The main trunk divides in a bifurcation in 78% of hemispheres and in a trifurcation in 12%. In the remaining 10%, the main trunk divides into multiple (four or more) branches. A, bifurcation: equal trunk pattern (18% of hemispheres). The main trunk divides into superior (red) and inferior (blue) trunks that are of approximately the same diameter and supply cortical areas of similar size. The superior trunk supplies the frontal and parietal areas and the inferior trunk supplies the temporal and temporo-occipital areas. B, bifurcation: inferior trunk dominant (32% of hemispheres). The inferior trunk (blue) has a larger diameter and area of supply than the superior trunk (red). The inferior trunk supplies the temporal, occipital, and parietal areas, and the superior trunk supplies the frontal areas. C, bifurcation: superior trunk dominant (28% of hemispheres). The superior trunk (red) has the largest diameter and area of supply; it supplies the frontal, parietal, temporo-occipital, and posterior temporal areas, and the smaller inferior trunk (blue) supplies the temporopolar through the middle temporal areas. D, trifurcation pattern (12% of hemispheres). The main trunk of the middle cerebral artery divides into three trunks. The superior trunk (red) supplies the frontal areas, the middle trunk (yellow) supplies the areas around the posterior end of the sylvian fissure, and the inferior trunk (blue) supplies the temporal areas. E, multiple trunks 10% of hemispheres). The main trunk gives rise to multiple smaller trunks. Two trunks supply the frontal areas (red and yellow), two supply the parietal areas (light green and dark green), and three supply the temporal and occipital areas (purple, brown, and blue). (Images courtesy of AL Rhoton, Jr.)