Duplicate origin and extremely long P1 segment of the posterior cerebral artery diagnosed by MR angiography.

We present what we believe is the first report of a patient with an anomalous artery arising from the distal basilar artery and fusing with the mid-portion of an extremely long P1 segment of the left posterior cerebral artery that was diagnosed on magnetic resonance (MR) angiography. Careful review of MR angiographic images is important to detect rare arterial variations, and partial maximum-intensity-projection images aid their identification on MR angiography.