Aneurysm growth occurs at region of low wall shear stress: patient-specific correlation of hemodynamics and growth in a longitudinal study

Abstract

Background and purpose: Evolution of intracranial aneurysmal disease is known to be related to hemodynamic forces acting on the vessel wall. Low wall shear stress (WSS) has been reported to have a negative effect on endothelial cells normal physiology and may be an important contributor to local remodeling of the arterial wall and to aneurysm growth and rupture.

Methods: Seven patient-specific models of intracranial aneurysms were constructed using MR angiography data acquired at two different time points (mean 16.4+/-7.4 months between the two time points). Numeric simulations of the flow in the baseline geometries were performed to compute WSS distributions. The lumenal geometries constructed from the two time points were manually coregistered, and the radial displacement of the wall was calculated on a pixel-by-pixel basis. This displacement, corresponding to the local growth of the aneurysm, was compared to the time-averaged wall shear stress (WSS TA) through the cardiac cycle at that location. For statistical analysis, radial displacement was considered to be significant if it was larger than half of the MR pixel resolution (0.3 mm).

Results: Mean WSS TA values obtained for the areas with a displacement smaller and greater than 0.3 mm were 2.55+/-3.6 and 0.76+/-1.5 Pa, respectively (P<0.001). A linear correlation analysis demonstrated a significant relationship between WSS TA and surface displacement (P<0.001).

Conclusions: These results indicate that aneurysm growth is likely to occur in regions where the endothelial layer lining the vessel wall is exposed to abnormally low wall shear stress.

Figure 1

Displacement map: The baseline shape (blue, left) is co-registered with the shape at the follow-up time point (green, middle). The displacement map is then generated (right): red indicates larger displacement; blue corresponds to an unchanged surface from one time point to the next.

Figure 2

Visual comparison of WSS map (left) and displacement map (right). Areas with low WSS (blue, left) match with areas of larger displacement (red, right). Similarly, areas of high WSS (red, left) correspond to no growth regions (blue, right).

Figure 3

Sampling on the aneurysm: Patches are placed at regular intervals over the surface. White lines indicate the cut planes. Only the body of the aneurysm is taken into account in the statistical analysis.

Figure 4

Scatter Plot of the local displacement as a function of the inverse of time-averaged WSS (WSS_TA) showing a clear trend for increased growth (high values of displacement) at patches that experience low WSS through the cardiac cycle (i.e. high values of inverse WSS_TA). A linear regression (black) fit to the data (p<0.001). The LOWESS curve (grey) closely follows the linear regression curve, confirming that linear regression is reasonable both locally and globally and is not biased by outliers.