Numerical modeling of the flow in intracranial aneurysms: prediction of regions prone to thrombus formation

Hemodynamic alterations of flow diverters on aneurysms at the fetal posterior communicating artery: A simulation study using CFD to compare the surpass streamline, pipeline flex, and tubridge devices

PURPOSE

Traditional flow diverters (FDs) for treating aneurysms at the fetal posterior communicating artery are unsatisfactory. Surpass Streamline is a novel FD with different mesh characteristics; however, the outcomes for such aneurysms remain unclear. This study aimed to compare hemodynamic alterations induced by Surpass Streamline, Pipeline Flex, and Tubridge devices and explore possible strategies for aneurysms at the fetal posterior communicating artery.

METHODS

Two simulated aneurysms (Case 1, Case 2) were constructed from digital subtraction angiography (DSA). The three FDs were virtually deployed, and hemodynamic analysis based on computational fluid dynamics was performed. Hemodynamic parameters, including the sac-averaged velocity magnitude (Velocity), high-flow volume (HFV), and wall shear stress (WSS), were compared between each FD and the untreated model (control). Surpass Streamline was performed in real life for two aneurysms and the clinical outcomes were collected for analysis.

RESULTS

Compared to the control, the Surpass resulted in the most significant reduction in flow. In Case 1, the Velocity, HFV, and WSS were reduced by 51.6%, 78.1%, and 64.3%, respectively. In Case 2, the Velocity, HFV, and WSS were reduced by 48.0%, 81.1%, and 65.3%, respectively. Tubridge showed slightly larger changes in hemodynamic parameters than Pipeline. In addition, our analysis suggested that metal coverage was correlated with the WSS, Velocity, and HFV. The postoperative DSA showed that the aneurysm was nearly occluded in Case 1 and decreased in Case 2.

CONCLUSION

Compared to that with the Pipeline and Tubridge, the Surpass resulted in the greatest reduction in hemodynamic parameters and might be effective for aneurysms at the fetal posterior communicating artery. Virtual FD deployment and computational fluid dynamics analysis may be used to predict the treatment outcomes.

Inflow Angle and Height-Width Ratio are Predictors of Incomplete Occlusion at One and Two Years After Flow Diverter Treatment for Small- and Medium-Sized Internal Carotid Artery Aneurysms

OBJECTIVE

We investigated the association between the inflow angle of aneurysms and their occlusion status at 1 and 2 years after flow diverter (FD) treatment.

METHODS

We retrospectively analyzed 42 consecutive patients from a single center with 43 untreated, unruptured internal carotid artery (ICA) proximal to communicating segment, saccular aneurysms sized <12 mm.

RESULTS

At 1 year posttreatment, the complete occlusion (CO) rate was 58.1%. On univariate analyses, the proportion of inflow angle >90° was significantly lower in the CO group than in the incomplete occlusion group (20.0% VS. 83.3%; P < 0.001). The CO incidence decreased with a height-width (H/W) ratio of <1.2 (P = 0.059). On multivariate analysis, an H/W ratio of <1.2 (odds ratio [OR], 0.076; P = 0.027) and an inflow angle of >90° (OR, 0.020; P = 0.0011) significantly influenced CO at 1 year post FD. At 2 years posttreatment, the CO rate was 76.3% (29/38 cases with available follow-up data). On univariate analyses, in the CO group compared to the incomplete occlusion group, the proportion of H/W ratio <1.2 was significantly lower (P = 0.005) and the proportion of inflow angle >90° was significantly lower (P = 0.021); aneurysm dome size tended to be larger (8.5 mm vs. 7.1 mm; P = 0.080). On multivariate analysis, an H/W ratio <1.2 (OR, 0.042; P = 0.015) and an inflow angle >90° (OR: 0.088; P = 0.031) significantly influenced CO at 2 years post FD.

CONCLUSIONS

The inflow angle of >90° and H/W ratio <1.2 may significantly influence the CO rate in small- or medium-sized internal carotid artery aneurysms 1 and 2 years post FD.

Clot embolization studies and computational framework for embolization in a canonical tube model

Despite recent advances in the development of computational methods of modeling thrombosis, relatively little effort has been made in developing methods of modeling blood clot embolization. Such a model would provide substantially greater understanding of the mechanics of embolization, as in-vitro and in-vivo characterization of embolization is difficult. Here, a method of computationally simulating embolization is developed. Experiments are performed of blood clots formed in a polycarbonate tube, where phosphate-buffered saline is run through the tube at increasing flow rates until the clot embolizes. The experiments revealed embolization can be initiated by leading edge and trailing edge detachment or by non-uniform detachment. Stress-relaxation experiments are also performed to establish values of constitutive parameters for subsequent simulations. The embolization in the tube is reproduced in silico using a multiphase volume-of-fluid approach, where the clot is modeled as viscoelastic. By varying the constitutive parameters at the wall, embolization can be reproduced in-silico at varying flow rates, and a range of constitutive parameters fitting the experiments is reported. Here, the leading edge embolization is simulated at flow rates consistent with the experiments demonstrating excellent agreement in this specific behavior.

Aortic hemodynamic and morphological analysis before and after repair of thoracoabdominal aortic aneurysm using a G-Branch endograft

Background and objective: The G-Branch endograft is a novel multibranched "off-the-shelf" device used to repair thoracoabdominal aortic aneurysms (TAAAs). This report describes the hemodynamic and morphological performance of the G-Branch endograft in a human patient with TAAA. Materials and methods: We retrospectively reviewed the computed tomography angiography scans and clinical data of a woman in whom TAAA was treated using a G-Branch endograft. Patient-specific three-dimensional models were reconstructed, and computational fluid dynamics and morphological and hemodynamic indicators were analyzed before and after implantation of the device. Results: From a morphological perspective, there was an increase in cross-sectional area in the G-Branch endograft and all bridging stent grafts over time. Blood flow was redistributed among the renovisceral arteries, with a decrease in flow rate in the celiac artery and an increase in the left renal artery. Laminar blood flow was smoother and more rapid after implantation of the G-Branch device and remained stable during follow-up. In the bridging stent grafts, flow recirculation zones were found in the bridging zones of the celiac artery and superior mesenteric artery as well as the distal sealing zones of both renal arteries. Furthermore, higher time-averaged wall shear stress and a lower oscillatory index and relative resident time were found in the G-Branch endograft and bridging stent grafts. Quantitative analysis showed obvious reduction in the surface area ratio of the elevated time-averaged wall shear stress area and surface area ratio of the relative resident time after G-branch implantation. Conclusion: The revascularization of branch vessels occurred following G-branch implantation, with improvements arising not only from morphological changes but also from hemodynamic alterations. The long-term performance of the G-Branch endograft needs further investigation and clinical validation.

Automatic 4D Flow MRI Segmentation Using the Standardized Difference of Means Velocity

We present a method to automatically segment 4D flow magnetic resonance imaging (MRI) by identifying net flow effects using the standardized difference of means (SDM) velocity. The SDM velocity quantifies the ratio between the net flow and observed flow pulsatility in each voxel. Vessel segmentation is performed using an F-test, identifying voxels with significantly higher SDM velocity values than background voxels. We compare the SDM segmentation algorithm against pseudo-complex difference (PCD) intensity segmentation of 4D flow measurements in in vitro cerebral aneurysm models and 10 in vitro Circle of Willis (CoW) datasets. We also compared the SDM algorithm to convolutional neural network (CNN) segmentation in 5 thoracic vasculature datasets. The in vitro flow phantom geometry is known, while the ground truth geometries for the CoW and thoracic aortas are derived from high-resolution time-of-flight (TOF) magnetic resonance angiography and manual segmentation, respectively. The SDM algorithm demonstrates greater robustness than PCD and CNN approaches and can be applied to 4D flow data from other vascular territories. The SDM to PCD comparison demonstrated an approximate 48% increase in sensitivity in vitro and 70% increase in the CoW, respectively; the SDM and CNN sensitivities were similar. The vessel surface derived from the SDM method was 46% closer to the in vitro surfaces and 72% closer to the in vitro TOF surfaces than the PCD approach. The SDM and CNN approaches both accurately identify vessel surfaces. The SDM algorithm is a repeatable segmentation method, enabling reliable computation of hemodynamic metrics associated with cardiovascular disease.

