Evaluating the Effectiveness of 2 Different Flow Diverter Stents Based on the Stagnation Region Formation in an Aneurysm Sac Using Lagrangian Coherent Structure

Computational fluid dynamics-based virtual angiograms for the detection of flow stagnation in intracranial aneurysms

The goal of this study was to test if CFD-based virtual angiograms could be used to automatically discriminate between intracranial aneurysms (IAs) with and without flow stagnation. Time density curves (TDC) were extracted from patient digital subtraction angiography (DSA) image sequences by computing the average gray level intensity inside the aneurysm region and used to define injection profiles for each subject. Subject-specific 3D models were reconstructed from 3D rotational angiography (3DRA) and computational fluid dynamics (CFD) simulations were performed to simulate the blood flow inside IAs. Transport equations were solved numerically to simulate the dynamics of contrast injection into the parent arteries and IAs and then the contrast retention time (RET) was calculated. The importance of gravitational pooling of contrast agent within the aneurysm was evaluated by modeling contrast agent and blood as a mixture of two fluids with different densities and viscosities. Virtual angiograms can reproduce DSA sequences if the correct injection profile is used. RET can identify aneurysms with significant flow stagnation even when the injection profile is not known. Using a small sample of 14 IAs of which seven were previously classified as having flow stagnation, it was found that a threshold RET value of 0.46 s can successfully identify flow stagnation. CFD-based prediction of stagnation was in more than 90% agreement with independent visual DSA assessment of stagnation in a second sample of 34 IAs. While gravitational pooling prolonged contrast retention time it did not affect the predictive capabilities of RET. CFD-based virtual angiograms can detect flow stagnation in IAs and can be used to automatically identify aneurysms with flow stagnation even without including gravitational effects on contrast agents.

Determining flow stasis zones in the intracranial aneurysms and the relation between these zones and aneurysms' aspect ratios after flow diversions

BACKGROUND

Flow diverter stents (FDSs) are widely used to treat aneurysms in the clinic. However, even the same flow diverter (FD) use on different patients' aneurysm sites can cause unexpected hemodynamics at the aneurysm region yielding low success rates for the overall treatment. Therefore, the present study aims to unfold why FDs do not work as they are supposed to for some patients and propose empirical correlation along with a contingency table analysis to estimate the flow stasis zones in the aneurysm sacs.

METHODS

The present work numerically evaluated the use of FRED4518 FDS on six patients' intracranial aneurysms based on patient-specific aneurysm geometries. Computational fluid dynamics (CFD) simulation results were further processed to identify the time evolution of weightless blood particles for six patients' aneurysms.

RESULTS

Stagnation zone formation, incoming and outgoing blood flow at the aneurysm neck, and statistical analysis of six patients indicated that FRED4518 showed a large flow stasis zone for an aspect ratio larger than 0.75. However, FRED4518, used for aneurysms with an aspect ratio of less than 0.65, caused small stagnant flow zones based on the number of blood particles that stayed in the aneurysm sac.

CONCLUSION

A patient-specific empirical equation is derived considering aneurysms' morphological characteristics to determine the amount of stagnated fluid flow zones and magnitude of the mean aneurysm velocity in the aneurysm sac for FRED4518 based on weightless fluid particle results for the first time in the literature. As a result, numerical simulation results and patient data-driven equation can help perceive stagnated fluid zone amount before FRED4518 placement by shedding light on neuro-interventional surgeons and radiologists.

Investigation of the effect of anticoagulant usage in the flow diverter stent treatment of the patient-specific cerebral aneurysm using the Lagrangian coherent structures

Anticoagulants are prescribed to the flow diverter treated patients to diminish the risk of embolism in the arteries. In the present study, digital subtraction angiography images of a 49-year-old female patient with a left paraophthalmic aneurysm were used to build a numerical model to investigate the effect of an anticoagulant on hemodynamics at the aneurysm site. The Carreau-Yasuda viscosity model was utilized to define blood viscosity, and the coefficients of the viscosity model were updated based on the usage of warfarin. The five-cardiac cycle-long numerical simulations were performed, and Lagrangian coherent structures, hyperbolic time, and fluid particle analyses were also employed in the numerical models. These analyses allowed us to evaluate the formation of stagnated regions, recirculation zones, and the number of jailed particles inside the aneurysm sac following a flow diverter placement. It is realized that anticoagulant use caused blood to be less viscous, yielding a substantial amount of incoming blood flow to enter the aneurysm sac. Only 12% of the nearly 25,000 fluid particles seeded from the artery inlet have stayed inside the sac. Furthermore, the deviation between warfarin added blood and normal blood flow becomes more extensive, with every heartbeat undermining the effectiveness of patient-specific CFD models when the use of anticoagulants is overlooked in the viscosity models.

