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Anatomical variations of the CoW. The black arrows point out the variant arteries compared with the normal CoW model. (a) normal-type circle or NT; (b) fetal-type posterior or FTP; (c) transitional-type or TransT; (d) full FTP or fuFTP; (e) bilateral FTP or bFTP; (f) bilateral PCoA hypoplasia or bhypPCoA; (g) unilateral PCoA hypoplasia or uhypPCoA; (h) unilateral PCoA absent or uabPCoA.
