MRA Study on Variation of the Circle of Willis in Healthy Chinese Male Adults

Figure 2

The types of the morphology and variation of Willis’ circle (COW). (1) Integral type (type I-a). All the blood vessels are shown, and A1 segment is balanced, PCoA<P1; (2) partial-integral type (type II-e). A1 segment on the right side is hypoplasia, and PCoA on the left side is absent; (3) nonintegral type (type IV-e). A1 segment on the right side and bilateral PCoA are absent; (4) partial-integral type (type I-f). Left side type I FTP; (5) partial-integral type (type I-g). Integral anterior circulation with multi-A2 right side type III FTP; (6) integral type (type I-c). Bilateral type II FTP; (7) partial-integral type (type I-j). Bilateral type III FTP; (8) partial-integral type (type I-k). Left side: double PCA and multi-A2 segments; (9) integral type (type II-a). Mild variation of A1 segment on the right side accompanied by A2 combined stem; (10) hypoplasia of basilar artery. Bilateral PCA is formed on the collateral branch of abnormal internal carotid artery.