Correct Closure of the Left Atrial Appendage Reduces Stagnant Blood Flow and the Risk of Thrombus Formation: A Proof-of-Concept Experimental Study Using 4D Flow Magnetic Resonance Imaging

OBJECTIVE

The study was conducted to investigate the effect of correct occlusion of the left atrial appendage (LAA) on intracardiac blood flow and thrombus formation in patients with atrial fibrillation (AF) using four-dimensional (4D) flow magnetic resonance imaging (MRI) and three-dimensional (3D)-printed phantoms.

MATERIALS AND METHODS

Three life-sized 3D-printed left atrium (LA) phantoms, including a pre-occlusion (i.e., before the occlusion procedure) model and correctly and incorrectly occluded post-procedural models, were constructed based on cardiac computed tomography images from an 86-year-old male with long-standing persistent AF. A custom-made closed-loop flow circuit was set up, and pulsatile simulated pulmonary venous flow was delivered by a pump. 4D flow MRI was performed using a 3T scanner, and the images were analyzed using MATLAB-based software (R2020b; Mathworks). Flow metrics associated with blood stasis and thrombogenicity, such as the volume of stasis defined by the velocity threshold (|V̅| < 3 cm/s), surface-and-time-averaged wall shear stress (WSS), and endothelial cell activation potential (ECAP), were analyzed and compared among the three LA phantom models.

RESULTS

Different spatial distributions, orientations, and magnitudes of LA flow were directly visualized within the three LA phantoms using 4D flow MRI. The time-averaged volume and its ratio to the corresponding entire volume of LA flow stasis were consistently reduced in the correctly occluded model (70.82 mL and 39.0%, respectively), followed by the incorrectly occluded (73.17 mL and 39.0%, respectively) and pre-occlusion (79.11 mL and 39.7%, respectively) models. The surface-and-time-averaged WSS and ECAP were also lowest in the correctly occluded model (0.048 Pa and 4.004 Pa^-1 , respectively), followed by the incorrectly occluded (0.059 Pa and 4.792 Pa^-1 , respectively) and pre-occlusion (0.072 Pa and 5.861 Pa^-1 , respectively) models.

CONCLUSION

These findings suggest that a correctly occluded LAA leads to the greatest reduction in LA flow stasis and thrombogenicity, presenting a tentative procedural goal to maximize clinical benefits in patients with AF.

Study and modeling of the thrombosis of small cerebral aneurysms, with and without flow diverter, by the lattice Boltzmann method

BACKGROUND AND OBJECTIVE

Small cerebral aneurysms are currently commonly treated non-invasively by flow diverter device. These stents lead to thrombotic occlusion of the aneurysm soon after their placement. The purpose of this work is to model clotting into intracranial aneurysms with and without stents, using a non-Newtonian of blood behavior, and to investigate the importance of stent to generate desired thrombus in intracranial aneurysms.

METHOD

The description of blood flow is made by the Boltzmann lattice equations, while thrombosis is modeled by the "fluid age" model. The lattice Boltzmann method is a computational technique for simulating fluid dynamics. The method is based on a mesoscopic approach, where the fluid is represented by a set of particles that move and interact on a grid. The model for blood coagulation is described by lattice Boltzmann Method, and it doesn't take into account the complicated coagulation pathway, this main idea is developed using the model of residence time of blood: all fluid in the domain is assumed to be capable of clotting, given enough time. The fluid age is measured by a passive scalar using a transport equation, and the node coagulates if the fluid age increases enough. Three small aneurysms of different sizes and shapes with three stents of various porosities were used to test the ability of the model to predict thrombosis. The "occlusion rate" parameter is used to assess the effectiveness of the flow diverter device.

RESULTS

For the large aspect ratio factor, the occlusion is: 91% for flow diverter devise with seven struts. For medium aspect ratio, a rate of 80% is achieved. An occlusion rate of slightly more than 30% is obtained for very small aneurysms with low aspect ratio. The Newtonian model underestimates the volume of thrombosis generated. The difference in the prediction of the thrombosis volume between the Newtonian and no-Newtonian Carreau-Yasuda models is approximately 10%.

CONCLUSION

The occlusion rate is proportional to the aspect ratio form factor. For the large and medium aspect ratio factors, the occlusion is satisfactory. Concerning very small aneurysms with low aspect ratio, aneurysm occlusion is low. This rate can be improved to almost complete occlusion if the flow diverter device is doubled. The generality of the model suggests its extensibility toward any other type of thrombosis (stenosis, thrombosis in aortic aneurysms).

Hemodynamic characteristics in a cerebral aneurysm model using non-Newtonian blood analogues

This study aims to develop an experimentally validated computational fluid dynamics (CFD) model to estimate hemodynamic characteristics in cerebral aneurysms (CAs) using non-Newtonian blood analogues. Blood viscosities varying with shear rates were measured under four temperatures first, which serves as the reference for the generation of blood analogues. Using the blood analogue, particle image velocimetry (PIV) measurements were conducted to quantify flow characteristics in a CA model. Then, using the identical blood properties in the experiment, CFD simulations were executed to quantify the flow patterns, which were used to compare with the PIV counterpart. Additionally, hemodynamic characteristics in the simplified Newtonian and non-Newtonian models were quantified and compared using the experimentally validated CFD model. Results showed the proposed non-Newtonian viscosity model can predict blood shear-thinning properties accurately under varying temperatures and shear rates. Another developed viscosity model based on the blood analogue can well represent blood rheological properties. The comparisons in flow characteristics show good agreements between PIV and CFD, demonstrating the developed CFD model is qualified to investigate hemodynamic factors within CAs. Furthermore, results show the differences of absolute values were insignificant between Newtonian and non-Newtonian fluids in the distributions of wall shear stress (WSS) and oscillatory shear index (OSI) on arterial walls. However, not only does the simplified Newtonian model underestimate WSS and OSI in most regions of the aneurysmal sac, but it also makes mistakes in identifying the high OSI regions on the sac surface, which may mislead the hemodynamic assessment on the pathophysiology of CAs.

Challenges in Modeling Hemodynamics in Cerebral Aneurysms Related to Arteriovenous Malformations

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Left atrial appendage occlusion in ventricular assist device patients to decrease thromboembolic events: A computer simulation study

Atrial fibrillation (AF) is a common comorbidity in left ventricular assist device (LVAD) patients and has been identified as a risk factor for thromboembolic stroke. Blood stagnation within the left atrial appendage (LAA) is considered a possible major source of thrombosis and clinical studies have shown reduced thromboembolic risk after LAA occlusion (LAAO). Therefore, this study aims to investigate the effect of LAAO on thrombosis-related parameters using patient-specific simulations. Left ventricular and left atrial geometries of an LVAD patient were obtained from computed tomography and combined with hemodynamic data with either sinus rhythm (SR) or AF generated by a lumped parameter model. In four simulations applying contractile walls, stagnation volume and blood residence times were evaluated with or without AF and with or without LAAO. Reduced atrial contraction in AF resulted in unfavorable flow dynamics within the left atrium. The average atrial velocity was lower for the AF simulation when compared to SR, resulting in a 55% increase in the atrial stagnation volume (from 4.2 to 6.5 cm³). Moreover, blood remained in the LAA for more than 8 cardiac cycles. After LAAO the atrial stagnation decreased from 4.2 to 1.4 cm³ for SR and from 6.5 to 2.3 cm³ for the AF simulation. A significant stagnation volume was found in the LAA for both SR and AF, with larger values occurring with AF. These regions are known as potential sources for thrombus formation and can be diminished by LAAO. This significantly improved the thrombus-related flow parameters and may also lower the risk of thromboembolic events from the appendage.

