We investigate whether blebs in cerebral aneurysms form in regions of low or high wall 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 bleb. Computational fluid dynamics simulations 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 inflow jet maintained a fairly concentrated structure 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 were formed inside the blebs and the blebs progressed to a state of low WSS.
Assuming that blebs form due to a focally damaged 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.