Fluid Flow Characteristics of Healthy and Calcified Aortic Valves Using Three-Dimensional Lagrangian Coherent Structures Analysis

Aortic valve calcification is an important cardiovascular disorder that deteriorates the accurate functioning of the valve leaflets. The increasing stiffness due to the calcification prevents the complete closure of the valve and therefore leads to significant hemodynamic alterations. Computational fluid dynamics (CFD) modeling enables the investigation of the entire flow domain by processing medical images from aortic valve patients. In this study, we computationally modeled and simulated a 3D aortic valve using patient-specific dimensions of the aortic root and aortic sinus. Leaflet stiffness is deteriorated in aortic valve disease due to calcification. In order to investigate the influence of leaflet calcification on flow dynamics, three different leaflet-stiffness values were considered for healthy, mildly calcified, and severely calcified leaflets. Time-dependent CFD results were used for applying the Lagrangian coherent structures (LCS) technique by performing finite-time Lyapunov exponent (FTLE) computations along with Lagrangian particle residence time (PRT) analysis to identify unique vortex structures at the front and backside of the leaflets. Obtained results indicated that the peak flow velocity at the valve orifice increased with the calcification rate. For the healthy aortic valve, a low-pressure field was observed at the leaflet tips. This low-pressure field gradually expanded through the entire aortic sinus as the calcification level increased. FTLE field plots of the healthy and calcified valves showed a variety of differences in terms of flow structures. When the number of fluid particles in the healthy valve model was taken as reference, 1.59 and 1.74 times more particles accumulated in the mildly and severely calcified valves, respectively, indicating that the calcified valves were not sufficiently opened to allow normal mass flow rates.

Wall Shear Stress Topological Skeleton Analysis in Cardiovascular Flows: Methods and Applications

A marked interest has recently emerged regarding the analysis of the wall shear stress (WSS) vector field topological skeleton in cardiovascular flows. Based on dynamical system theory, the WSS topological skeleton is composed of fixed points, i.e., focal points where WSS locally vanishes, and unstable/stable manifolds, consisting of contraction/expansion regions linking fixed points. Such an interest arises from its ability to reflect the presence of near-wall hemodynamic features associated with the onset and progression of vascular diseases. Over the years, Lagrangian-based and Eulerian-based post-processing techniques have been proposed aiming at identifying the topological skeleton features of the WSS. Here, the theoretical and methodological bases supporting the Lagrangian- and Eulerian-based methods currently used in the literature are reported and discussed, highlighting their application to cardiovascular flows. The final aim is to promote the use of WSS topological skeleton analysis in hemodynamic applications and to encourage its application in future mechanobiology studies in order to increase the chance of elucidating the mechanistic links between blood flow disturbances, vascular disease, and clinical observations.

Computational fluid dynamics analysis of intracranial aneurysms treated with flow diverters: A case report

INTRODUCTION

Intracranial aneurysms (IAs) are localized dilatations of intracranial arteries due to weaknesses of the endothelial layer. IAs may be treated by flow diverters (FDs), alternatively to stents and coils combination. FD is a method for the treatment of IAs especially for large, wide-necked or fusiform aneurysms. In this case report, we described a 65-year-old woman with IA who were treated by FD.

CASE PRESENTATION

A 65-year-old woman was diagnosed with a giant aneurysm at the posterior inferior cerebellar artery segment of the left internal carotid artery. Then based on the computed tomography data of this woman, aneurysm vascular stent model was constructed before and after FD, and internal pressure, velocity, wall shear stress (WSS) of aneurysms were determined by CFD analysis. Standard boundary conditions were applied. It was found that a single FD stent and double FD stents decreased the blood flow and WSS of aneurysm. The effect of single FD stent+30% filling on blood flow was more obvious, but the aneurysm rupture was caused by excessive coil packing. So, a single stent+10% coil packing rate was the best option for treating aneurysms.

CONCLUSIONS

CFD analysis for flow velocity and WSS have protection on aneurysms.

Understanding the effect of effective metal surface area of flow diverter stent's on the patient-specific intracranial aneurysm numerical model using Lagrangian coherent structures

The effective metal surface area (EMSA) of flow diversions plays an essential role in the occlusion mechanism inside the aneurysm since the value of EMSA determines the amount of blood flow into the aneurysm sac. In the present study, three different models of a flow diverter stent, namely FRED 4017, FRED 4038, and FRED 4539, were virtually placed at the aneurysm neck of a 52-years-old female patient to identify the effect of EMSA on stagnation region formation inside the aneurysm sac. Lagrangian coherent structures (LCSs), hyperbolic time, and particle tracking analysis were employed to the velocity vectors obtained from computational fluid dynamics (CFD). It is noticed that use of FRED 4017 stent with 0.42 EMSA value caused nearly 40% of the weightless blood flow particles (more than FRED 4038 and FRED 4539) to stay inside the aneurysm while only 0.35% of the blood flow was remaining inside the aneurysm sac when no stent was placed into the aneurysm site. Furthermore, hyperbolic time computations illustrated the formation of stagnation fluid flow zones that can be associated with the residence time of the blood flow particles. Lastly, the results of hyperbolic time analysis are in good agreement with digital subtraction angiography (DSA) images taken in the clinic a few minutes after a FRED 4017 implantation.