Hemodynamic Comparison of Treatment Strategies for Intracranial Vertebral Artery Fusiform Aneurysms

Objective

This study comparatively analyzed the hemodynamic changes resulting from various simulated stent-assisted embolization treatments to explore an optimal treatment strategy for intracranial vertebral artery fusiform aneurysms. An actual vertebral fusiform aneurysm case treated by large coil post-stenting (PLCS) was used as a control.

Materials and Methods

A single case of an intracranial vertebral artery fusiform aneurysm underwent a preoperative and eight postoperative finite element treatment simulations: PLCS [single and dual Low-profile Visualized Intraluminal Support (LVIS)], Jailing technique (single and dual LVIS both simulated twice, Pipeline Embolization Device (PED) with or without large coils (LCs). Qualitative and quantitative assessments were performed to analyze the most common hemodynamic risk factors for recurrence.

Results

Jailing technique and PED-only had a high residual flow volume (RFV) and wall shear stress (WSS) on the large curvature of the blood flow impingement region. Quantitative analysis determined that PLSC and PED had a lower RFV compared to preoperative than did the jailing technique [PED+LC 2.46% < PLCS 1.2 (dual LVIS) 4.75% < PLCS 1.1 (single LVIS) 6.34% < PED 6.58% < Jailing 2.2 12.45% < Jailing 1.2 12.71% < Jailing 1.1 14.28% < Jailing 2.1 16.44%]. The sac-averaged flow velocity treated by PLCS, PED and PED+LC compared to preoperatively was significantly lower than the jailing technique [PED+LC = PLCS 1.2 (dual LVIS) 17.5% < PLCS 1.1 (single LVIS) = PED 27.5% < Jailing 1.2 = Jailing 2.2 32.5% < Jailing 1.1 37.5% < Jailing 2.1 40%]. The sac-averaged WSS for the PLCS 1.2 (dual LVIS) model was lower than the PED+LC, while the high WSS area of the Jailing 1 model was larger than for Jailing 2 [PLCS 1.2 38.94% (dual LVIS) < PED+LC 41% < PLCS 1.1 43.36% (single LVIS) < PED 45.23% < Jailing 2.1 47.49% < Jailing 2.2 47.79% < Jailing 1.1 48.97% < Jailing 1.2 49.85%].

Conclusions

For fusiform aneurysms, post large coil stenting can provide a uniform coil configuration potentially reducing the hemodynamic risk factors of recurrence. Flow diverters also may reduce the recurrence risk, with long-term follow-up required, especially to monitor branch blood flow to prevent postoperative ischemia.

Computational fluid dynamics (CFD) analysis in a ruptured vertebral artery dissecting aneurysm implanted by Pipeline when recurrent after LVIS-assisted coiling treatment: Case report and review of the literatures

BACKGROUNDS

Hemodynamics plays an important role in the natural history of the process of rupture and recurrence of intracranial aneurysms. This study aimed to investigate the role of hemodynamics for recurrence in a vertebral artery dissecting aneurysm (VADA).

METHODS

A patient with a ruptured VADA firstly treated by low-profile visualized intraluminal support (LVIS)-assisted coiling, and was implanted with a Pipeline Embolization Device (PED) after aneurysm recurrence. Finite element analysis and computational fluid dynamics simulations were conducted in 6 serial imaging procedures, and the calculated hemodynamics was correlated with aneurysm recurrence.

RESULTS

Wall shear stress (WSS) was not effectively suppressed, resulting in aneurysm recurrence with initial entry tear to occur above the protuberance after 7 months of LVIS stent-assisted coiling. With the implantation of PED, WSS, inflow stream and velocity at the aneurysm neck significantly decreased. During the 3-month follow-up after PED deployment, there was significant shrinkage of the sac and the blood flow in the sac was reduced considerably. The 27-month follow-up after PED deployment indicated the aneurysm was stable.

CONCLUSIONS

The present case study suggests that insufficient suppression of high WSS and high inflow velocity at the neck of the parent artery, especially near the posterior inferior cerebellar artery, might be associated with aneurysm recurrence.

Computational Investigation of Anastomosis Options of a Right-Heart Pump to Patient Specific Pulmonary Arteries

Patients with Fontan circulation have increased risk of heart failure, but are not always candidates for heart transplant, leading to the development of the subpulmonic Penn State Fontan Circulation Assist Device. The aim of this study was to use patient-specific computational fluid dynamics simulations to evaluate anastomosis options for implanting this device. Simulations were performed of the pre-surgical anatomy as well as four surgical options: a T-junction and three Y-grafts. Cases were evaluated based on several fluid-dynamic quantities. The impact of imbalanced left-right pulmonary flow distribution was also investigated. Results showed that a 12-mm Y-graft was the most energy efficient. However, an 8-mm graft showed more favorable wall shear stress distribution, indicating lower risk of thrombosis and endothelial damage. The 8-mm Y-grafts also showed a more balanced pulmonary flow split, and lower residence time, also indicating lower thrombosis risk. The relative performance of the surgical options was largely unchanged whether or not the pulmonary vascular resistance remained imbalanced post-implantation.

Influence of postural changes on haemodynamics in internal carotid artery bifurcation aneurysm using numerical methods

Cerebral intracranial aneurysms are serious problems that can lead to stroke, coma, and even death. The effect of blood flow on cerebral aneurysms and their relationship with rupture are unknown. In addition, postural changes and their relevance to haemodynamics of blood flow are difficult to measure in vivo using clinical imaging alone. Computational simulations investigating the detailed haemodynamics in cerebral aneurysms have been developed in recent times not only to understand the progression and rupture but also for clinical evaluation and treatment. In the present study, the haemodynamics of a patient-specific case of a large aneurysm on the left side internal carotid bifurcation (LICA) and no aneurysm on the right side internal carotid bifurcation (RICA) was investigated. The simulation of these patient-specific models using fluid–structure interaction provides a valuable comparison of flow behavior between normal and aneurysm models. The influences of postural changes were investigated during standing, sleeping, and head-down (HD) position. Significant changes in flow were observed during the HD position and quit high arterial blood pressure in the internal carotid artery (ICA) aneurysm model was established when compared to the normal ICA model. The velocity increased abruptly during the HD position by more than four times (LICA and RICA) and wall shear stress by four times (LICA) to ten times (RICA). The complex spiral flow and higher pressures prevailing within the dome increase the risk of aneurysm rupture.

