Anatomical segments

P1: PCA from the origin to posterior communicating artery (AKA mesencephalic, precommunicating, circular, peduncular, basilar...). The long and short circumflex and thalamoperforating arteries arise from P1
P2: PCA from origin of PComA to the origin of inferior temporal arteries (AKA ambient, post-communicating, perimesencephalic), P2 traverses the ambient cistern, hippocampal, anterior temporal, peduncular perforating, and medial posterior choroidal arteries arise from P2
P3: PCA from the origin of the inferior temporal branches to the origin of the terminal branches (AKA quadrigeminal segment). P3 traverses the quadrigeminal cistern
P4: segment after the origin of the parieto-occipital and calcarine arteries, includes the cortical branches of the PCA

P1 segment