A, Anterolateral view of the brainstem and right petrosal cerebellar surface. The right AICA passes below the abducens and between the facial and vestibulocochlear nerves before reaching the cerebellopontine fissure and petrosal cerebellar surface. B, The right AICA arises just above the vertebrobasilar junction and passes below the pontomedullary junction before turning upward to reach the surface of the middle cerebellar peduncle. It passes above the floccular and along the cerebellopontine fissure to reach the petrosal surface. C, The left AICA passes above the abducens nerve and below the facial and vestibulocochlear nerves, where it gives rise to a recurrent perforating branch to the brainstem. The SCA passes above the posterior trigeminal root. D, The right AICA loops into the porus of the meatus and between the facial and vestibulocochlear nerves. E, Another brainstem and cerebellum. The right vertebral artery is a duplicate artery and gives rise to duplicate PICAs. The AICAs arise from the lower part of the basilar artery. The left AICA is larger than the right. The rostral duplicate PICA loops upward into the cerebellopontine angle. The left vertebral artery loops upward into the left cerebellopontine angle. (Images courtesy of AL Rhoton, Jr.)