Diagram of the functional vascular anatomy of the head and neck with the 3 major extracranial–intracranial anastomotic pathway regions: the orbital, petrous-cavernous-clival, and upper cervical regions. Diagram of the functional vascular anatomy of the head and neck with the 3 major extracranial–intracranial anastomotic pathway regions: the orbital, petrous-cavernous-clival, and upper cervical regions. S. Geibprasert et al. AJNR Am J Neuroradiol 2009;30:1459-1468 ©2009 by American Society of Neuroradiology