A, A left frontotemporal bone flap has been elevated and the dura opened. The left frontal and temporal lobes have been retracted to expose the carotid artery entering the dura medial to the anterior clinoid process. The carotid bifurcation has been exposed. Lenticulostriate arteries arise from the M1. The M1 splits in a trifurcation pattern. B, The exposure has been extended between the chiasm and frontal lobe to the AComA and the contralateral A1 and A2s. A recurrent artery arising near the AComA passes laterally above the carotid bifurcation. C, The basilar bifurcation has been exposed through the opticocarotid triangle located between the internal carotid artery, A1, and optic nerve. D, The carotid bifurcation has been depressed to expose the basilar apex in the interval between the carotid bifurcation and the lower margin of the optic tract. Perforating branches crossing the area can make the approach hazardous. A thalamoperforating artery arises from the ipsilateral P1. E, The temporal pole has been retracted posteriorly for a pretemporal exposure. The carotid and anterior choroidal arteries have been elevated to expose the PComA, which gives rise to a large perforating branch referred to as a premamillary artery. The M1 gives rise to an early branch proximal to the trifurcation. The P2 extends above and the superior cerebellar artery (SCA) extends below the oculomotor nerve. F, Anterior subtemporal view. The temporal pole and the carotid artery have been elevated to the expose the origin of the normal sized PComA. The AChA passes backward along the medial edge of the uncus. A large MPChA arises from the P1 and loops downward as it passes to the quadrigeminal cistern. (Images courtesy of AL Rhoton, Jr.)