ObjectiveTo describe the rate and characteristics of air bubble retention (ABR) within cerebral aneurysms treated by flow diversion.MethodsProcedural flat detector CT (FDCT) studies were scrutinized for the presence of air bubbles (density<-200HU) in patients treated by flow diversion. Patients with intrasaccular treatment or previous clipping were excluded. Clinical outcomes, procedural angiograms, aneurysm characteristics and imaging features of air within the aneurysm were evaluated.ResultsBubbles were noted in 17.1% of 105 aneurysms in 85 FDCTs. Aneurysms with ABR were significantly larger (mean diameter: 18.9 versus 7.5mm, P<0.0001). There was a trend for the use of multiple devices during the treatment of these aneurysms (72.2% vs 49.4%, P=0.071). All of the bubbles were located rostrally in the aneurysm sac and were smaller than 6mm (mean diameter: 2.11.3mm). None of the patients had post-procedural neurological deterioration. The air had spontaneously disappeared on follow-up CT images (available in 12 patients) obtained at a mean follow-up duration of 48h.Conclusions ABR is not infrequent in cerebral flow diversion procedures. It is a clinically silent and self-limited technical complication. We propose air entrapment or filtration through the flow diverter as causative factors.