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Head and Neck: MRA, Circle of Willis III Anterior Cerebral A.
1-horizontal [A1] segment
2-vertical [A2] segment
This MRA of the region of the Circle of Willis is just slightly less than a true axial projection and demonstrates the vessels of the Circle of Willis well. The horizontal segment of the anterior cerebral artery is identified passing medially where it joins the contralateral ACA and then passes superiorly within the midline interhemispheric fissure, the A2 segment. The small anterior communicating artery between the two anterior cerebral arteries is patent and allows the filling of both anterior cerebral arteries in this patient since the right A1 segment is small. The Circle of Willis is a pentagon shape of anastomotic vessels within the suprasellar cistern which connect the anterior and posterior blood supplies and helps to provide collateral flow. The circle is composed of both anterior cerebral arteries, the single anterior communicating artery, both internal carotid arteries, both posterior cerebral arteries, the basilar tip and two posterior communicating arteries. The ring is complete in only 20% of patients. In this case the right anterior communicating artery is small and the left posterior communicating artery is absent.