Influence of intracranial aneurysm-to-parent vessel size ratio on hemodynamics and implication for rupture: results from a virtual experimental study

Abstract

Objective: The effectiveness of intracranial aneurysm (IA) size as a predictor for rupture has been debated. We recently performed a retrospective analysis of IA morphology and found that a new index, namely, aneurysm-to-parent vessel size ratio (SR), was strongly correlated with IA rupture, with 77% of ruptured IAs showing an SR of more than 2, and 83% of unruptured IAs showing an SR of 2 or less. As hemodynamics have been implicated in both IA development and rupture, we examine how varying SR influences intra-aneurysmal hemodynamics.

Methods: One sidewall and 1 terminal IA were virtually reconstructed from patient 3-dimensional angiographic images. In 2 independent in silico experiments, the SR was varied from 1.0 to 3.5 by virtually changing either aneurysm size or vessel diameter while keeping the other parameter constant. Pulsatile computational fluid dynamics simulations were performed on each model for hemodynamics analysis.

Results: Low SR (</=2) aneurysm morphology consistently demonstrated simple flow patterns with a single intra-aneurysmal vortex, whereas higher SR (>2) aneurysm morphology presented multiple vortices and complex flow patterns. The aneurysm luminal area that was exposed to low wall shear stress increased with increasing SR. Complex flow, multiple vortices, and low aneurysmal wall shear stress have been associated with ruptured IAs in previous studies.

Conclusion: Higher SR, irrespective of aneurysm type and absolute aneurysm or vessel size, gives rise to flow patterns typically observed in ruptured IAs. These results provide hemodynamic support for the existing correlation of SR with rupture risk.

FIGURE 1

Drawings showing geometry models of experiment 1 (constant intracranial aneurysm [IA] size and varying size ratio [SR]). A, sidewall IA case; B, terminal IA case.

FIGURE 2

Drawings showing geometry models of experiment 2 (constant vessel size and varying SR). A, sidewall IA case; B, terminal IA case.

FIGURE 3

Drawings showing intra- aneurysmal flow patterns (time averaged) from experiment 1 illustrated by vector plots on planes cutting through the aneurysm volume for sidewall IA (A) and terminal (B) IA cases. Velocities are normalized with the corresponding inlet velocity, thereby allowing comparison of A and B. Vector lengths have been held constant to enhance visibility of vortex structures; color scales indicate velocity magnitude. Flow patterns change from a simple, single vortex to multiple vortices when SR is more than 2. Time-dependent animations of the intra-aneurysmal flow over a cardiac cycle have been included as supplemental files (see Videos, Supplementary Digital Content 1, http://links.lww.com/A779, and Supplementary Digital Content 2, http://links.lww.com/A780). Dynamics of the vortices observed can be seen in the animations.

FIGURE 4

Surface plots of scaled wall shear stress (WSS _scaled) on the aneurysmal walls in experiment 1 showing an increase in the area exposed to low WSS_scaled (<0.5 Pa) with increasing SR for sidewall (A) and terminal (B) IA cases.

FIGURE 5

Drawings showing intra-aneurysmal flow patterns (time averaged) from experiment 2 illustrated by vector plots on planes cutting through the aneurysm volume for sidewall IA (A) and terminal (B) IA cases. Vector lengths have been held constant to enhance visibility of vortex structures; color scales indicate velocity magnitude. Flow patterns change from a simple, single vortex to multiple vortices when SR is more than 2. Time- dependent animations of the intra- aneurysmal flow over a cardiac cycle have been included as supplemental files (see Videos, Supplementary Digital Content 3, http://links.lww.com/A781, and Supplementary Digital Content 4, http://links.lww.com/A782). Dynamics of observed vortices can be seen in the animations.

FIGURE 6

Surface plots of WSS on aneurysmal walls in experiment 2 showing an increase in the area exposed to low WSS (<0.5 Pa) with increasing SR for sidewall (A) and terminal (B) IA cases.