Crural, ambient and quadrigeminal cisterns. The posterior cerebral artery serves as an anatomic landmark within these cisterns. The terminal branch of the basilar artery, the P1 segment is defined as the section of the PCA from the basilar apex to the posterior communicating artery. The P2 segment is sub-divided into the P2a (anterior) segment within the crural cistern and the P2p (posterior) segment within the ambient cistern. The P3 segment lies within the quadrigeminal cistern and the P4 segments are cortical segments. Within the ambient cistern lies the P2p segment of the PCA, the superior cerebellar arteries, basal veins of Rosenthal, and the trochlear nerve (CN IV). Within the quadrigeminal cistern, the basal veins of Rosenthal merge with the internal cerebral veins to form the vein of Galen. (Image courtesy of AL Rhoton, Jr.)