Black Belt Sign on Diluted-Contrast 3D Rotational Angiography as a Potential Neointimal Marker associated with Intracranial Aneurysm Occlusion after Flow Diversion

BACKGROUND AND PURPOSE: Neointimal formation at the aneurysm neck is essential after flow diverter (FD) treatment; however, reliable imaging markers are lacking. White collar sign (WCS), an established endothelialization marker after coil embolization, is paradoxically associated with incomplete occlusion after FD. We aimed to propose the "Black Belt Sign" (BBS)-an endoluminal band-like filling defect on 3D rotational angiography (3DRA)-as a neointimal marker, determine the optimal imaging modality, and assess its association with aneurysm occlusion. MATERIALS AND METHODS: This single-center, retrospective cohort study included consecutive patients who underwent FD placement from April 2015 to December 2024, completed 6- and 12-month follow-ups, and underwent diluted 3DRA, pure 3DRA, and high-resolution cone-beam CT (HR-CBCT) at 6 months. Three blinded assessors graded the 6-month BBS (non, partial, full). The optimal modality was determined by inter-rater reliability. BBS stage was compared across 6-month O'Kelly-Marotta (OKM) grades using the Cochran-Armitage test. Among 6-month incompletely occluded aneurysms, 12-month complete occlusion rates were compared by BBS stage, and risk ratios (RRs) were estimated using modified Poisson regression with cluster-robust standard errors at the patient level. WCS on 6-month 2D-DSA was recorded, and detection rates were examined across BBS stages and OKM grades. RESULTS: 142 aneurysms in 123 patients were analyzed. Diluted 3DRA showed superior inter-rater reliability (Light's κ = 0.82). The predominant BBS stage transitioned from non to partial to full as the 6-month OKM grades improved (non/partial/full: OKM A, 62/38/0%; B, 25/75/0%; C, 3/59/38%; D, 1/7/91%, respectively; P for trend < .01). Among the 61 incompletely occluded aneurysms at 6 months, the 12-month complete occlusion rates were 10% for non-BBS, 22% for partial-BBS, and 71% for full-BBS. Compared to non-BBS, the RR for 12-month occlusion was 2.16 (95% confidence interval [CI]: 0.30-15.5) for partial-BBS and 7.14 (95% CI: 1.07-47.8) for full-BBS. WCS was most frequent in aneurysms with partial-BBS and OKM grade B (75%). CONCLUSIONS: Diluted 3DRA was identified as the optimal modality. BBS aligned with the concurrent aneurysm occlusion status and predicted subsequent complete occlusion, supporting its potential as a neointimal biomarker after FD treatment. WCS may reflect immature neointimal formation after FD.