The left internal carotid artery is occluded at the bifurcation.  An enlarged ascending pharyngeal artery originates from the external carotid artery, 3 cm above the bifurcation.  Numerous distal collateral branches arise from the ascending pharyngeal artery.  The collaterals pass through the medial, anterior portion of the left middle cranial fossa intracranially.  They are probably going through either the foramen rotundum or foramen vesalianum.

The internal carotid artery is reconstituted at the petrous portion from the ascending pharyngeal collaterals.  This collateral route supplies a reasonable contrast volume to the left middle cerebral group.  The intracranial flow is, however, somewhat slowed as the meningeal vessels are filling distally more rapidly (straightened distal meningeal branches just adjacent to the left calvarium inner table).