Identification of Vortex Cores in Cerebral Aneurysms on 4D Flow MRI

Abstract

Background and purpose: The complexity and instability of the vortex flow in aneurysms are factors related to the rupture risk of unruptured cerebral aneurysms. We identified aneurysm vortex cores on 4D flow MR imaging and examined the relationship of these factors with the characteristics of cerebral aneurysms.

Materials and methods: We subjected 40 aneurysms (37 unruptured, 3 ruptured) to 4D flow MR imaging. We 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. Complexity and stability were compared in aneurysms with a smooth surface and those with blebs or daughter sacs.

Results: The threshold for visualizing vortex cores ranged from 3% to 13% of the maximum inflow velocity. Vortex cores could be visualized in 38 aneurysms; in 2, they were not visualized through the cardiac cycle. 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 (P = .006). Of the 3 ruptured aneurysms, 1 aneurysm had an unstable vortex core; in the other 2, the vortex core was not visualized.

Conclusions: The identification of vortex cores on 4D flow MR imaging may help to stratify the rupture risk of unruptured cerebral aneurysms.

Fig 1.

4D flow MR images of flow streamlines with velocities below the threshold determined by the percentage value of the maximum inflow velocity. A, When the threshold was decreased from 30% to 10% of the maximum inflow velocity, a single vortex core was visualized as a thin, streamline bundle. At a threshold of 5%, the vortex core was a single line. B, A thin, streamline bundle passed through the center of vortical flow vectors on a cutting plane of the aneurysm dome.

Fig 2.

4D flow MR images of an unruptured aneurysm on the paraclinoid segment of the right ICA. A, Flow vector map. B, The inflow jet is visualized as a layer of streamlines with high velocities. A single stable vortex core (yellow arrow) is visualized in the diastolic (C) and systolic (D) phases of the cardiac cycle. The vortex core is visualized as a bundle of streamlines with velocities below 7% (C) and 10% (D) of the maximum inflow velocity. The aneurysm flow pattern is simple and stable.

Fig 3.

An unruptured right MCA bifurcation aneurysm with a daughter sac. Vortex cores in the diastolic (A) and systolic (B) phases of the cardiac cycle. A single vortex core is visualized in the diastolic phase (A, yellow arrow), and another, in the systolic phase (B, red arrow). The vortex cores are visualized as bundles of streamlines with velocities below 4% and 7% of the maximum inflow velocity in the diastolic and systolic phases, respectively. The flow pattern was recorded as complex.

Fig 4.

A ruptured right ICA aneurysm with a daughter sac. The vortex core in the diastolic (A) and systolic (B) phases of the cardiac cycle. The vortex core in both phases is visualized as a bundle of streamlines with velocities below 4% of the maximum inflow velocity. The direction of the tip of the vortex core markedly changes during the cardiac cycle (yellow arrows). The flow pattern was recorded as unstable.

Fig 5.

A large ruptured aneurysm on the tip of the basilar artery. A, Flow vector map. Streamlines with velocities below 7% of the maximum inflow velocity in the diastolic (B) and systolic (C) phases. Although multiple small vortices beneath the aneurysm surface and irregular streamlines are visualized, no vortex cores are visualized through the cardiac cycle.