From the case:
Dominant A1 with aneurysm
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A saccular aneurysm can be seen at the bifurcation of the dominant A1 segment on the right. The left A1 segment is hypoplastic.
Case Discussion
Aneurysms occur at branch points and when anatomy is variant (e.g fenestrations, azygous ACA etc..) the incidence of aneurysms is higher than normal, presumably due to altered hemodynamic forces.