Circle of Willis

Overview

The Circle of Willis is an arterial ring, formed as the internal carotid and vertebrobasilar systems anastomose in the suprasellar cistern. The communicating pathways allow for anastomotic circulation and equalisation of blood flow between the hemispheres.

Location

• Suprasellar cistern of the basal cistern
• se below optic tracts, above oculomotor nerves CN3

Relations

• Surrounds optic chiasm, pituitary infundibulum and mammillary bodies
• Inferiorly: pituitary gland
• Inferolateral to the hypothalamus
• Horizontal (A1) ACA segments normally course above optic nerves

Vascular territory

• Entire central base of the brain (including the hypothalamus, internal capsule, optic tracts, thalamus and midbrain)
• Branches of the COW supply:
  • Optic chiasm and tracts, infundibulum, hypothalamus and other structures of the base of the brain

Structure

Anterior circulation

• Internal carotid arteries
  • Terminates by bifurcation into ACA and middle cerebral arteries (MCA)
• A1 segments of anterior cerebral arteries (ACA)
  • Medial lenticulostriate arteries – supplies lentiform nuclei
  • Recurrent artery of Heubner (usually A2 branch) – supplies basal ganglia and anterior limb of the internal capsule
  • Takes a rostromedial course, soon united in the midline by ACOM, marking the end of A1 segment of ACA 
• Anterior communicating artery (ACOM)

Posterior circulation

• P1 segment of posterior cerebral arteries (PCA)
• Posterior communicating arteries (PCOM)
  • arises from supraclinoid/communicating segment of each ICA at the anterior perforated substances and passes posterior through the interpeduncular cistern to join the ipsilateral PCA, marking the end of P1 segment of PCA.
  • Anterior thalamoperforating arteries
  • Cross the diencephalic leaf to enter the interpeduncular cistern
• Basilar Artery
  • Divides in the interpeduncular cistern ventral to midbrain, into left and right PCA, which diverge superior to oculomotor and trochlear nerves
  • Posterior thalamoperforating arteries
  • Thalamogeniculate arteries

Variants

Anterior cerebral artery

• Duplicated, hypoplastic, fenestrated or absent ACOM
• Hypoplastic or absent A1 – two ACAs supplied by one ICA trunk
• Assymetry of A1 segment – associated with ACA aneurysm
• Azygos anterior cerebral artery – both A1 segments of the anterior cerebral arteries unite to form a single midline A2 trunk, resulting in no anterior communicating artery.
• Trifurcation of ACA – three A2 segments
• Multiple ACOMs

Middle cerebral artery

• MCA duplication
• Accessory MCA

Posterior cerebral artery

• Hypoplastic or absent PCOM
• Fetal origin of PCA – where PCOM is larger than P1 segment of ipsilateral PCA
• Artery of Percheron – a rare anatomic variant of P1 segment that can supply the thalamus and midbrain bilaterally.