Pictorial review: magnetic resonance angiography of arterial variants at the Circle of Willis.

As intracranial MR angiography becomes more widely used and spatial resolution improves, anomalies at the Circle of Willis which have been previously well described on angiographic studies and anatomic dissections will become more frequently appreciated by MR angiography. Recognition of these variants is important to avoid confusion of the anomalies with aneurysms, evaluate collateral pathways in the intracerebral circulation, and enhance pre-operative planning in patients undergoing surgery at the skull base. In this review, we illustrate several of the more common types of anomalies at the Circle of Willis and discuss the possible clinical significance of each.