Marcelo Raschi

Aneurysm Rupture Following Treatment with Flow-Diverting Stents: Computational Hemodynamics Analysis of Treatment

BACKGROUND AND PURPOSE: Flow-diverting approaches to intracranial aneurysm treatment had many promising early results, but recent apparently successful treatments have been complicated by later aneurysm hemorrhage. We analyzed 7 cases of aneurysms treated with flow diversion to explore the possible rupture mechanisms. MATERIALS AND METHODS: CFD analysis of pre- and posttreatment conditions was performed on 3 giant aneurysms that ruptured after treatment and 4 successfully treated aneurysms. Pre- and posttreatment hemodynamics were compared including WSS, relative blood flows, vascular resistances, and pressures, to identify the effects of flow-diverter placements. RESULTS: Expected reductions in aneurysm velocity and WSS were obtained, indicating effective flow diversion from the sac into the parent artery, consistent with periprocedural observations. In each case with postaneurysm rupture, the result of flow diversion led to an increase in pressure within the aneurysm. This pressure increase is related to larger effective resistance in the parent artery from placement of the devices and, in 2 cases, the reduction of a preaneurysm stenosis. CONCLUSIONS: Flow-diversion devices can cause intra-aneurysmal pressure increases, which can potentially lead to rupture, especially for giant aneurysms. This relates both to changes in the parent artery configuration, such as reduction of a proximal stenosis, and to the flow diversion into higher resistance parent artery pathways combined with cerebral autoregulation, leading to higher pressure gradients. These may be important effects that should be considered when planning interventions. Potentially dangerous cases could be identified with angiography and/or patient-specific CFD models.

Suggested Connections Between Risk Factors of Intracranial Aneurysms: A Review

The purpose of this article is to review studies of aneurysm risk factors and the suggested hypotheses that connect the different risk factors and the underlying mechanisms governing the aneurysm natural history. The result of this work suggests that at the center of aneurysm evolution there is a cycle of wall degeneration and weakening in response to changing hemodynamic loading and biomechanic stress. This progressive wall degradation drives the geometrical evolution of the aneurysm until it stabilizes or ruptures. Risk factors such as location, genetics, smoking, co-morbidities, and hypertension seem to affect different components of this cycle. However, details of these interactions or their relative importance are still not clearly understood.

Association between hemodynamic conditions and occlusion times after flow diversion in cerebral aneurysms

Background Evaluation of flow diversion treatment of intracranial aneurysms is difficult owing to lack of knowledge of the target hemodynamic environment. Objective To identify hemodynamic conditions created after flow diversion that induce fast aneurysm occlusion. Methods Two groups of aneurysms treated with flow diverters alone were selected: (a) aneurysms completely occluded at 3 months (fast occlusion), and (b) aneurysms patent or incompletely occluded at 6 months (slow occlusion). A total of 23 aneurysms were included in the study. Patient-specific computational fluid dynamics models were constructed and used to characterize the hemodynamic environment immediately before and after treatment. Average post-treatment hemodynamic conditions between the fast and slow occlusion groups were statistically compared. Results Aneurysms in the fast occlusion group had significantly lower post-treatment mean velocity (fast=1.13cm/s, slow=3.11cm/s, p=0.02), inflow rate (fast=0.47mL/s, slow=1.89mL/s, p=0.004) and shear rate (fast=20.52 1/s, slow=32.37 1/s, p=0.02) than aneurysms in the slow occlusion group. Receiver operating characteristics analysis showed that mean post-treatment velocity, inflow rate, and shear rate below a certain threshold could discriminate between aneurysms of the fast and slow occlusion groups with good accuracy (84%, 77%, and 76%, respectively). Conclusions The occlusion time of cerebral aneurysms treated with flow diverters can be predicted by the hemodynamic conditions created immediately after device implantation. Specifically, low post-implantation flow velocity, inflow rate, and shear rate are associated with fast occlusion times.

Analysis of hemodynamics and aneurysm occlusion after flow-diverting treatment in rabbit models.

BACKGROUND AND PURPOSE: Predicting the outcome of flow diversion treatment of cerebral aneurysms remains challenging. Our aim was to investigate the relationship between hemodynamic conditions created immediately after flow diversion and subsequent occlusion of experimental aneurysms in rabbits. MATERIALS AND METHODS: The hemodynamic environment before and after flow-diversion treatment of elastase-induced aneurysms in 20 rabbits was modeled by using image-based computational fluid dynamics. Local aneurysm occlusion was quantified by using a voxelization technique on 3D images acquired 8 weeks after treatment. Global and local voxel-by-voxel hemodynamic variables were used to statistically compare aneurysm regions that later thrombosed to regions that remained patent. RESULTS: Six aneurysms remained patent at 8 weeks, while 14 were completely or nearly completely occluded. Patent aneurysms had statistically larger neck sizes (P = .0015) and smaller mean transit times (P = .02). The velocity, vorticity, and shear rate were approximately 2.8 times (P < .0001) larger in patent regions—that is, they had larger “flow activity” than regions that progressed to occlusion. Statistical models based on local hemodynamic variables were capable of predicting local occlusion with good precision (84% accuracy), especially away from the neck (92%–94%). Predictions near the neck were poorer (73% accuracy). CONCLUSIONS: These results suggests that the dominant healing mechanism of occlusion within the aneurysm dome is related to slow-flow-induced thrombosis, while near the neck, other processes could be at play simultaneously.