An integrated fluid-structure interaction and thrombosis model for type B aortic dissection

False lumen thrombosis (FLT) in type B aortic dissection has been associated with the progression of dissection and treatment outcome. Existing computational models mostly assume rigid wall behavior which ignores the effect of flap motion on flow and thrombus formation within the FL. In this study, we have combined a fully coupled fluid–structure interaction (FSI) approach with a shear-driven thrombosis model described by a series of convection–diffusion reaction equations. The integrated FSI-thrombosis model has been applied to an idealized dissection geometry to investigate the interaction between vessel wall motion and growing thrombus. Our simulation results show that wall compliance and flap motion can influence the progression of FLT. The main difference between the rigid and FSI models is the continuous development of vortices near the tears caused by drastic flap motion up to 4.45 mm. Flap-induced high shear stress and shear rates around tears help to transport activated platelets further to the neighboring region, thus speeding up thrombus formation during the accelerated phase in the FSI models. Reducing flap mobility by increasing the Young’s modulus of the flap slows down the thrombus growth. Compared to the rigid model, the predicted thrombus volume is 25% larger using the FSI-thrombosis model with a relatively mobile flap. Furthermore, our FSI-thrombosis model can capture the gradual effect of thrombus growth on the flow field, leading to flow obstruction in the FL, increased blood viscosity and reduced flap motion. This model is a step closer toward simulating realistic thrombus growth in aortic dissection, by taking into account the effect of intimal flap and vessel wall motion.

Cerebral Aneurysm Occlusion at 12-Month Follow-Up After Flow-Diverter Treatment: Statistical Modeling for V&V With Real-World Data

Background: Flow-Diverter (FD) porosity has been pointed as a critical factor in the occlusion of cerebral aneurysms after treatment.

Objective: Verification and Validation of computational models in terms of predictive capacity, relating FD porosity and occlusion after cerebral aneurysms treatment.

Methods: Sixty-four aneurysms, with pre-treatment and follow-up images, were considered. Patient demographics and aneurysm morphological information were collected. The computational simulation provided by ANKYRAS provided FD porosity, expansion, and mesh angle. FD occlusion was assessed and recorded from follow-up images. Multiple regression Logit and analysis of covariance (ANCOVA) models were used to model the data with both categorical and continuous models.

Results: Occlusion of the aneurysm after 12 months was affected by aneurysm morphology but not by FD mesh morphology. A Time-To-Occlusion (TTO) of 6.92 months on average was observed with an SE of 0.24 months in the aneurysm population surveyed. TTO was estimated with statistical significance from the resulting model for the data examined and was capable of explaining 92% of the data variation.

Conclusions: Porosity was found to have the most correction power when assessing TTO, proving its importance in the process of aneurysm occlusion. Still, further Verification and Validation (V&V) of treatment simulation in more extensive, multi-center, and randomized databases is required.

Effect of Pulsatility on the Transport of Thrombin in an Idealized Cerebral Aneurysm Geometry

Computational models of cerebral aneurysm thrombosis are designed for use in research and clinical applications. A steady flow assumption is applied in many of these models. To explore the accuracy of this assumption a pulsatile-flow thrombin-transport computational fluid dynamics (CFD) model, which uses a symmetrical idealized aneurysm geometry, was developed. First, a steady-flow computational model was developed and validated using data from an in vitro experiment, based on particle image velocimetry (PIV). The experimental data revealed an asymmetric flow pattern in the aneurysm. The validated computational model was subsequently altered to incorporate pulsatility, by applying a data-derived flow function at the inlet boundary. For both the steady and pulsatile computational models, a scalar function simulating thrombin generation was applied at the aneurysm wall. To determine the influence of pulsatility on thrombin transport, the outputs of the steady model were compared to the outputs of the pulsatile model. The comparison revealed that in the pulsatile case, an average of 10.2% less thrombin accumulates within the aneurysm than the steady case for any given time, due to periodic losses of a significant amount of thrombin-concentrated blood from the aneurysm into the parent vessel’s bloodstream. These findings demonstrate that pulsatility may change clotting outcomes in cerebral aneurysms.

Numerical modelling of blood rheology and platelet activation through a stenosed left coronary artery bifurcation

Coronary bifurcations are prone to atherosclerotic plaque growth, experiencing regions of reduced wall shear stress (WSS) and increased platelet adhesion. This study compares effects across different rheological approaches on hemodynamics, combined with a shear stress exposure history model of platelets within a stenosed porcine bifurcation. Simulations used both single/multiphase blood models to determine which approach best predicts phenomena associated with atherosclerosis and atherothrombosis. A novel Lagrangian platelet tracking model was used to evaluate residence time and shear history of platelets indicating likely regions of thrombus formation. Results show a decrease in area of regions with pathologically low time-averaged WSS with the use of multiphase models, particularly in a stenotic bifurcation. Significant non-Newtonian effects were observed due to low-shear and varying hematocrit levels found on the outer walls of the bifurcation and distal to the stenosis. Platelet residence time increased 11% in the stenosed artery, with exposure times to low-shear sufficient for red blood cell aggregation (>1.5 s). increasing the risk of thrombosis. This shows stenotic artery hemodynamics are inherently non-Newtonian and multiphase, with variations in hematocrit (0.163-0.617) and elevated vorticity distal to stenosis (+15%) impairing the function of the endothelium via reduced time-averaged WSS regions, rheological properties and platelet activation/adhesion.

Effects of the atrium on intraventricular flow patterns during mechanical circulatory support

Simulations of the ventricular flow patterns during left ventricular assist device (LVAD) support are mainly performed with idealized cylindrical inflow, neglecting the influence of the atrial vortex. In this study, the influence of the left atrium (LA) on the intra-ventricular flow was investigated via Computational Fluid Dynamics (CFD) simulations. Ventricular flow was simulated by a combined Eulerian (carrier flow)/Lagrangian (particles) approach taking into account either the LA or a cylindrical inflow section to mimic a fully support condition. The flow deviation at the mitral valve, the blood low-velocity volume as well as the residence time and shear stress history of the particles were calculated. Inclusion of the LA deflects the flow at the mitral valve by 25°, resulting in an asymmetric flow jet entering the left ventricle. This reduced the ventricular low-velocity volume by 40% (from 6.4 to 3.9 cm³), increased (40%) the shear stress experienced by particles and correspondingly increased (27%) their residence time. Under the studied conditions, the atrial geometry plays a major role in the development of intraventricular flow patterns. A reliable prediction of blood flow dynamics and consequently thrombosis risk analysis within the ventricle requires the consideration of the LA in computational simulations.

Self-healing polyurethane-elastomer with mechanical tunability for multiple biomedical applications in vivo

The unique properties of self-healing materials hold great potential in the field of biomedical engineering. Although previous studies have focused on the design and synthesis of self-healing materials, their application in in vivo settings remains limited. Here, we design a series of biodegradable and biocompatible self-healing elastomers (SHEs) with tunable mechanical properties, and apply them to various disease models in vivo, in order to test their reparative potential in multiple tissues and at physiological conditions. We validate the effectiveness of SHEs as promising therapies for aortic aneurysm, nerve coaptation and bone immobilization in three animal models. The data presented here support the translation potential of SHEs in diverse settings, and pave the way for the development of self-healing materials in clinical contexts.

Modeling Microglia Activation and Inflammation-Based Neuroprotectant Strategies During Ischemic Stroke

Neural inflammation immediately follows the onset of ischemic stroke. During this process, microglial cells can be activated into two different phenotypes: the M1 phenotype, which can worsen brain injury by producing pro-inflammatory cytokines; or the M2 phenotype, which can aid in long term recovery by producing anti-inflammatory cytokines. In this study, we formulate a nonlinear system of differential equations to model the activation of microglia post-ischemic stroke, which includes bidirectional switching between the microglia phenotypes, as well as the interactions between these cells and the cytokines that they produce. Further, we explore neuroprotectant-based modeling strategies to suppress the activation of the detrimental M1 phenotype, while promoting activation of the beneficial M2 phenotype. Through use of global sensitivity techniques, we analyze the effects of the model parameters on the ratio of M1 to M2 microglia and the total number of activated microglial cells in the system over time. Results demonstrate the significance of bidirectional microglia phenotype switching on the ratio of M1 to M2 microglia, in both the absence and presence of neuroprotectant terms. Simulations further suggest that early inhibition of M1 activation and support of M2 activation leads to a decreased minimum ratio of M1 to M2 microglia and allows for a larger number of M2 than M1 cells for a longer time period.

