Left vertebral injection fills only the posterior inferior cerebellar artery (PICA)
Filling of only the PICA from a small vertebral artery is a fairly commonly encountered (0.2% of population) normal variant. If the vertebral artery arises directly from the aortic arch, it frequently is the smaller of the two vertebral arteries and can fill only a PICA intracranial group.
This is one of the reasons that the larger vertebral artery should always be selected as the artery of preferable injection for a posterior fossa study.