Background: Visualization of the clot is key to the endovascular treatment of ischemic stroke, but identification of the distal part of the clot and distal arteries during mechanical thrombectomy (MT) remains challenging with conventional intraoperative imaging. We aimed to leverage the potential of contrast-enhanced cone beam CT (CE-CBCT) to better visualize the distality ('dark side') of the clot.

Methods: We retrospectively analyzed all patients treated with MT for medium vessel occlusion (MeVO) guided by three-dimensional (3D) imaging to identify those who underwent either CE-CBCT or 3D rotational angiography (3DRA). Our primary outcome was the visualization of the proximal and distal edges of the clot and the arteries beyond the occlusion on the different types of intraoperative imaging: digital subtraction angiography (DSA), 3DRA, and CE-CBCT. Secondary outcomes included the efficacy, safety, and timing of MT between CE-CBCT versus 3DRA groups.

Results: The proximal edge of the clot was well visualized by DSA, 3DRA, and CE-CBCT. However, the distal edge of the clot and distal arteries were significantly better visualized with CE-CBCT (60.0%) than 3DRA (2.7%) and DSA (11.4-13.5%). Moreover, MT efficacy was better with CE-CBCT than 3DRA, with higher first-pass effect (65.7% vs 37.8%) and final recanalization grade (expanded Treatment In Cerebral Infarction (eTICI) 3: 71.4% vs 43.2%). Finally, the visualization of the distal edge of the clot and distal arteries correlated with better collateralization grade.

Conclusion: CE-CBCT localizes the clot and reveals the underlying vascular anatomy better than 3DRA or DSA, thereby enhancing the efficacy of MT. The improved filling of collaterals with CE-CBCT may explain this better visualization of the 'dark side' of the clot.