At first, vessel diameter and COW calcification of the 13 excluded subjects were recorded by two experienced neuroradiologists (TCW and TYC with 12 and 17 years of experience, respectively) for the interobserver reliability tests. Due to substantial to almost perfect reproducibility (ICC values of 0.82–0.97), the vessel diameter measurements of all 210 included subjects were completed by only one reader (TCW). For COW classification, the diameter ratios between the bilateral A1 segments and between the ipsilateral PcomA and P2 segments were recorded (Fig. 1a, b). The diameters of the bilateral distal cervical ICAs were measured at 1 cm below the petrous segment of the ICA (Fig. 1c). In each case, the diameter of the dominant A1 segment was set as 100%, and the diameter of the non-dominant A1 segment was set as the percentage compared to the dominant A1 segment. Bilateral PcomA diameters were transformed into the percentage of the ipsilateral P2 segment diameters. F-PCA was defined as a PcomA diameter equal to the ipsilateral P2 segment diameter with an absence of the ipsilateral P1 segment.
Fig. 1 Examples of vascular diameter measurement. a) Typical location of bilateral A1 diameter measurement at in the middle of the A1 segment. b) Typical location of PcomA and P2 diameter measurement at in the middle of the PcomA and proximal P2 segment, respectively. Note there was no PcomA on the contralateral side. c) Typical location of bilateral distal cervical ICA diameter measurement at 1 cm below the petrous ICA segment