Efficient simulation of a low-profile visualized intraluminal support device: a novel fast virtual stenting technique

Hemodynamic analysis and implantation strategies of delayed intracranial aneurysm rupture after flow diverter treatment

BACKGROUND

Delayed aneurysm rupture after flow diverters (FDs) is a serious complication which mechanism remains unclear. The hemodynamics of FDs with proximal or distal densification implantation strategies have rarely been reported. In this study, we investigated not only the hemodynamic factors involved in postoperative rupture, but also the hemodynamic effects of different FDs implantation strategies on avoiding this complication.

METHODS

We selected 2 internal carotid artery (ICA) aneurysms with similar morphological characteristics, both of which were treated with FDs but had opposite therapeutic outcomes (Case 1, ruptured after FD treatment; Case 2, recovered). The FDs strategies we designed were strategy A [with homogeneous 30% metal coverage ratio (MCR)], strategy B (with distal densification of 40% and proximal 30% MCR) and strategy C (with proximal densification of 40% and distal 30% MCR). Virtually FDs deployment and computational fluid dynamics (CFD) method were performed to simulate FDs implantation strategies and analyze the hemodynamics associated with postoperative rupture.

RESULTS

After FDs implantation, the velocity of blood entering the aneurysm decreased (Case 1, 25.4%; Case 2, 30.6%), but the inflow jet impingement still existed in Case 1. The overall WSS decreased similarly in both cases, but the high WSS region hardly diminished in Case 1. For overall wall pressure, Case 2 decreased slightly but increased in Case 1. Of the three FDs implantation strategies, strategy C had the best hemodynamic effects, including the maximum blood velocity reduction and a tendency to form a more stable flow pattern, the maximum reduction rate of overall WSS and the effective diminish of high WSS area as well as the overall decrease of wall pressure.

CONCLUSIONS

Not significant decrease of blood flow velocity entering the aneurysm adding persistent impact of inflow jet impingement, high WSS area that did not diminish and abnormal increase of pressure on the aneurysm wall may be causative of postoperative rupture and bleeding of ICA aneurysms. In addition, the hemodynamic effects were favorable when the FD was improved to proximal densification, which may reduce the risk of delayed aneurysm rupture following FDs treatment.

KEYWORDS

Delayed rupture; flow diverter (FD); computational fluid dynamics (CFD); intracranial aneurysm (IAs); internal carotid artery (ICA).

Cerebral aneurysm flow diverter modeled as a thin inhomogeneous porous medium in hemodynamic simulations

Rapid and accurate simulation of cerebral aneurysm flow modifications by flow diverters (FDs) can help improving patient-specific intervention and predicting treatment outcome. However, when FD devices are explicitly represented in computational fluid dynamics (CFD) simulations, flow around the stent wires must be resolved, leading to high computational cost. Classic porous medium (PM) methods can reduce computational expense but cannot capture the inhomogeneous FD wire distribution once implanted on a cerebral artery and thus cannot accurately model the post-stenting aneurysmal flow. We report a novel approach that models the FD flow modification as a thin inhomogeneous porous medium (iPM). It improves over the classic PM approaches in two ways. First, the FD is more appropriately treated as a thin screen, which is more accurate than the classic 3D-PM-based Darcy-Forchheimer relation. Second, pressure drop is calculated cell-by-cell using the local FD geometric parameters across an inhomogeneous PM. We applied the iPM technique to simulating the post-stenting hemodynamics of three patient-specific aneurysms. To test its accuracy and speed, we compared the results from the iPM technique against CFD simulations with explicit FD devices. The iPM CFD ran 500% faster than the explicit CFD while achieving 94%-99% accuracy; thus, iPM is a promising clinical bedside modeling tool to assist endovascular interventions with FD and stents.

Imbalanced flow changes of distal arteries: An important factor in process of delayed ipsilateral parenchymal hemorrhage after flow diversion in patients with cerebral aneurysms

BACKGROUND AND OBJECTIVE

Hemodynamic forces may play a role in symptomatic delayed ipsilateral parenchymal hemorrhage (DIPH) of intracranial aneurysm (IA) after flow diverter placement. We aimed to investigate the hemodynamic risk factors in the postsurgical DIPH process.

METHODS

Six patients with internal carotid artery (ICA) aneurysm developed to DIPH and 12 patients without DIPH (1:2 matched controls) after flow diverter were included between January 2015 to January 2019. Postsurgical hemodynamics of distal arteries (terminal ICA, middle cerebral artery (MCA), anterior cerebral artery (ACA)) were investigated using computational fluid dynamics, as well as the hemodynamic alteration between pre- and post-treatment. The DIPH related and unrelated distal arteries (either MCA or ACA) were discriminated and compared. Definition of imbalance index is the difference in increased velocity post-flow diverter between MCA and ACA and was used to evaluate the blood flow distribution of distal arteries.

RESULTS

The mean and maximum flow velocities in the terminal ICA increased significantly after treatment in both groups. In DIPH group, the increase rate of mean velocity in the DIPH-related artery was significantly higher than that in DIPH-unrelated artery after the treatment (20.98 ± 15.38% vs -6.40 ± 7.74%; p = 0.028). Between the DIPH and control group, the baseline characteristics were well matched. However, a higher imbalance index of mean velocity was found in DIPH group (27.38 ± 13.03% vs 10.85 ± 14.12%; p = 0.031).

CONCLUSION

The mean velocity of DIPH related artery increased more, and the imbalance in increased blood flow distribution of distal arteries might play an important role in DIPH after flow diverter of IAs.

COMPUTATIONAL INVESTIGATION OF THROMBIN CONCENTRATION IN CEREBRAL ANEURYSMS TREATED WITH FLOW-DIVERTING STENTS

Flow-diverting stent is an ongoing embolization device to treat cerebral aneurysms, and it diverts the flow direction to reduce the flow velocity inside the aneurysmal sacs and promote the thrombus formation. However, its effect for aneurysm embolization is controversial. A hemodynamic-biomedical coupling model was constructed to describe the generation and transport of thrombin in arteries, and the model was applied to investigate the variation of thrombin concentration, which plays a key role in thrombus formation, in two patient-specific cerebral aneurysm models when they are treated with Pipeline flow diverting stents. It is observed from computational fluid dynamics simulations that thrombin concentration in the aneurysmal sac without collateral artery increases significantly after Pipeline implantation, however, it has hardly any variation in the aneurysmal sac without collateral artery or in the giant aneurysmal sac after Pipeline implantation. Therefore, we believe that single Pipeline is very effective to embolize a small aneurysm without collateral artery, but cannot embolize a giant aneurysm or a small aneurysm with a collateral artery on its sac effectively.