WSS distribution, and a more heterogeneous WSS field (p<0.0001), than aneurysms without blebs. They were also larger (p<0.0001), more elongated (p<0.0001), had wider necks (p=0.0002), and more distorted and irregular shapes (p<0.0001).

CONCLUSIONS

Strong and concentrated inflow jets, high-speed, complex, and unstable flow patterns, and concentrated, oscillatory, and heterogeneous WSS patterns favor the formation of blebs in IAs. Blebs are more likely to form in large, elongated, and irregularly shaped aneurysms. These adverse characteristics could be considered signs of aneurysm instability when evaluating aneurysms for conservative observation or treatment.

Computational Hemodynamic Modeling of Arterial Aneurysms: A Mini-Review

Arterial aneurysms are pathological dilations of blood vessels, which can be of clinical concern due to thrombosis, dissection, or rupture. Aneurysms can form throughout the arterial system, including intracranial, thoracic, abdominal, visceral, peripheral, or coronary arteries. Currently, aneurysm diameter and expansion rates are the most commonly used metrics to assess rupture risk. Surgical or endovascular interventions are clinical treatment options, but are invasive and associated with risk for the patient. For aneurysms in locations where thrombosis is the primary concern, diameter is also used to determine the level of therapeutic anticoagulation, a treatment that increases the possibility of internal bleeding. Since simple diameter is often insufficient to reliably determine rupture and thrombosis risk, computational hemodynamic simulations are being developed to help assess when an intervention is warranted. Created from subject-specific data, computational models have the potential to be used to predict growth, dissection, rupture, and thrombus-formation risk based on hemodynamic parameters, including wall shear stress, oscillatory shear index, residence time, and anomalous blood flow patterns. Generally, endothelial damage and flow stagnation within aneurysms can lead to coagulation, inflammation, and the release of proteases, which alter extracellular matrix composition, increasing risk of rupture. In this review, we highlight recent work that investigates aneurysm geometry, model parameter assumptions, and other specific considerations that influence computational aneurysm simulations. By highlighting modeling validation and verification approaches, we hope to inspire future computational efforts aimed at improving our understanding of aneurysm pathology and treatment risk stratification.

Longitudinal computational fluid dynamics study of stenosis and aneurysmal degeneration of an aortorenal bypass

Saphenous vein graft (SVG) bypass placement is regarded as the optimal option for renal artery stenosis, which usually causes secondary hypertension and poor renal perfusion. Using computational fluid dynamics, this study aimed to investigate the underlying hemodynamic mechanism of the vein aneurysm and stenosis after aortorenal bypass surgery. Three-dimensional models were reconstructed based on computed tomographic angiography images of a 20-year-old female patient who suffered from uncontrollable hypertension using the image processing package Mimics (Materialise). The morphology and hemodynamic parameters in the healthy state, at initial presentation and at post-operative 9-month and 2-year follow-ups after surgery were analysed. The hemodynamic parameters became normal in the left and right renal arteries after bypass surgery. However, flow separation and stagnation occurred at the post-operative 9-month aorta-vein anastomosis, which caused asymmetrical flow and extremely high wall shear stress (WSS) and WSS gradients at the outflow vein tract, where the stenosis occurred 2 years later. In addition, the graft bending produced an asymmetrical flow pattern downstream. This research revealed that the abnormal hemodynamics, including flow separation and extremely high WSS values and gradients, caused by the retrograde flow of aortorenal bypass may be responsible for the SVG degeneration. In addition, flow asymmetry due to vessel bending is a potential risk factor for SVG aneurysm dilation.

Sex-related differences in wall remodeling and intraluminal thrombus resolution in a rat saccular aneurysm model

OBJECTIVE

Intracranial aneurysms (IAs) are more often diagnosed in women. Hormones and vessel geometry, which influences wall shear stress, may affect pathophysiological processes of the arterial wall. Here, the authors investigated sex-related differences in the remodeling of the aneurysm wall and in intraluminal thrombus resolution.

METHODS

A well-characterized surgical side-wall aneurysm model was used in female, male, and ovariectomized rats. Decellularized grafts were used to model highly degenerated and decellularized IA walls and native grafts to model healthy IA walls. Aneurysm growth and thrombus composition were analyzed at 1, 7, 14, and 28 days. Sex-related differences in vessel wall remodeling were compared with human IA dome samples of men and pre- and postmenopausal women.

