Hemodynamics before and after bleb formation in cerebral aneurysms

Abstract

ABSTRACT We investigate whether blebs in cerebral aneurysms form in regions of low or high wa ll shear stress (WSS), and how the intraaneurysmal hemodynamic pattern changes after bleb formation. Seven intracranial aneurysms harboring well defined blebs were selected from our database and subject-specific computational models were constructed from 3D rotational angiography. For each patient, a second anatomical model representing the aneurysm before bleb formation was constructed by smoothing out the bl eb. Computational fluid dynamics simula tions were performed under pulsatile flow conditions for both models of each aneurysm. In six of the seven aneurysms, the blebs formed in a region of elevated WSS associated to the inflow jet impaction zone. In one, the bleb formed in a region of low WSS associated to the outflow zone. In this case, the infl ow jet maintained a fairly concentrated st ructure all the way to the outflow zone, while in the other six aneurysms it dispersed after impacting the aneurysm wall. In all aneurysms, once the blebs formed, new flow recirculation regions we re formed inside the blebs and the bleb s progressed to a state of low WSS. Assuming that blebs form due to a focally da maged arterial wall, these results seem to indicate that the localized injury of the vessel wall may be caused by elevated WSS associated with the inflow jet. However, the final shape of the aneurysm is probably also influenced by the peri-aneurysmal environment that can provide extra structural support via contact with structures such as bone or dura matter. Keywords: Cerebral aneurysms, rotational angiography, co mputational fluid dynamics, hemodynamics, blebs 1. INTRODUCTIONPrevious studies found that unruptured aneurysms tended to have large flow impingement regions (more dispersed inflow jets) and simple stable flow patterns, while ruptured aneurysms tended to have small impaction zones (more concentrated inflow jets) and complex or unstable intra-aneurysmal flow patterns [1]. This suggests that elevated wall shear stress caused by the impaction of a concentrated inflow jet may have a damaging effect on the arterial wall. A recent study of the relationship between wall shear stress and aneurysm formation in animal models showed that elevated wall shear stress gradients tend to damage the arterial wall promoting aneury sm formation [2]. Another study of the motion of the arterial wall of cerebral aneurysms based on dynamic digital subtraction angiography (DSA) showed that the bleb of an aneurysm had a larger deformation am plitude (by a factor of about two) than the rest of the aneurysm sac [3]. Similar observations were reported by study of wall motion based on dynamic computed tomography angiography (CTA) [4]. These observations suggest that blebs deform at a larger rate because of a locally weaker aneurysm wall. Therefore, a focalized injury of the arterial wall is likely the reason of the formation of the bleb. So the question is: what caused this focalized injury to the arterial wall ? Our hypothesis is that mechanobiological processes modulated by elevated wall shear stress caused by the impaction of the inflow jet on the aneurysm wall are responsible for this local damage and thus for the formation of a bleb. This paper represents the first attempt at testing this hypothesis. For this purpose, patient-specific computational fluid dynamics (CFD) models were used to investigate whether blebs formed in regions previously exposed to infl ow jet impaction and elevated wall shear stress for a number of intracranial aneurysms ha rboring well defined blebs.