A, Normal configuration of the posterior half of the circle of Willis; both P1s are larger than communicating arteries and the latter are not hypoplastic (diameter more than 1 mm). The right superior cerebellar artery is duplicated. The largest right P1 branch gives rise to both the thalamoperforating and the posterior choroidal arteries. Only two perforating arteries arise on the right P1. The left posterior choroidal arises on P2. Both premamillary arteries (largest communicating trunk to premamillary area) arise from the middle third of the PComAs. AChAs arise as a single trunk. B, Hypoplastic left communicating artery. Thalamoperforating artery arises on P1 medial to the posterior choroidal arteries on both sides. The left premamillary artery arises from the posterior and the right from the anterior portion of the PComA. The superior cerebellar arteries are duplicated on both sides. C, PComAs are hypoplastic bilaterally. The largest right P1 branch gives rise to both the thalamoperforating and the PChAs. The thalamoperforating artery arises medial to the posterior choroidal artery on the left P1. The premamillary artery arises from the anterior third of the right PComA and from the middle third on the left. The left anterior choroidal arises from the carotid as two trunks. D, Fetal origin of the right posterior cerebral artery. The thalamoperforating artery on the right arises near the basilar bifurcation. The right posterior choroidal artery arises on P2. The left posterior choroidal artery arises medial to the thalamoperforating artery. The right premamillary artery arises from the anterior portion of the communicating artery. The left premamillary area is supplied by a group of nearly equal-sized arteries. The anterior choroidal artery bifurcates immediately after origin on the left. E, Bilateral fetal origin of the posterior cerebral artery. The right posterior choroidal artery arises lateral to the thalamoperforating artery. The largest left P1 branch gives rise to the thalamoperforating and choroidal arteries. The right premamillary artery arises from the middle portion of the communicating artery. A premamillary arterial complex is present on the left. F, Fetal type of right posterior cerebral origin and hypoplastic left communicating artery. The right posterior choroidal artery arises lateral to the well-developed thalamoperforating artery. No thalamoperforating branches are present on the left. The right premamillary artery arises from the anterior and the left from the posterior portion of the communicating artery. (Images courtesy of AL Rhoton, Jr.)