Classification of Early Branches Arising From the Prebifurcation Part of the M1 Segment of the MCA. The early branches have been classified according to their sites and patterns of origin. The proposed classification system takes into account the proximity of the early branch to the ICA bifurcation, the relative diameter of the early branch, the number of LSAs arising from the early branches, and the cortical distribution. Four groups and their subgroups have been identified. These drawings, which are based on anatomical specimens, depict the characteristics of the eight different patterns of the early branches of the MCA. Group I: The prebifurcation part of the M1 segment does not give rise to an early branch. Group IIA: A single ETB arises from the proximal half of the prebifurcation M1 segment. The proximal ETB has a larger diameter than ETBs arising distally. Lenticulostriate arteries arise from the initial portion of the ETB. The ETBs in this group generally give rise to multiple cortical arteries, which supply a large area of the cortex. Group IIB: The prebifurcation part of the M1 segment gives rise to only one ETB, but unlike Group IIA, the branch is located on the distal half of the single main trunk. The diameter of the ETB is smaller in Group IIB than in Group IIA. Group IIC: Multiple ETBs arise from the prebifurcation part of the M1 segment. The diameter of the proximal ETB is usually larger than that of the more distal ETB. The nearer the ETB is to the ICA bifurcation, the greater its chance of giving rise to an LSA. Group III: The prebifurcation M1 segment gives rise only to an EFB, a pattern identified in less than 5% of cases. On average, an EFB gives rise to more LSAs per vessel than an ETB. The EFB always passes upward and forward to supply the anterosuperior portion of the insula. Group IVA: The prebifurcation part of the M1 segment gives rise to one ETB and one EFB, with the ETB being closer to the ICA bifurcation than the EFB. The chance of observing more than one LSA arising from a distal EFB is low, whereas LSAs often arise from the proximal ETB in this subdivision. More cortical arteries arise from the proximal ETB than from the distal EFB. Group IVB: The prebifurcation M1 segment also gives rise to only one ETB and one EFB; however, the EFB in this group is closer to the ICA bifurcation than the ETB. The EFB usually arises on the proximal half of the prebifurcation M1 segment. The segment diameter of the EFB is larger, and it gives rise to more LSAs than the distal EFB in Group IVA. The orbitofrontal branch of the EFB may give rise to LSAs, the course of which is longer than those of LSAs arising from the prebifurcation M1 segment and the main trunk of the EFB. The small ETB, which arises distal to the EFB, resembles those seen in Group IIB. Group IVC: The prebifurcation M1 segment has multiple ETBs accompanying an EFB, which can be located proximal to or in between the ETBs. This group is the least common of all eight different patterns. The diameter of the distal ETB is smaller than that of the proximal ETB in most cases. The first ETB usually gives rise to more cortical arteries than the distal ETB. An EFB in this group that arises proximal to the ETBs gives rise to more LSAs and has a larger cortical representation than an EFB arising in between the ETBs. (Images courtesy of AL Rhoton, Jr.)