Recent advances in computational modeling of fibrin clot formation: A review

The field of thrombosis and hemostasis is crucial for understanding and developing new therapies for pathologies such as deep vein thrombosis, diabetes related strokes, pulmonary embolisms, and hemorrhaging related diseases. In the last two decades, an exponential growth in studies related to fibrin clot formation using computational tools has been observed. Despite this growth, the complete mechanism behind thrombus formation and hemostasis has been long and rife with obstacles; however, significant progress has been made in the present century. The computational models and methods used in this context are diversified into different spatiotemporal scales, yet there is no single model which can predict both physiological and mechanical properties of fibrin clots. In this review, we list the major strategies employed by researchers in modeling fibrin clot formation using recent and existing computational techniques. This review organizes the computational strategies into continuum level, system level, discrete particle (DPD), and multi-scale methods. We also discuss strengths and weaknesses of various methods and future directions in which computational modeling of fibrin clots can advance.

Two closely-spaced Aneurysms of the Supraclinoid Internal Carotid Artery: How Does One Influence the Other?

The objective of this study was to use image-based CFD simulation techniques to analyze the impact that multiple closely spaced IAs of the supra-clinioid segment of the ICA have on each other's hemodynamic characteristics. The vascular geometry of fifteen (15) subjects with 2 IAs were gathered using a 3D clinical system. Two groups of computer models were created for each subject's vascular geometry: both IAs present (Model A) and after removal of one IA (Model B). Models were separated into two groups based on IA separation: tandem (one proximal and one distal) and tandem (aneurysms directly opposite on a vessel). Simulations using a pulsatile velocity waveform were solved by a commercial CFD solver. Proximal IAs altered flow into distal IAs (5 of 7), increasing flow energy and spatial-temporally averaged wall shear stress (STA-WSS: 3-50\% comparing Model A to B) while decreasing flow stability within distal IAs. Thus, proximal IAs may ``protect" a distal aneurysm from destructive remodeling due to flow stagnation. Among adjacent IAs, the presence of both IAs decreased each other's flow characteristics, lowering WSS (Model A to B) and increasing flow stability: all changes statistically significant (t-test p < 0.05). A negative relationship exists between the mean percent change in flow stability in relation to adjacent IA volume and ostium area. Closely spaced IAs impact hemodynamic alterations onto each other concerning flow energy, stressors and stability. Understanding these alterations may improve clinical management of closely-spaced IAs.

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.

Safety and efficacy of anterior communicating artery compromise during endovascular coil embolization of adjoining aneurysms

OBJECTIVE

In the presence of symmetric A1 flow, the safety and efficacy of compromising the anterior communicating artery (ACoA) during coil embolization of ACoA aneurysms has yet to be evaluated. Herein, the authors describe their experience, focusing on procedural safety.

METHODS

Between October 2012 and July 2017, 285 ACoA aneurysms with symmetric A1 flows were treated at the authors' institution by endovascular coil embolization. Clinical and angiographic outcome data were subjected to binary logistic regression analysis.

RESULTS

ACoA compromise was chosen in the treatment of 71 aneurysms (24.9%), which were completely (n = 15) or incompletely (n = 56) compromised. In the remaining 214 lesions, the ACoA was preserved. Although 9 patients (3.2%) experienced procedure-related thromboembolisms (compromised, 4; preserved, 5), all but 1 patient (with ACoA compromise) were asymptomatic. In multivariate analysis, subarachnoid hemorrhage at presentation was the sole independent risk factor for thromboembolism (OR 15.98, p < 0.01), with ACoA compromise being statistically unrelated. In 276 aneurysms (96.8%) with follow-up of > 6 months (mean 20.9 ± 13.1 months, range 6-54 months), recanalization was confirmed in 21 (minor, 15; major, 6). A narrow (≤ 4 mm) saccular neck (p < 0.01) and ACoA compromise (p = 0.04) were independently linked to prevention of recanalization.

CONCLUSIONS

During coil embolization of ACoA aneurysms, the ACoA may be compromised without serious complications if A1 flows are symmetric. This approach may also confer some long-term protection from recanalization, serving as a valid treatment option for such lesions.

Patient-Specific Computational Fluid Dynamics in Ruptured Posterior Communicating Aneurysms Using Measured Non-Newtonian Viscosity : A Preliminary Study

Objective

The objective of this study was to analyze patient-specific blood flow in ruptured aneurysms using obtained non-Newtonian viscosity and to observe associated hemodynamic features and morphological effects.

Methods

Five patients with acute subarachnoid hemorrhage caused by ruptured posterior communicating artery aneurysms were included in the study. Patients’ blood samples were measured immediately after enrollment. Computational fluid dynamics (CFD) was conducted to evaluate viscosity distributions and wall shear stress (WSS) distributions using a patient-specific geometric model and shear-thinning viscosity properties.

Results

Substantial viscosity change was found at the dome of the aneurysms studied when applying non-Newtonian blood viscosity measured at peak-systole and end-diastole. The maximal WSS of the non-Newtonian model on an aneurysm at peaksystole was approximately 16% lower compared to Newtonian fluid, and most of the hemodynamic features of Newtonian flow at the aneurysms were higher, except for minimal WSS value. However, the differences between the Newtonian and non-Newtonian flow were not statistically significant. Rupture point of an aneurysm showed low WSS regardless of Newtonian or non-Newtonian CFD analyses.

Conclusion

By using measured non-Newtonian viscosity and geometry on patient-specific CFD analysis, morphologic differences in hemodynamic features, such as changes in whole blood viscosity and WSS, were observed. Therefore, measured non-Newtonian viscosity might be possibly useful to obtain patient-specific hemodynamic and morphologic result.

Towards optimal flow diverter porosity for the treatment of intracranial aneurysm

PURPOSE

Low-porosity endovascular stents, known as flow diverters (FDs), have been proposed as an effective and minimally invasive treatment for sidewall intracranial aneurysms (IAs). Although it has been reported that the efficacy of a FD is substantially influenced by its porosity, clinical doctors would clearly prefer to do their interventions optimally based on refined quantitative data. This study focuses on the association between the porosity configurations and the FD efficacy, in order to provide practical data to help the clinical doctors optimize the interventions.

METHOD

Numerical simulations in fluid dynamics were performed using four patient-specific IA geometries, pulsatile velocity profiles and braided fully resolved FDs. The variation of velocity and wall shear stress within the IAs, were investigated in this study. Lattice Boltzmann method (LBM) was used to solve the main challenge centered on the diversity of spatial scales since the typical diameter of struts of FDs is only 25μm while the artery normally can be larger by a hundred times.

RESULTS

Numerical simulations revealed that the blood flow within IA sac was substantially reduced when the porosity is less than 86%. In particular, the flow condition within each IA sac is favorite to initialize thrombus formation when porosity is less than 70%.

CONCLUSION

Our study suggests the existence of a porosity threshold below which the efficacy of a FD will be sufficient for the patients to initialize the thrombus formation. Therefore, by estimating the porosity of FD on patient-specific information, it may be potentially to predict whether or the blood flow condition will successfully become prothrombotic after the FD intervention.

Accounting for residence-time in blood rheology models: do we really need non-Newtonian blood flow modelling in large arteries?

