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The authors subjected 40 aneurysms (37 unruptured, 3 ruptured) to 4D flow MR imaging. They visualized streamlines with velocities below the threshold—that is, a percentage value of the aneurysm maximum inflow velocity—and progressively decreased the threshold to identify vortex cores as thin, streamline bundles with minimum velocities. A simple flow pattern (single vortex core) was identified in 27 aneurysms; the other 13 exhibited a complex flow pattern. The cores were stable in 32 and unstable in 8 aneurysms. Significantly more aneurysms with-than-without blebs or daughter sacs had a complex flow pattern. The identification of vortex cores on 4D flow MR imaging may help to stratify the rupture risk of unruptured cerebral aneurysms.