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Figure 1: (a) Schematic diagrams of anatomic variations in the anterior part of the CW (modifi ed from Chen et al.[11]). (a) Normal adult pattern. There is a single AcomA (red). The ICA bifurcates into the pre-communicating segment of the ACA and the MCA. (b) Two or more AcomAs. (c) Median artery of the corpus callosum arises from the AcomA. (d) Fusion of the ACAs over a short distance. (e) ACA forms a common trunk and split distally into two post communicating segments. (b) (f) MCA originates from the ICA as two separate trunks. (g) Hypoplasia or absence of an AcomA. (h) One pre-communicating segment of an ACA is hypoplastic or absent, the other precommunicating segment gives rise to both post-communicating segments of the ACAs. (i) Hypoplasia or absence of an ICA. ACA gives rise to both postcommunicating segments, supplies retrograde flow to the ipsilateral precommunicating segment. (j) Hypoplasia or absence of an anterior communication. The MCA arises as two separate trunks. CW = Circle of willis, ICA = Internal carotid artery, MCA = Middle cerebral artery, AcomA = Anterior communicating artery, ACA = Anterior cerebral artery

Figure 1: (a) Schematic diagrams of anatomic variations in the anterior part of the CW (modifi ed from Chen <i>et al</i>.[11]). (a) Normal adult pattern. There is a single AcomA (red). The ICA bifurcates into the pre-communicating segment of the ACA and the MCA. (b) Two or more AcomAs. (c) Median artery of the corpus callosum arises from the AcomA. (d) Fusion of the ACAs over a short distance. (e) ACA forms a common trunk and split distally into two post communicating segments. (b) (f) MCA originates from the ICA as two separate trunks. (g) Hypoplasia or absence of an AcomA. (h) One pre-communicating segment of an ACA is hypoplastic or absent, the other precommunicating segment gives rise to both post-communicating segments of the ACAs. (i) Hypoplasia or absence of an ICA. ACA gives rise to both postcommunicating segments, supplies retrograde flow to the ipsilateral precommunicating segment. (j) Hypoplasia or absence of an anterior communication. The MCA arises as two separate trunks. CW = Circle of willis, ICA = Internal carotid artery, MCA = Middle cerebral artery, AcomA = Anterior communicating artery, ACA = Anterior cerebral artery