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Branches of the internal carotid and basilar arteries form an anastomotic ring on the ventral surface of the brain, known as the circle of Willis

The circle of Willis is the ‘central’ anastomotic network linking the internal carotid circulations on each side and the vertebrobasilar circulation

This affords some protection against cerebral infarction in the event of arterial occlusion

The participating arteries are:

  1. The terminal ICAs
  2. The first part of the anterior cerebral arteries (A1 segments)
  3. The anterior communicating artery
  4. The posterior communicating arteries
  5. The first part of the posterior cerebral arteries (P1 segments)
  6. The basilar artery

Small perforating arteries arise from the communicating arteries

In the axial plane the ‘circle’ has a polygonal configuration within the suprasellar cistern

Hypoplasia or aplasia of it components parts is common and the circle is complete in only a minority of individuals

This anastomosis between right and left internal carotid arteries, their branches and the posterior cerebral arteries forms a circle that encloses the optic chasm and the pituitary stalk in the suprasellar and interpeduncular cisterns.

The circle of Willis is subject to many variations. Although it is complete in 90% of subjects, variation of at least one vessel, enough to affect its role as a collateral route, is found in 60% of people.

The reported incidence of each variant depends on the methods used to identify the vessels of the circle of Willis.

The most commonly variant vessel is the posterior communicating artery

Commonest variants of the circle of Willis:

  1. Hypoplastic posterior communicating artery (22%)
  2. Large posterior communicating artery associated with a reduction in size of the proximal part of the ipsilateral posterior cerebral artery so that the posterior cerebral artery effectively receives its supply from the middle cerebral artery (reports of incidence vary from 6% to 40%). Such an artery is called a ‘fetal’ posterior communicating artery
  3. Hypoplastic proximal segment of the anterior cerebral artery (the precommunicating horizontal A1 segment) on one side, with both anterior cerebral vessels supplied from that of the other side (30%)
  4. Hypoplastic anterior communicating artery (3%)
  5. Anterior cerebral artery may be fused as a single trunk, an azygos anterior cerebral artery