Figure 4: Three-dimensional reconstructed computed tomography. (a) View from behind: The left vertebral artery was pinched at the level of a fibro-osseous canal behind the lateral mass of atlas (arrow). Note the regular course of the right vertebral artery in its arterial groove. A nonunion of the atlas may also be seen. (b) Circulus arteriosus Wilisii seen from a left oblique view. Note the fetal confi guration on the right side and an aplastic p-com on the left side