Accessory posterior cerebral artery-posterior cerebral artery anastomosis located just distal to the P1-P2 junction incidentally diagnosed by magnetic resonance angiography.

PURPOSE: To describe a case of an accessory posterior cerebral artery (PCA)-PCA anastomosis located just distal to the P1-P2 junction. METHODS: A 73-year-old man underwent cranial magnetic resonance (MR) angiography using a 1.5-Tesla scanner. MR angiography was performed using a standard three-dimensional (3D) time-of-flight technique. RESULTS: Maximum-intensity-projection (MIP) MR angiography showed no pathological arterial lesions. However, the parieto-occipital branch of the right PCA arose from the internal carotid artery (ICA). On MR angiographic source images, a tiny posterior communicating artery (PCoA) arose from the ICA just proximal to the origin of this anomalous artery, indicating that the anomalous artery represented an accessory PCA. Partial MIP images demonstrated a short anastomotic channel between the accessory PCA and normally originating right PCA that supplying the temporal branch located just distal to the P1-P2 junction. CONCLUSION: The author reports a case of a congenital short anastomosis between an accessory PCA and the PCA. The accessory PCA most frequently supplies the temporal branch of the PCA; however, in the present case, it supplied the parieto-occipital branch. Although an accessory PCA-PCA anastomosis at the distal P2 segment has previously been reported, the present case demonstrated an anastomosis just distal to the P1-P2 junction. Careful observation of MR angiographic source images is useful for identifying tiny arteries that are not visible on MIP images. Generation of partial MIP images is also valuable for detecting arterial variations.