Apparent posterior cerebral artery territory perfusion asymmetry on arterial spin labeling MRI is a common non-pathologic finding in patients with a unilateral fetal posterior cerebral artery.

Apparent PCA hypoperfusion on ASL is not uncommon in patients with a contralateral fetal PCA who have no clinical or conventional imaging findings to suggest true hypoperfusion. This phenomenon is likely due to differential blood velocities between the carotid and vertebral arteries that result in differential arterial transit times and labeling efficiency. It is important for radiologists to know that apparent hypoperfusion may arise from variant circle of Willis anatomy.