Left Pterional Exposure of MCA Segments. A, Left pterional exposure. The sylvian fissure has been opened and the temporal lobe has been retraced to expose an ETB originating on the proximal half of the M1 segment (Group IIA). The ETB arises at a right angle to the main trunk. Several LSAs arise from the early branch. The superior trunk immediately bifurcates following the major MCA bifurcation giving the impression of a trifurcation. B, The prebifurcation part of the M1 segment gives rise to a proximal ETB on its initial portion. The branch diameter is large and the ETB arises at a right angle with respect to the main trunk (Group IIA). The prebifurcation trunk of the MCA has been elevated to expose the LSAs arising distal to the origin of the early branch. C, Left pterional exposure. An ETB arises from the distal part of the prebifurcation M1 segment (Group IIB). The diameter of the early branch at its origin is smaller in comparison with those of the proximal ETBs. Lenticulostriate arteries arise from the prebifurcation M1 segment, distal to the origin of the ETB. D, Left pterional exposure, another specimen. The prebifurcation M segment gives rise to a proximal EFB and a more distal ETB (Group IVB). The EFB has a large diameter and gives rise to several LSAs within its initial 1 cm. The EFB has been elevated to expose the LSAs originating distally. The common trunk of the EFB passes upward and forward and gives rise to prefrontal, orbitofrontal, and precentral arteries. The prebifurcation trunk of the MCA gives rise to LSAs distal to the origin of the ETB. E and F, Right pterional exposure. Two ETBs and one EFB arise proximal to the bifurcation (Group IVC). The EFB passes upward and forward to supply the anterosuperior portion of the insula. F, The EFB arises between the two ETBs, and the proximal ETB gives rise to LSAs. Lenticulostriate arteries also arise from the initial segment of the EFB. The main bifurcation is located at the limen insulae. (Images courtesy of AL Rhoton, Jr.)