Circle, Willis

The circle of Willis (CoW) connects the anterior circulation with the posterior circulation via posterior communicating arteries, as well as the left hemisphere with the right hemisphere via anterior communicating artery. It is estimated that only 1/3 of the population has the textbook presentation.[1]



This CTA head (thickness: 10 mm) presents an intact CoW and a normal variant of the ACOM.


This 3D reconstruction image from a CTA head showed an incomplete circle of Willis—hypoplastic right P1 (14), and absence of left PCOM and right A1.

The same patient underwent a diagnostic cerebral angiography and flow diverter embolization of the aneurysm.

  1. Right A2
  2. Left A2
  3. ACOM
  4. Left A1
  5. Right ophthalmic artery
  6. Left ICA
  7. Right ophthalmic artery aneurysm
  8. Right ICA
  9. Left M1
  10. Right M1
  11. Right PCOM
  12. Left P1
  13. Right P2
  14. Right P1
  15. BA
  16. Right VA
  17. Left VA
  18. Left PICA
  19. Left STA
  20. Right STA
  21. Left occipital artery


The DSA reveals two key features. Firstly, it shows that both P1 segments of the PCAs (A and B) are underdeveloped, a condition known as bilateral hypoplastic P1s. Secondly, it indicates the presence of bilateral fetal PCOM arteries (C and D), which means these arteries are the primary source of blood to the PCAs, a configuration reminiscent of the fetal brain’s circulation.

  1. Anatomy Atlases: Illustrated Encyclopedia of Human Anatomic Variation: Opus II: Cardiovascular System: Arteries: Head, Neck, and Thorax: Circle of Willis ↩︎