True fenestration of the anterior communicating artery diagnosed by magnetic resonance angiography.

Variations of the anterior cerebral artery-anterior communicating artery (ACoA) complex are common. Most are duplicated or partially duplicated ACoAs that are confused with fenestration of the ACoA. We report here an extremely rare case of true fenestration of the ACoA diagnosed by magnetic resonance angiography. Tiny true fenestrations of the ACoA may be overlooked easily by MR angiography. Partial maximum-intensity-projection image and volume-rendering image are useful in identifying true fenestration of the ACoA.