Patient-specific computational fluid dynamics (CFD) is a promising tool that provides highly resolved haemodynamics information. The choice of blood rheology is an assumption in CFD models that has been subject to extensive debate. Blood is known to exhibit shear-thinning behaviour, and non-Newtonian modelling has been recommended for aneurysmal flows. Current non-Newtonian models ignore rouleaux formation, which is the key player in blood's shear-thinning behaviour. Experimental data suggest that red blood cell aggregation and rouleaux formation require notable red blood cell residence-time (RT) in a low shear rate regime. This study proposes a novel hybrid Newtonian and non-Newtonian rheology model where the shear-thinning behaviour is activated in high RT regions based on experimental data. Image-based abdominal aortic and cerebral aneurysm models are considered and highly resolved CFD simulations are performed using a minimally dissipative solver. Lagrangian particle tracking is used to define a backward particle RT measure and detect stagnant regions with increased rouleaux formation likelihood. Our novel RT-based non-Newtonian model shows a significant reduction in shear-thinning effects and provides haemodynamic results qualitatively identical and quantitatively close to the Newtonian model. Our results have important implications in patient-specific CFD modelling and suggest that non-Newtonian models should be revisited in large artery flows.

Thrombosis in Cerebral Aneurysms and the Computational Modeling Thereof: A Review

Thrombosis is a condition closely related to cerebral aneurysms and controlled thrombosis is the main purpose of endovascular embolization treatment. The mechanisms governing thrombus initiation and evolution in cerebral aneurysms have not been fully elucidated and this presents challenges for interventional planning. Significant effort has been directed towards developing computational methods aimed at streamlining the interventional planning process for unruptured cerebral aneurysm treatment. Included in these methods are computational models of thrombus development following endovascular device placement. The main challenge with developing computational models for thrombosis in disease cases is that there exists a wide body of literature that addresses various aspects of the clotting process, but it may not be obvious what information is of direct consequence for what modeling purpose (e.g., for understanding the effect of endovascular therapies). The aim of this review is to present the information so it will be of benefit to the community attempting to model cerebral aneurysm thrombosis for interventional planning purposes, in a simplified yet appropriate manner. The paper begins by explaining current understanding of physiological coagulation and highlights the documented distinctions between the physiological process and cerebral aneurysm thrombosis. Clinical observations of thrombosis following endovascular device placement are then presented. This is followed by a section detailing the demands placed on computational models developed for interventional planning. Finally, existing computational models of thrombosis are presented. This last section begins with description and discussion of physiological computational clotting models, as they are of immense value in understanding how to construct a general computational model of clotting. This is then followed by a review of computational models of clotting in cerebral aneurysms, specifically. Even though some progress has been made towards computational predictions of thrombosis following device placement in cerebral aneurysms, many gaps still remain. Answering the key questions will require the combined efforts of the clinical, experimental and computational communities.

In vitro measurement of platelet adhesion to intact endothelial cells under low shear conditions

BACKGROUND

Prediction of thrombus formation at intact arterial walls under low shear flow conditions is clinically important particularly for better prognoses of embolisation in cerebral aneurysms. Although a new mathematical model for this purpose is necessary, little quantitative information has been known about platelet adhesion to intact endothelial cells.

OBJECTIVE

The objective of this study is to measure the number of platelets adhering to intact endothelial cells with a focus upon the influence of the shear rate.

METHODS

Endothelial cells disseminated in μ-slides were exposed to swine whole blood at different shear rates. Adenosine diphosphate (ADP) was used as an agonist. Adherent platelets were counted by means of scanning electron microscopy.

RESULTS

At an ADP concentration of 1 µM, 20.8 ± 3.1 platelets per 900 µm2 were observed after 30-minute perfusion at a shear rate of 0.8 s-1 whereas only 3.0 ± 1.4 per 900 µm2 at 16.8 s-1.

CONCLUSIONS

The number of adherent platelets is determined by a balance between the shear and the degree of stimulation by the agonist. At an ADP concentration of 1 µM, a limit to the shear rate at which platelets can adhere to intact endothelial cells is considered to be slightly higher than 16.8 s-1.

Phase-contrast MRI versus numerical simulation to quantify hemodynamical changes in cerebral aneurysms after flow diverter treatment

Cerebral aneurysms are a major risk factor for intracranial bleeding with devastating consequences for the patient. One recently established treatment is the implantation of flow-diverters (FD). Methods to predict their treatment success before or directly after implantation are not well investigated yet. The aim of this work was to quantitatively study hemodynamic parameters in patient-specific models of treated cerebral aneurysms and its correlation with the clinical outcome. Hemodynamics were evaluated using both computational fluid dynamics (CFD) and phase contrast (PC) MRI. CFD simulations and in vitro MRI measurements were done under similar flow conditions and results of both methods were comparatively analyzed. For preoperative and postoperative distribution of hemodynamic parameters, CFD simulations and PC-MRI velocity measurements showed similar results. In both cases where no occlusion of the aneurysm was observed after six months, a flow reduction of about 30-50% was found, while in the clinically successful case with complete occlusion of the aneurysm after 6 months, the flow reduction was about 80%. No vortex was observed in any of the three models after treatment. The results are in agreement with recent studies suggesting that CFD simulations can predict post-treatment aneurysm flow alteration already before implantation of a FD and PC-MRI could validate the predicted hemodynamic changes right after implantation of a FD.

Partial thrombosis of an anterior communicating artery aneurysm prior to endovascular coiling, with intra-procedural distal thrombus embolization

Thromboembolic stroke from migration of thrombus formed in non-giant intracranial aneurysms is a recognized but rare event. We describe a case of partial thrombosis of a 7 mm anterior communicating artery aneurysm, which embolized to the right callosomarginal artery in the brief time interval between two sequential diagnostic angiograms performed as part of elective endovascular coiling, and before any instrumentation had been advanced into the intracranial circulation. To our knowledge, this is the first reported case of aneurysmal thrombus embolization observed angiographically in near real time.

Non-Newtonian versus numerical rheology: Practical impact of shear-thinning on the prediction of stable and unstable flows in intracranial aneurysms

Computational fluid dynamics (CFD) shows promise for informing treatment planning and rupture risk assessment for intracranial aneurysms. Much attention has been paid to the impact on predicted hemodynamics of various modelling assumptions and uncertainties, including the need for modelling the non-Newtonian, shear-thinning rheology of blood, with equivocal results. Our study clarifies this issue by contextualizing the impact of rheology model against the recently demonstrated impact of CFD solution strategy on the prediction of aneurysm flow instabilities. Three aneurysm cases were considered, spanning a range of stable to unstable flows. Simulations were performed using a high-resolution/accuracy solution strategy with Newtonian and modified-Cross rheology models and compared against results from a so-called normal-resolution strategy. Time-averaged and instantaneous wall shear stress (WSS) distributions, as well as frequency content of flow instabilities and dome-averaged WSS metrics, were minimally affected by the rheology model, whereas numerical solution strategy had a demonstrably more marked impact when the rheology model was fixed. We show that point-wise normalization of non-Newtonian by Newtonian WSS values tended to artificially amplify small differences in WSS of questionable physiological relevance in already-low WSS regions, which might help to explain the disparity of opinions in the aneurysm CFD literature regarding the impact of non-Newtonian rheology. Toward the goal of more patient-specific aneurysm CFD, we conclude that attention seems better spent on solution strategy and other likely "first-order" effects (eg, lumen segmentation and choice of flow rates), as opposed to "second-order" effects such as rheology.

