Fenestrations of the basilar artery demonstrated on magnetic resonance angiograms: an analysis of 212 cases.

Fenestration of the basilar artery (BA) is a rare variant of the intracranial artery, well demonstrated in autopsy and angiographic studies. Some angiographic series show a high incidence of associated aneurysms at the basilar fenestration site. The purpose of this study is to report the incidence of BA fenestration, its configurations, associated aneurysms, and arterial anomalies in a large series of intracranial MR angiograms (MRAs). A total of 16,416 MRAs were retrospectively reviewed to identify the location, size and associated intracranial arterial anomalies of BA fenestrations. All images were obtained with the time-of-flight (TOF) technique. Of the 16,416 MRAs, 215 fenestrations were found in 212 cases (1.29%). Most fenestrations were located in the proximal BA. The average length of the fenestration was 4.6 mm; the largest was 15.6 mm. No aneurysm was found at the site of the fenestration. Thirteen aneurysms were found in nine cases at locations other than the BA: seven in the middle cerebral artery (MCA), one in the anterior cerebral artery (ACA), one in the anterior communicating artery (Acom), one in the vertebral artery (VA), one at the carotid siphon, and two at the internal carotid-posterior communicating artery (IC-PC). Arterial anomalies in other locations were found in 26 cases. BA fenestrations were found in 1.29% of the 16,416 cases studied. There were no aneurysms at the BA fenestration site. Aneurysms at the BA fenestration site may be an exceedingly rare phenomenon.