Where is the Circle of Willis (CoW) located?

Circle of Willis Location

The Circle of Willis is located at the base of the brain, beneath the hypothalamus, forming a ring-like arterial structure that sits directly above the thalamus in the diencephalon. 1

Anatomical Position

• The Circle of Willis is positioned at the base of the brain as a pentagonal network comprised of fourteen cerebral vessels 2
• It sits beneath the hypothalamus as a ring-like anastomotic structure 3, 4
• The arterial circle is located directly above the thalamus, with the thalamus receiving its blood supply from perforating arteries that arise from components of the Circle of Willis 1

Relationship to Surrounding Structures

• The Circle of Willis has a critical spatial relationship with the thalamus, which sits as a deep brain structure in the diencephalon directly above this arterial ring 1
• The proximal Circle of Willis represents the classic location for saccular aneurysms, which preferentially occur at arterial branch points and bifurcations 5
• Basal atherosclerosis assessment requires noting similarities and differences between the anterior versus posterior portions of the circle of Willis 6

Clinical Access and Visualization

• The temporal bone acoustic window allows ultrasound visualization of the basal cerebral arteries and proximal branches of the Circle of Willis 6
• The circle can be accessed during autopsy by preserving the base of the brain including the brain stem with intact arterial circle 7, 8
• Three-dimensional CT reconstruction demonstrates the course of ultrasound beams toward the middle, anterior, and posterior cerebral arteries, showing their relationship to the lesser sphenoid wing and acoustic window 6