Posterior communicating artery aneurysms demonstrate faster interval growth than other growing aneurysms

Background Posterior communicating artery aneurysms are often associated with a high rupture risk. This study compares the differences in the rate of growth and morphological characteristics between growing posterior communicating artery aneurysms and other types of growing aneurysms. Materials and methods Thirteen patients with growing internal carotid artery aneurysms were scanned using Siemens 64 slice computed tomography scanners. Three patients had ophthalmic aneurysms, three had superior hypophyseal aneurysms and seven had posterior communicating artery aneurysms. Each aneurysm case had three distinct time points, with an average separation time of 1.3 ± 0.6 years. Annual aneurysm dimensional growth, annual volume growth, annual increase in surface area, size ratio, aspect ratio, and non-sphericity index were analysed. Results Posterior communicating artery aneurysms demonstrated significantly higher annual increases in dimensional growth, volume and surface area when compared to other internal carotid artery aneurysms. Posterior communicating artery aneurysms also demonstrated a significantly higher increase in aspect ratio, size ratio and lower non-sphericity index. Discussion Posterior communicating artery aneurysms had significantly greater growth per year when compared to other types of internal carotid artery aneurysms, and had different morphological characteristics.

Virtual endovascular treatment of intracranial aneurysms: models and uncertainty

Virtual endovascular treatment models (VETMs) have been developed with the view to aid interventional neuroradiologists and neurosurgeons to pre-operatively analyze the comparative efficacy and safety of endovascular treatments for intracranial aneurysms. Based on the current state of VETMs in aneurysm rupture risk stratification and in patient-specific prediction of treatment outcomes, we argue there is a need to go beyond personalized biomechanical flow modeling assuming deterministic parameters and error-free measurements. The mechanobiological effects associated with blood clot formation are important factors in therapeutic decision making and models of post-treatment intra-aneurysmal biology and biochemistry should be linked to the purely hemodynamic models to improve the predictive power of current VETMs. The influence of model and parameter uncertainties associated to each component of a VETM is, where feasible, quantified via a random-effects meta-analysis of the literature. This allows estimating the pooled effect size of these uncertainties on aneurysmal wall shear stress. From such meta-analyses, two main sources of uncertainty emerge where research efforts have so far been limited: (1) vascular wall distensibility, and (2) intra/intersubject systemic flow variations. In the future, we suggest that current deterministic computational simulations need to be extended with strategies for uncertainty mitigation, uncertainty exploration, and sensitivity reduction techniques. WIREs Syst Biol Med 2017, 9:e1385. doi: 10.1002/wsbm.1385 For further resources related to this article, please visit the WIREs website.

Experimental and CFD flow studies in an intracranial aneurysm model with Newtonian and non-Newtonian fluids

BACKGROUND

According to the clinical data, flow conditions play a major role in the genesis of intracranial aneurysms. The disorder of the flow structure is the cause of damage of the inner layer of the vessel wall, which leads to the development of cerebral aneurysms. Knowledge of the alteration of the flow field in the aneurysm region is important for treatment.

OBJECTIVE

The aim is to study quantitatively the flow structure in an patient-specific aneurysm model of the internal carotid artery using both experimental and computational fluid dynamics (CFD) methods with Newtonian and non-Newtonian fluids.

METHODS

A patient-specific geometry of aneurysm of the internal carotid artery was used. Patient data was segmented and smoothed to obtain geometrical model. An elastic true-to-scale silicone model was created with stereolithography. For initial investigation of the blood flow, the flow was visualized by adding particles into the silicone model. The precise flow velocity measurements were done using 1D Laser Doppler Anemometer with a spatial resolution of 50 μ m and a temporal resolution of 1 ms. The local velocity measurements were done at a distance of 4 mm to each other. A fluid with non-Newtonian properties was used in the experiment. The CFD simulations for unsteady-state problem were done using constructed hexahedral mesh for Newtonian and non-Newtonian fluids.

RESULTS

Using 1D laser Doppler Anemometer the minimum velocity magnitude at the end of systole -0.01 m/s was obtained in the aneurysm dome while the maximum velocity 1 m/s was at the center of the outlet segment. On central cross section of the aneurysm the maximum velocity value is only 20% of the average inlet velocity. The average velocity on the cross-section is only 11% of the inlet axial velocity. Using the CFD simulation the wall shear stresses for Newtonian and non-Newtonian fluid at the end of systolic phase (t= 0.25 s) were computed. The wall shear stress varies from 3.52 mPa (minimum value) to 10.21 Pa (maximum value) for the Newtonian fluid. For the non-Newtonian fluid the wall shear stress minimum is 2.94 mPa; the maximum is 9.14 Pa. The lowest value of the wall shear stress for both fluids was obtained at the dome of the aneurysm while the highest wall shear stress was at the beginning of the outlet segment. The vortex in the aneurysm region is unstable during the cardiac cycle. The clockwise rotation of the streamlines at the inlet segment for Newtonian and non-Newtonian fluid is shown.

CONCLUSION

The results of the present study are in agreement with the hemodynamics theory of aneurysm genesis. Low value of wall shear stress is observed at the aneurysm dome which can cause a rupture of an aneurysm.

Computer modeling of ischemic stroke

The occlusion of a blood vessel in the brain causes an ischemic stroke. Current treatment relies restoration of blood flow within 3 hours. Substantial research has focused on neuroprotection to spare compromised neural tissue and extend the treatment time window. Despite success with animal models and extensive associated clinical testing, there are still no therapies of this kind. Ischemic stroke is fundamentally a multiscale phenomenon where a cascade of changes triggered by loss of blood flow involves processes at spatial scales from molecular to centimeters with damage occurring in milliseconds to days and recovery into years. Multiscale computational modeling is a technique to assist understanding of the many agents involved in these multitudinous interacting pathways to provide clues for in silico development of multi-target polypharmacy drug cocktails.

Computational Fluid Dynamics modeling of contrast transport in basilar aneurysms following flow-altering surgeries

In vivo measurement of blood velocity fields and flow descriptors remains challenging due to image artifacts and limited resolution of current imaging methods; however, in vivo imaging data can be used to inform and validate patient-specific computational fluid dynamics (CFD) models. Image-based CFD can be particularly useful for planning surgical interventions in complicated cases such as fusiform aneurysms of the basilar artery, where it is crucial to alter pathological hemodynamics while preserving flow to the distal vasculature. In this study, patient-specific CFD modeling was conducted for two basilar aneurysm patients considered for surgical treatment. In addition to velocity fields, transport of contrast agent was simulated for the preoperative and postoperative conditions using two approaches. The transport of a virtual contrast passively following the flow streamlines was simulated to predict post-surgical flow regions prone to thrombus deposition. In addition, the transport of a mixture of blood with an iodine-based contrast agent was modeled to compare and verify the CFD results with X-ray angiograms. The CFD-predicted patterns of contrast flow were qualitatively compared to in vivo X-ray angiograms acquired before and after the intervention. The results suggest that the mixture modeling approach, accounting for the flow rates and properties of the contrast injection, is in better agreement with the X-ray angiography data. The virtual contrast modeling assessed the residence time based on flow patterns unaffected by the injection procedure, which makes the virtual contrast modeling approach better suited for prediction of thrombus deposition, which is not limited to the peri-procedural state.

Effect of a Flared Renal Stent on the Performance of Fenestrated Stent-Grafts at Rest and Exercise Conditions

PURPOSE

To quantify the hemodynamic impact of a flared renal stent on the performance of fenestrated stent-grafts (FSGs) by analyzing flow patterns and wall shear stress-derived parameters in flared and nonflared FSGs in different physiologic scenarios.

METHODS

Hypothetical models of FSGs were created with and without flaring of the proximal portion of the renal stent. Flared FSGs with different dilation angles and protrusion lengths were examined, as well as a nonplanar flared FSG to account for lumbar curvature. Laminar and pulsatile blood flow was simulated by numerically solving Navier-Stokes equations. A physiologically realistic flow rate waveform was prescribed at the inlet, while downstream vasculature was modeled using a lumped parameter 3-element windkessel model. No slip boundary conditions were imposed at the FSG walls, which were assumed to be rigid. While resting simulations were performed on all the FSGs, exercise simulations were also performed on a flared FSG to quantify the effect of flaring in different physiologic scenarios.

