Anatomical scheme and MRI performed in a fetus with normal posterior fossa. According to Ghi et al. [2], the BV angle is defined as the angle formed by a line tangential to the dorsal part of the brainstem (red line) and a line starting from the tip of the pons and crossing the lower edge of the cerebellar vermis (green line); whereas the BT angle is defined as the angle formed by a line tangential to the dorsal part of the brainstem and a line starting from the upper limit of the quadrigeminal plate, and following the tentorial surface down to the occipital bone (continuous blue line). The dotted blue line starts at the posterior tentorial insertion and ends at the anterior tentorial insertion. The dash-dotted blue line starts at the posterior tentorial insertion and ends at the quadrigeminal plate.