ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

In addition to the analyses of averaged wall shear stresses, the temporal change of their
direction has been considered as well. The previously defined parameter OSI can vary
between a value of 0 and 0.5. With increasing OSI more intense direction changes
occur. As presented in Figure 5, elevated OSI regions are mainly noticeable close to the
bifurcations and can be explained by the existence of a recirculating flow in these areas.
Due to a narrow ostium and to sharp changes in direction leading to a helical flow
pattern, multiple regions with increased OSI values are visible in case 1. In contrast, the
ostium area of case 2 is much wider and the transition between the aneurysm and the
surrounding vessels is considerably smoother. Therefore, almost no temporal changes of
the shear directions appear in this case.
Fig.: 5: Oscillatory shear index of case 1 (left) and case 2 (right)
5. CONCLUSION
Within this study the hemodynamics in two ruptured cerebral aneurysms were
investigated. In both cases the place of rupture was known and therefore in the focus of
this work. In order to carry out CFD simulations, raw patient-specific imaging datasets
were reconstructed and subsequently spatially discretized using block-structured
meshing algorithms. Analyses regarding velocity and pressure distribution as well as
arterial wall shear stresses were performed. Additionally, the temporal change of the
direction of shear stresses was assessed using the OSI.
Analyses of velocity and pressure distribution demonstrated that the most prominent
values do not appear at the aneurysms domes. However, an oscillating motion of the
blood within one cardiac cycle was identified in the ROIs. This leads to the assumption
that the flow direction and its temporal variation are more likely to influence the rupture
probability than just the velocity magnitude.
Considering now wall shear stresses averaged during one cardiac cycle, areas with high
values are found below the aneurysms and mostly at the intersection between the
dilation and the surrounding arteries. Although high wall shear stresses are sometimes
considered in the literature as a driving factor regarding rupture, the present
observations do not support this hypothesis.
The analysis of OSI revealed again elevated values close to the bifurcations and at the
aneurysms neck. However, no major difference was identified in the ROI compared to
the healthy, straight vessel walls. Although the shear index may indicate a higher load
on endothelial tissue no direct correlation with the rupture probability was found in the
present case.
Further investigations aim onto the experimental verification of these observations. In
this context, four-dimensional (time-dependent 3D velocity vectors) measurements
using a 7 Tesla-MRT or Particle Tracing Velocimetry techniques appear most suitable
[13]. In order to quantify the observed flow oscillation in the ROI further post-