Study of variations in the posterior part of the circle of Willis using magnetic resonance angiography

The Circle of Willis is an arterial anastomosis uniting the internal carotid artery with the vertebro-basilar system. This cerebral arterial circle has ten components namely- the two anterior cerebral arteries, branches of internal carotid arteries, connected by the anterior communicating artery anteriorly, and the two posterior cerebral arteries, branches of basilar artery, connected to the internal carotid arteries via the two posterior communicating arteries. The aim of this study was to study the variations in the formation of the posterior part of the circle of Willis using Magnetic Resonance Angiography (MRA), and to review the embryological and clinical perspectives. The study was conducted over 18 months in the Department of Anatomy in collaboration with the Department of Radio-diagnosis at Pushpagiri Institute of Medical Sciences and Research Centre, Tiruvalla. MRA study was performed on 215 images, which were obtained from the electronic database at the Department of Radio-diagnosis. All the circles were further subdivided into complete and incomplete, both as a whole, and pertaining to the posterior parts, and analyzed. In this study, it was observed that the entire circle was complete in approximately 62-66% of the total, the posterior part being complete in approximately 66-76% of the total. The incidences of variations were observed to be higher on the right side than on the left. The most frequent variation was the partial foetal variant of posterior cerebral artery (PCA) in the posterior part of the circle of Willis.