Fig. 2.

An example of manually labeled arteries for one subject. For ACA_distal, all main branches distal to the anterior communicating artery were selected (1–3 branches of the pericallosal artery depending on morphology, A2–A3 level). PCA was cropped at P3 level, distal to pons, to get a uniform length (Osborn 1999). MCA was divided into a proximal (MCA) and a distal (MCA_distal) part. The proximal part consists of the M1 segment, pre- and post-bifurcation. The MCA_distal includes the full visible length of MCA, or until it reaches the cortex (M2 and M3 segments). The border between the proximal and distal part was set at the genu where the MCA takes a turn in the posterior direction (Osborn 1999). Only the branches that extend posteriorly (M2) and laterally (M3) were included. For M1, branches forming/preceding the main M2 branches, or having the same direction as those doing so, were included. The direction and continuity of the arteries were decided by visual inspection. Since MCA_distal consists of several branches, the individual variation at M3 level was too large for it to be useful to construct a separate probability map. Note that in the vascular segmentation process, gaps sometimes arise in low-flow arteries, here seen in the MCA_distal on the left side of the figure