We present a qualitative analysis of the flow pattern in cerebral aneurysms and the effect on the velocity reduction of stent implantation. We use the Lattice Boltzmann (LB) method, which implements a mesoscopic dynamics of the interactions between fictitious particles. LB provides an efficient numerical solver for complex boundary problems such as the flow in cerebral aneurysms. Several numerical and empirical studies show that the flow reduction mechanism of the stent in the saccular aneurysm is not simple to understand in detail. We have focused on the flow pattern in the aneurysm dome and shown that several characteristic properties allow to predict the effect of a treatment by a stent.