A1 segment hypoplasia


Cerebral MR angiography shows absent right A1 segment, a common variant of circus of willis accounting for 10% of individuals. The contralateral A1 segment supplies ipsilateral A2 segment by a prominent anterior communicating artery.

Fetal origin of right posterior cerebral aretry also incidentally noted, a common variant of the posterior cerebral circulation in which a prominent ipsilateral PCOM supplies the posterior cerebral artery rather than basilar artery. 

This study was done in conjunction with a brain MRI study which was normal.