False positive dynamic imaging of the cerebral circulation due to a congenital anomaly.

False positive dynamic imaging of the cerebral circulation simulating vascular occlusive disease resulted from a clinically insignificant congenital variation in several patients at our institutions. The anomaly consisted of the asymmetrical caliber of the internal carotid and proximal anterior cerebral arteries. The 1 mm difference in the diameter of the internal carotid arteries was sufficient to be appreciated on dynamic imaging. The frequency of this anomaly was found to be 5% in a series of 200 bilateral carotid angiograms.