Quantitative characterization of the hemodynamic environment in ruptured and unruptured brain aneurysms

Abstract

Background and purpose: Hemodynamics are thought to play an important role in the mechanisms of aneurysm pathogenesis, progression, and rupture. The purpose of this study was to define quantitative measures related to qualitative flow characteristics previously analyzed and to investigate their relationship to aneurysm rupture.

Materials and methods: The hemodynamic environments in 210 cerebral aneurysms were analyzed by using image-based CFD under different flow conditions. Quantitative hemodynamic variables were defined and extracted from the simulation results. A statistical analysis of the relationship to the previous history of aneurysm rupture was performed, and the variability with flow conditions was assessed.

Results: Ruptured aneurysms were more likely to have larger inflow concentrations, larger MWSS, larger shear concentrations, and lower viscous dissipation ratios than unruptured aneurysms. Areas under low WSS and measures of abnormally low shear force distributions of ruptured and unruptured aneurysms were not statistically different. Although the values of hemodynamic quantities changed with different flow conditions, the statistical differences or ratios between their mean values over the ruptured and unruptured groups were maintained, for both pulsatile and steady flows.

Conclusions: Concentrated inflow streams and WSS distributions with elevated levels of MWSS and low aneurysmal viscous dissipation are statistically associated with a clinical history of prior aneurysm rupture. In contrast, the area and total viscous shear force applied in the aneurysm region subjected to abnormally low WSS levels are not. This study highlights the potential for image-based CFD for investigating aneurysm-evolution mechanisms and for clinical assessment of aneurysm risks.

Fig 1.

Subdivision of the volumetric vascular domain and its surface into geographic regions: aneurysm, near-parent vessel, far-parent vessel, and ostium surface.

Fig 2.

Hemodynamic data reduction. Top row, left to right: aneurysm neck delineation, aneurysm orifice triangulation, and vascular model geographic subdivision. Middle row: flow visualization, left to right: flow streamlines, isovelocity surface, WSS distribution. Bottom row, left to right: normal velocity magnitude at the aneurysm orifice; inflow (red) and outflow (blue) regions of the ostium; and regions of high (red), normal (gray), and low (blue) WSS.

Fig 3.

Ratio of mean hemodynamic variables of ruptured aneurysms to unruptured aneurysms for each flow condition considered. The green bars represent the mean values over the unruptured aneurysm group used as reference values (ie, normalized to 1). The asterisks indicate that the differences in mean values between ruptured and unruptured groups are statistically significant (for all flow conditions).