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Background Middle meningeal artery (MMA) embolization is an established treatment for chronic subdural hematoma (CSDH). Procedural endpoints are typically determined by angiographic stasis, although the true distribution of embolic particles within distal microvasculature cannot be directly visualized. We sought to demonstrate the discrepancy between fluoroscopic appearance and actual particle behavior during polyvinyl alcohol (PVA) and embolic sphere embolization.Methods A transparent in-vitro vascular model replicating the MMA and distal branches was constructed under physiologic flow conditions. Polyvinyl alcohol particles (150-1000 μm across multiple size ranges) and embolic shere were injected using standard technique. Particle motion, aggregation, and lodging were recorded using high-resolution magnified video and compared with fluoroscopic endpoints used in clinical practice.Results Fluoroscopic stasis showed proximal flow stagnation and apparent occlusion. However, magnified visualization demonstrated continued distal particle migration beyond the angiographically visible field. Fine particles redistributed into smaller branches after apparent stasis, and delayed settling occurred even after injection cessation. These findings indicate a mismatch between the visible angiographic endpoint and ongoing distal microvascular embolization.Conclusions Angiographic stasis does not represent completion of embolization during MMA embolization for CSDH. Recognition of ongoing invisible embolization may improve endpoint interpretation and help optimize particle size selection and injection technique to enhance procedural safety.Disclosures Y. Kim: 2; C; Medtronic.