Large Neck and Strong Ostium Inflow as the Potential Causes for Delayed Occlusion of Unruptured Sidewall Intracranial Aneurysms Treated by Flow Diverter

Abstract

Background and purpose: Flow diverter-induced hemodynamic change plays an important role in the mechanism of intracranial aneurysm occlusion. Our aim was to explore the relationship between aneurysm features and flow-diverter treatment of unruptured sidewall intracranial aneurysms.

Materials and methods: MR imaging, 4D phase-contrast, was prospectively performed before flow diverter implantation in each patient with unruptured intracranial aneurysm. Two postprocedure follow-ups were scheduled at 6 and 12 months. Responses were grouped according to whether the aneurysms were occluded or remnant. Preprocedural aneurysm geometries and ostium hemodynamics in 38 patients were compared between the 2 groups at 6 and 12 months. Receiver operating characteristic curve analyses were performed for significant geometric and hemodynamic continuous parameters.

Results: After the 6-month assessment, 21 of 41 intracranial aneurysms were occluded, and 9 additional aneurysms were occluded at 12 months. Geometrically, the ostium maximum diameter was significantly larger in the remnant group at 6 and 12 months (both P < .001). Hemodynamically, the proximal inflow zone was more frequently observed in the remnant group at 6 months. Several preprocedural ostium hemodynamic parameters were significantly higher in the remnant group. As a prediction for occlusion, the areas under the curve of the ostium maximum diameter (for 6 and 12 months), systolic inflow rate ratio (for 6 months), and systolic inflow area (for 12 months) reached 0.843, 0.883, 0.855, and 0.860, respectively.

Conclusions: Intracranial aneurysms with a large ostium and strong ostium inflow may need a longer time for occlusion. Preprocedural 4D flow MR imaging can well illustrate ostium hemodynamics and characterize aneurysm treatment responses.

Fig 1.

Different inflow zones at the aneurysm neck during systolic peak on (Paraview software). All blood flow directions are shown from the bottom to the top of the image. A, Proximal: aneurysm (black arrows) located at the medial C6 (ophthalmic/supraclinoid) segment of the right ICA and the inflow zone (red region in the aneurysm ostium) on the proximal part of the neck. B, Lateral side: aneurysm (black arrows) located at the dorsal C6 segment of the left ICA and inflow zone (red region in the aneurysmal ostium) on the lateral side of the neck. C, Distal: aneurysm (black arrows) located at the dorsal C6 segment of the right ICA and inflow zone (red region in the aneurysmal ostium) on the distal part of the neck.

Fig 2.

A 50-year-old man with an unruptured sidewall aneurysm at the medial C6 segment of the right ICA (the same case shown in Fig 1A). A, An initial anteroposterior angiogram of the intracranial segment of the ICA of the aneurysm. B, A follow-up at 6 months by using an axial 3D TOF sequence, showing partial thrombosis (*); the dark portion in the right ICA was the artifact that resulted from the FD. C, Axial 3D TOF sequence with gadolinium (acquired immediately after image B) showed enhancement inside the aneurysm compared with image B, and described a remnant. D, A follow-up anteroposterior angiogram of the intracranial segment of the ICA at 12 months did not show any residual, which represented a complete occlusion.

Fig 3.

The receiver operating characteristic (ROC) curves of the systolic inflow rate ratio (dark line) and ostium maximum diameter (light line) against the occlusion at 6 months have areas under the curve (AUC) of 0.855 for the systolic inflow rate ratio and 0.843 for the ostium maximum diameter.

Fig 4.

The receiver operating characteristic (ROC) curves of the systolic inflow area (dark line) and ostium maximum diameter (light line) against the occlusion at 12 months have areas under the curve (AUC) of 0.860 for the systolic inflow area and 0.883 for the ostium maximum diameter.

Fig 5.

Illustrations of late and early occluded types of aneurysms reported in our results. A, A diagram of a late occluded aneurysm shows obvious sac size, height, ostium in geometries, and intense ostium hemodynamics with proximal neck inflow. B, Diagram of an early occluded aneurysm shows a smaller sac size, height, ostium in geometries, and mild ostium hemodynamics, with distal neck inflow. P and A represent parent vessels and aneurysms, respectively. Red dotted arrows symbolize inward blood flows. Oval red and purple regions at the ostium represent the inward and outward blood flow areas, respectively.