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The circle of Willis, the brain's ring road
Four arteries supply the brain, and at its base they join in a ring. In this person the ring is not complete.
Two internal carotid arteries (front) and two vertebral arteries (back; they join into the basilar artery) supply the brain. At its base they are linked in a ring: anterior cerebral arteries (A1) + anterior communicating artery in front, internal carotids and posterior communicating arteries on the sides, posterior cerebral arteries (P1) behind. The middle cerebral arteries branch off the ring but are not part of it.
If one inflow is slowly narrowed or blocked, blood can reach that side through the communicating arteries. But many people have a thin or missing link, so the ring does not protect everyone equally.
Sources and licence
These are one real person's arteries, traced from an MR angiogram (TOF-MRA). Their right posterior communicating artery is not visible on the scan (absent or too thin): the copy you see while the ring is built is a mirror of the left one. The left P1 segment is also reconstructed (the patient's own is very thin). Data: TopBrain Challenge Data Release v3 (doi:10.5281/zenodo.21972006); Yang et al., TopBrain challenge, medRxiv 2026; The TopCoW Challenge, NEJM AI 2026. Brain: Lüsebrink et al., Sci Data 2021 (CC0).
Educational content, not medical advice.
