Fluid-Structure Simulations of a Ruptured Intracranial Aneurysm: Constant versus Patient-Specific Wall Thickness

Abstract

Computational Fluid Dynamics is intensively used to deepen the understanding of aneurysm growth and rupture in order to support physicians during therapy planning. However, numerous studies considering only the hemodynamics within the vessel lumen found no satisfactory criteria for rupture risk assessment. To improve available simulation models, the rigid vessel wall assumption has been discarded in this work and patient-specific wall thickness is considered within the simulation. For this purpose, a ruptured intracranial aneurysm was prepared ex vivo, followed by the acquisition of local wall thickness using μCT. The segmented inner and outer vessel surfaces served as solid domain for the fluid-structure interaction (FSI) simulation. To compare wall stress distributions within the aneurysm wall and at the rupture site, FSI computations are repeated in a virtual model using a constant wall thickness approach. Although the wall stresses obtained by the two approaches-when averaged over the complete aneurysm sac-are in very good agreement, strong differences occur in their distribution. Accounting for the real wall thickness distribution, the rupture site exhibits much higher stress values compared to the configuration with constant wall thickness. The study reveals the importance of geometry reconstruction and accurate description of wall thickness in FSI simulations.

Figure 1

Specimen with Acom aneurysm and adjacent vessels (a). (1) Anterior cerebral arteries. (2) Anterior communicating artery. (b1) Rupture site with magnification (b2).

Figure 2

Slice image of the μCT data with the aneurysm wall (a). A detached tissue part of the ex vivo preparation is highlighted (see blue inlay). The resulting surface meshes for the inner vessel wall ((b) top left), the outer vessel wall ((b) bottom left), and the combination of both ((b) right) are illustrated.

Figure 3

The fluid mesh consists of polyhedral and prism cells (a). Hexahedral finite elements are used for the solid domain (b).

Figure 4

Visualization of the flow pattern (streamlines) and wall shear stresses of the patient-specific configuration at peak-systole.

Figure 5

Front view of the effective stress at the outer ((a) and (b)) and inner ((c) and (d)) surface of the constant ((a) and (c)) and the patient-specific ((b) and (d)) wall thickness (WT) configuration, respectively.

Figure 6

Second perspective of the effective stress at the outer ((a) and (b)) and inner ((c) and (d)) surface of the constant ((a) and (c)) and the patient-specific ((b) and (d)) wall thickness configuration, respectively. Very different stress levels are found at the location of the rupture site (indicated by the black arrow).

Figure 7

Histogram comparing wall stresses based on approx. 29,000 points in the aneurysm wall. The single bars indicate the number of points in a wall stress range of 500 Pa. As illustrated by the vertical lines, the average stress value obtained with the constant wall thickness configuration (dashed line) nearly matches the level obtained in the patient-specific configuration (solid line).

Figure 8

Histogram comparing wall stresses based on approx. 6,000 points around the rupture site. Bars of the constant WT configuration (indicated by the hatching) show a much lower stress level in the rupture zone, compared to the values found with patient-specific wall thickness. The dashed (constant WT) and solid (patient-specific WT) lines depict the mean stress found in the considered region of interest.