Left anterior inferior cerebellar artery supplied by an anomalous branch arising from the right vertebral artery: a case diagnosed by magnetic resonance angiography

Purpose To describe a case of a left anterior inferior cerebellar artery (AICA) supplied by an anomalous branch arising from the contralateral right vertebral artery (VA). Methods An 88-year-old woman presenting with vomiting underwent cranial magnetic resonance imaging (MRI), MR angiography (MRA), and MR cisternography using a 3-T scanner. MRA was performed using a standard three-dimensional (3D) time-of-flight technique. MR cisternography was performed using 3D heavily T2-weighted imaging. Results MRI showed acute and chronic cerebellar infarctions in the territory of the left posterior inferior cerebellar artery (PICA). MRA showed nonvisualization of the left VA, including the left PICA. The left AICA was supplied by an anomalous branch arising from the V4 segment of the right VA. No normally arising left AICA was identified. MR cisternography showed fusion of the anomalous artery with the occluded left VA. Conclusion The AICA usually arises from the proximal basilar artery. Several variations in the origin of the AICA have been reported. We report the first case of a left AICA supplied by the contralateral right VA. The proximal segment of the anomalous left AICA appears to represent a connecting artery between the right VA and the occluded left VA. The anastomotic artery between the bilateral VAs may have developed as a variation of the anterior spinal artery system. MR cisternography is useful for identifying an occluded artery within the cistern. Careful observation of MRA, together with the generation of partial volume-rendered images, may facilitate the identification of this rare arterial variation.