RESULTS

At 28 days, more aneurysm growth was observed in ovariectomized rats than in males or non-ovariectomized female rats. The parent artery size was larger in male rats than in female or ovariectomized rats, as expected. Wall inflammation increased over time in all groups and was most severe in the decellularized female and ovariectomized groups at 28 days compared with the male group. Likewise, in these groups the most elastin fragmentation was seen at 28 days. In female rats, on days 1, 7, and 14, the intraluminal thrombus was mainly composed of red blood cells and fibrin. On days 14 and 28, macrophage and smooth muscle cell invasion inside the thrombus was shown, leading to the removal of red blood cells and deposition of collagen and elastin. On days 14 and 28, similar profiles of thrombus reorganization were observed in male and ovariectomized female rats. However, collagen content in thrombi and vessel wall macrophage content were higher in aneurysms of male rats at 28 days than in those of female rats. On day 28, thrombus coverage by endothelial cells was lower in ovariectomized than in female or male rats. Finally, analysis of human IA domes showed that endothelial cell coverage was lower in men and postmenopausal women than in younger women.

CONCLUSIONS

Aneurysm growth and intraluminal thrombus resolution show sex-dependent differences. While certain processes (endothelial cell coverage and collagen deposition) point to a strong hormonal dependence, others (wall inflammation and aneurysm growth) seem to be influenced by both hormones and parent artery size.

Hemodynamic Characteristics Associated With Paraclinoid Aneurysm Recurrence in Patients After Embolization

Objective: To investigate the hemodynamic features before and after embolization of paraclinoidal aneurysms using hemodynamic numerical simulation and the influence of embolization on recurrence after embolization.

Methods: From January 2016 to December 2017, we enrolled a total of 113 paraclinoidal aneurysms treated with embolization. They were divided into recurrent group and stable group depending on follow-up results. An aneurysm model was generated based on 3D-DSA before and after embolization. The hemodynamic characteristics were analyzed between two groups using Computational fluid dynamic (CFD).

Results: In the recurrent group, the peak systolic WSS, OSI and velocity around the aneurysm neck areas prior to embolization were 20.47 ± 3.04 Pa, 0.06 ± 0.02 and 0.07 ± 0.03 m/s, respectively. These values were 23.50 ± 4.11 Pa, 0.06 ± 0.01 and 0.11 ± 0.02 m/s, respectively in the stable group (P > 0.05). The WSS, OSI, velocity around the same areas in the recurrent group after embolization were 35.59 ± 8.75 Pa, 0.07 ± 0.02 and 0.12 ± 0.03 m/s, respectively (P < 0.01). In the stable group, the WSS, OSI and velocity were 13.08 ± 2.89 Pa, 0.04 ± 0.01 and 0.07 ± 0.02 m/s, respectively (P < 0.01). After embolization, the WSS, OSI and velocity around the aneurysm neck areas in the recurrent group were significantly higher than those in the stable group.

Conclusions: High peak systolic WSS, OSI and velocity around aneurysm neck areas after embolization of paraclinoidal aneurysms may be important factors leading to recurrence.

Inflammatory Smooth Muscle Cells Induce Endothelial Cell Alterations to Influence Cerebral Aneurysm Progression via Regulation of Integrin and VEGF Expression

Cerebral aneurysm growth is characterized by vessel wall frailness, although the underlying cellular mechanisms are unclear. Here, we examined the relationship between inflammatory smooth muscle cells (SMCs) and endothelial cells (ECs) in cerebral aneurysms, including the mechanisms underlying inflammatory SMC-induced changes in ECs. Five saccular cerebral aneurysms were collected and five temporal artery samples were used as controls. Cells and cytokines were detected by immunohistochemistry and TUNEL (transferase dUTP nick end labeling) assays performed to evaluate apoptosis. Human umbilical vein endothelial cells (HUVECs) were seeded on collagen I, IV, and VI-coated plates for cell adhesion assays and inflammatory SMCs (iSMCs) were established by culture in flexible silicone chambers subjected to cyclic mechanical stretch. HUVECs were cultured in iSMC-conditioned medium, followed by evaluation of their viability, apoptosis, and function, and determination of VEGF (vascular endothelial growth factor) -A and integrin levels by western blotting. Aneurysm tissue contained fewer SMCs and lacked ECs. In aneurysm walls, more matrix metalloproteinase (MMP) -1, MMP-3, and apoptotic cells were detected, accompanied by decreased collagen IV and VI levels. Cell adhesion assays revealed that more HUVECs were attached in collagen IV and VI-coated plates compared with controls. iSMC-conditioned medium significantly reduced HUVEC viability and apoptosis showed an increased trend; however, the difference was not significant. iSMC medium also reduced tube formation and migration of HUVECs. Moreover, iSMC medium reduced HUVEC expression of VEGF-A, integrin α1, integrin α2, and integrin β. Our data demonstrate a lack of SMCs and ECs in aneurysm walls, accompanied by elevated MMP and decreased collagen levels. In vitro assays showed that iSMCs induced reduction in EC adhesion, and caused EC dysfunction. Understanding of the relationships among SMC, EC, and collagens during aneurysm progression provides an additional therapeutic option for prevention of cerebral aneurysm progression.