A, Both PICAs arise outside the dura as the vertebral arteries course behind the atlanto-occipital joints. The PICAs enter the dura at the level of the dorsolateral medulla and do not have an anterior medullary or a full lateral medullary segment. The left PICA loops downward in front of the posterior arch of the atlas. B, Enlarged view. The left PICA gives off a posterior meningeal artery, penetrates the dura by passing through the dural cuff around the vertebral artery, and loops downward behind the accessory nerve and the C1 and C2 roots before ascending to enter the cerebellomedullary fissure. The right PICA passes through the dura and courses along the side of the medulla in front of the rootlets of the accessory nerve. C, The left PICA penetrates the dural cuff with the vertebral artery and the C1 nerve root. The accessory nerve passes posterior to both the vertebral artery and the PICA. The rostral attachment of the dentate ligament ascends between the PICA and the vertebral artery to attach to the dura at the level of the foramen magnum. D, The C1 nerve root passes through the dural cuff with the vertebral artery and the PICA. The accessory nerve ascends posterior to both the vertebral artery and PICA. A small posterior spinal artery arises from the PICA and courses along the dorsolateral aspect of the spinal cord. (Images courtesy of AL Rhoton, Jr.)