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FIG 1. Schematic views of anatomic variations of the anterior (A) and posterior (B) parts of the circle of Willis. Only the right side is presented. The left side is a mirror image of the right side.
A, Demonstrable anterior communicating artery (a), approximated both proximal A1 segments without demonstrable anterior communicating artery (b), hypoplastic contralateral A1 segment (c), widely separated both A1 segments with no demonstrable anterior communicating artery (d), absence of contralateral A1 segment (e). The anterior part of the circle of Willis is considered as complete in types a and b, incomplete in types d and e, or indeterminate in type c.
B, Demonstrable ipsilateral posterior communicating artery and P1 segment (a), hypoplastic ipsilateral posterior communicating artery or ipsilateral P1 segment (b1 and b2), no demonstrable ipsilateral posterior communicating artery (c), no demonstrable ipsilateral P1 segment (d). The posterior part of the circle of Willis is considered as complete in type a, incomplete in types c and d, or indeterminate in type b (b1 or b2).
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