RESULTS

For cycle-averaged inflow of 2.94 L/min (rest) and 4.63 L/min (exercise), 27% of blood flow was channeled into each renal branch at rest and 21% under exercise for all the flared FSGs examined. Although the renal flow waveform was not affected by flaring, flow within the flared FSGs was disturbed. This flow disturbance led to high endothelial cell activation potential (ECAP) values at the renal ostia for all the flared geometries. Reducing the dilation angle or protrusion length and exercise lowered the ECAP values for flared FSGs.

CONCLUSION

Flaring of renal stents has a negligible effect on the time dependence of renal flow rate waveforms and can maintain sufficient renal perfusion at rest and exercise. Local flow patterns are, however, strongly dependent on renal flaring, which creates a local flow disturbance and may increase the thrombogenicity at the renal ostia. Smaller dilation angles, shorter protrusion lengths, and moderate lower limb exercise are likely to reduce the risk of thrombosis in flared geometries.

Computational Hemodynamic Analysis for the Diagnosis of Atherosclerotic Changes in Intracranial Aneurysms: A Proof-of-Concept Study Using 3 Cases Harboring Atherosclerotic and Nonatherosclerotic Aneurysms Simultaneously

This was a proof-of-concept computational fluid dynamics (CFD) study designed to identify atherosclerotic changes in intracranial aneurysms. We selected 3 patients with multiple unruptured aneurysms including at least one with atherosclerotic changes and investigated whether an image-based CFD study could provide useful information for discriminating the atherosclerotic aneurysms. Patient-specific geometries were constructed from three-dimensional data obtained using rotational angiography. Transient simulations were conducted under patient-specific inlet flow rates measured by phase-contrast magnetic resonance velocimetry. In the postanalyses, we calculated time-averaged wall shear stress (WSS), oscillatory shear index, and relative residence time (RRT). The volume of blood flow entering aneurysms through the neck and the mean velocity of blood flow inside aneurysms were examined. We applied the age-of-fluid method to quantitatively assess the residence of blood inside aneurysms. Atherosclerotic changes coincided with regions exposed to disturbed blood flow, as indicated by low WSS and long RRT. Blood entered aneurysms in phase with inlet flow rates. The mean velocities of blood inside atherosclerotic aneurysms were lower than those inside nonatherosclerotic aneurysms. Blood in atherosclerotic aneurysms was older than that in nonatherosclerotic aneurysms, especially near the wall. This proof-of-concept study demonstrated that CFD analysis provided detailed information on the exchange and residence of blood that is useful for the diagnosis of atherosclerotic changes in intracranial aneurysms.

Larger inflow angle and incomplete occlusion predict recanalization of unruptured paraclinoid aneurysms after endovascular treatment

BACKGROUND

Unruptured paraclinoid aneurysms have a high incidence of aneurysm recanalization (AR) after endovascular treatment. We aimed to identify the incidence and predictors of AR in these lesions.

METHODS

We retrospectively analyzed consecutive patients with unruptured paraclinoid aneurysms who underwent endovascular treatment between January 2013 and December 2014. Patients with fusiform aneurysms, dissection aneurysms, traumatic aneurysms, or without digital subtraction angiography (DSA) at follow-up, were excluded. AR was defined as any aneurysm remnant that had increased in size or contrast filling that was observed via DSA at the follow-up. Univariate and multivariate logistic regression analyses were performed to assess the predictors of AR.

RESULTS

We included 145 patients with 150 unruptured paraclinoid aneurysms in the analysis. The incidence of AR was 8.7% (95% confidence interval (CI): 4.7-13.3%) at a mean follow-up of 7.4 months. In the univariate analysis, AR was associated with aneurysm size (odd ratio (OR): 6.098; 95% CI: 1.870-19.886; p = 0.003), location (OR: 3.88; 95% CI: 1.196-12.583; p = 0.024), inflow angle (OR: 6.852; 95% CI: 1.463-32.087; p = 0.015), and Raymond scale (OR: 12.473; 95% CI: 2.7496-56.59; p < 0.001). In the adjusted multivariate analysis, AR was independently predicted by Raymond scale (OR: 9.136; 95% CI: 1.683-49.587; p = 0.001) and inflow angle (OR: 16.159; 95% CI: 3.211-81.308; p = 0.01).

CONCLUSIONS

Unruptured paraclinoid aneurysms had a high incidence of AR after endovascular treatment. An inflow angle of ≥90 degrees and incomplete occlusion were significant predictors of AR.

Growth and subsequent disappearance of a ruptured small saccular intracranial aneurysm: A morphometric and flow-dynamic analysis

The growth of a ruptured small saccular aneurysm has rarely been documented. Also rare are reports of spontaneous thrombosis of ruptured small intracranial saccular aneurysms. However, there are no reported instances of ruptured small saccular aneurysms that have demonstrated an increase in size after rupture, subsequently thrombosed and disappeared from circulation. We report one such aneurysm in a patient who presented with subarachnoid hemorrhage from a ruptured small saccular aneurysm of the anterior communicating artery. The possible reasons for the initial growth and subsequent thrombosis of the aneurysm from morphometric and flow dynamic points of view are discussed.

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

Background

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

Methods

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

Results

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

Conclusions

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

Bypass Surgery for the Treatment of Dolichoectatic Basilar Trunk Aneurysms: A Work in Progress

BACKGROUND

The treatment of dolichoectatic basilar trunk aneurysms has been ineffectual or morbid due to nonsaccular morphology, deep location, and involvement of brainstem perforators. Treatment with bypass surgery has been advocated to eliminate malignant hemodynamics and to stabilize aneurysm growth.

OBJECTIVE

To validate that flow alteration with bypass and parent artery occlusion favorably impacts aneurysm progression.

METHODS

Surgical management evolved in 3 phases, each with different hemodynamic alterations.

RESULTS

During a 17-year period, 37 patients with dolichoectatic basilar trunk aneurysms were retrospectively identified, of whom 21 patients were observed, 12 treated immediately, and 4 selected for treatment after clinical progression. In phase 1, flow reversal was overly thrombogenic, despite heparin (N = 5, final mortality, 100%). In phase 2, flow reduction with intracranial-to-intracranial bypass was safer than flow reversal, but did not prevent progressive aneurysm enlargement (N = 3, final mortality 67%). In phase 3, distal clip occlusion of the basilar trunk aneurysm preserved anterograde flow in the aneurysm without rupture, but reduced flow threatened perforator patency, despite treatment with clopidogrel (N = 8, final mortality 62%).

CONCLUSION

Shifting treatment strategy for dolichoectatic basilar trunk aneurysms improved surgical (80% to 50%) and final mortalities (100% to 62%), with stabilization of aneurysms in the phase 3 survivors. Good outcomes are determined by perforator preservation and mitigating aneurysm thrombosis. Occlusion techniques with increased distal run-off seem to benefit perforators. The treatment of dolichoectatic basilar trunk aneurysms can advance through concentrated management in dedicated centers, concerted efforts to study morphology and hemodynamics with computational methods, and widespread collection of registry data.

ABBREVIATIONS

4D PC-MRI, time-resolved phase-contrast MRIAICA, anterior inferior cerebellar arteryCE-MRA, high-resolution contrast-enhanced MR angiographyEC-IC, extracranial-to-intracranial bypassMCA, middle cerebral arteryMR, magnetic resonancemRS, modified Rankin ScalePCA, posterior cerebral arteryPICA, posterior inferior cerebellar arterySCA, superior cerebellar arterySTA, superficial temporal arteryVA, vertebral artery.