Analysis of flow changes in side branches jailed by flow diverters in rabbit models

Understanding the flow alteration in side branches during flow diversion treatment of cerebral aneurysms is important to prevent ischemic complications and improve device designs. Flow diverters were placed in the aorta of four rabbits crossing the origin of side arteries. Subject-specific computational models were constructed from 3D angiographies and Doppler ultrasounds (DUSs). Flow simulations were run before and after virtually deploying the flow diverters, assuming distal resistances remained unchanged after treatment. All jailed arteries remained patent angiographically 8 weeks after treatment. The computational models estimated decreases compared to pretreatment in the mean flow rates between 2% and 20% and in peak flow rates between 5% and 36%. The major changes were observed during systole. Flow patterns did not exhibit recirculation zones before treatment. Implantation of the flow diverters altered the flow structure only locally near the device wires. No major recirculation regions were created or destroyed. Flow diverters seem safe with respect to perforator or side branch occlusion. Relatively small changes in flow rates through jailed arteries are expected, even for moderate to large degrees of coverage of their origins. These results seem consistent with previous clinical experiences where no or very few complications related to perforator occlusion have been reported.

Strategy for analysis of flow diverting devices based on multi‐modality image‐based modeling

Quantification and characterization of the hemodynamic environment created after flow diversion treatment of cerebral aneurysms is important to understand the effects of flow diverters and their interactions with the biology of the aneurysm wall and the thrombosis process that takes place subsequently. This paper describes the construction of multi-modality image-based subject-specific CFD models of experimentally created aneurysms in rabbits and subsequently treated with flow diverters. Briefly, anatomical models were constructed from 3D rotational angiography images, flow conditions were derived from Doppler ultrasound measurements, stent models were created and virtually deployed, and the results were compared with in vivo digital subtraction angiography and Doppler ultrasound images. The models were capable of reproducing in vivo observations, including velocity waveforms measured in the parent artery, peak velocity values measured in the aneurysm, and flow structures observed with digital subtraction angiography before and after deployment of flow diverters. The results indicate that regions of aneurysm occlusion after flow diversion coincide with slow and smooth flow patterns, whereas regions still permeable at the time of animal sacrifice were observed in parts of the aneurysm exposed to larger flow activity, that is, higher velocities, more swirling, and more complex flow structures.

Strategy for modeling flow diverters in cerebral aneurysms as a porous medium

Simulations using the patient-specific geometry of the aneurysm may help in a better planning of the treatment and in a consequent reduction of the associated risks. We propose, evaluate, and implement a methodology for the simulation of flow diverter (FD) devices in intracranial aneurysms by using a porous medium method (PMM), which greatly reduces the computational cost of these simulations compared with immersed method (IMM) approaches used to model complex FDs. The method relies on parameters from an empirical correlation derived from experimental observations in wire screens, consistent with CFD simulations. The verification of our PMM strategy was carried out by comparing the results of simulations in different (patient-specific) geometries and FDs, to those obtained under identical conditions by the IMM. Overall, both quantitative and qualitative results are consistent between IMM and PMM in cases where the local porosity remains roughly uniform throughout the neck, with differences in the reduction of the observables lower than 10%. This PMM strategy is up to 10 times faster than the IMM, which allows for a runtime of hours instead of days, bringing it closer for its application in the clinic.

Flow Diversion in Rabbit Aneurysm Models

Flow diversion treatment of cerebral aneurysms has increasingly been considered, especially for large complex aneurysms with wide necks that are difficult to treat with coils of by clipping. However, the relationship between the hemodynamic environment created immediately after implantation of flow diverters (FD) and the subsequent aneurysm occlusion is poorly understood [1].Copyright

Influence of arterial geometry on a model for growth rate of atheromas

Atherosclerosis is a disease that affects medium and large size arteries and it can partially or totally obstruct blood flow through them. The lack of blood supply to the heart or the brain can cause an infarct or a stroke with fatal consequences or permanent effects. This disease involves the proliferation of cells and the accumulation of fat, cholesterol, cell debris, calcium and other substances in the artery wall. Such accumulation results in the formation of atherosclerotic plaques called atheromas, which may cause the obstruction of the blood flow. Cardiovascular diseases, among which atherosclerosis is the most frequent, are the first cause of death in developed countries. The published works in the subject suggest that hemodynamic forces on arterial walls have influence on the localization, initial development and growth rate of atheromas. This paper presents a model for this growth rate, and explores the influence of the bifurcation angle on the blood flow patterns and on the predictions of the model in a simplified carotid artery. The choice of the carotid bifurcation as the subject for this study obeys the fact that atheromas in this artery are often responsible for strokes. Our model predicts a larger initial growth rate in the external walls of the bifurcation and smaller growth area and lower growth rates as the bifurcation angle is increased. The reason for this seems to be the appearance of helical flow patterns as the angle is increased.

Analysis of Hemodynamics and Aneurysm Occlusion after Flow Diverting Treatment in Rabbit Models

Understanding the flow alteration in side branches during flow diversion treatment of cerebral aneurysms is important to prevent ischemic complications and improve device designs. Flow diverters were placed in the aorta of four rabbits crossing the origin of side arteries. Subject-specific computational models were constructed from 3D angiographies and Doppler ultrasounds (DUSs). Flow simulations were run before and after virtually deploying the flow diverters, assuming distal resistances remained unchanged after treatment. All jailed arteries remained patent angiographically 8 weeks after treatment. The computational models estimated decreases compared to pretreatment in the mean flow rates between 2% and 20% and in peak flow rates between 5% and 36%. The major changes were observed during systole. Flow patterns did not exhibit recirculation zones before treatment. Implantation of the flow diverters altered the flow structure only locally near the device wires. No major recirculation regions were created or destroyed. Flow diverters seem safe with respect to perforator or side branch occlusion. Relatively small changes in flow rates through jailed arteries are expected, even for moderate to large degrees of coverage of their origins. These results seem consistent with previous clinical experiences where no or very few complications related to perforator occlusion have been reported.