Fig. 2.

Atlas-based landmarks of the fourth ventricle as defined in the stereotactic atlas of the human brainstem and cerebellum by Afshar et al. [55]. Variability in the angle of the mesencephalic flexure (a) may lead to increased variability in the spatial relationship of brainstem structures to the traditional AC-PC line. Brainstem structures may therefore enjoy a more constant relationship with fourth ventricular landmarks. A line is drawn tangentially to the floor of the fourth ventricle in the midline (VFL); a second line passes through the fastigium, perpendicular to the first. The intersection of these two lines (B) and the fastigial point (F) define two points in a new reference plane in a similar manner to that defined by the more traditional AC-PC points. Extensions of the VFL and the AC-PC line subtend an angle a: the mesencephalic angle. Adapted from Zrinzo et al. [29], with permission.