Introduction:
The Circle of Willis is a critical intracranial arterial ring located at the base of the brain, providing a vital collateral pathway between the anterior and posterior circulations. Its integrity ensures continued cerebral perfusion in case of arterial occlusion. It is also the most common site of intracranial aneurysms, and its branches are frequently involved in subarachnoid hemorrhage (SAH).
Anatomy of the Circle of Willis:
Components:
The Circle of Willis is a polygonal anastomotic arterial network at the base of the brain, located in the interpeduncular cistern around the optic chiasm and hypothalamus. It is formed by contributions from both the anterior and posterior circulations.
Arterial Components:
- Anterior cerebral arteries (ACAs) – Right and left; arise from internal carotid arteries.
- Anterior communicating artery (AComA) – Connects the two ACAs.
- Internal carotid arteries (ICAs) – Terminal branches give rise to ACAs and middle cerebral arteries.
- Posterior cerebral arteries (PCAs) – Arise from the basilar artery.
- Posterior communicating arteries (PComAs) – Connect the PCAs with the internal carotid arteries on each side.
Key Supporting Arteries (not part of the circle proper but relevant):
Middle cerebral arteries (MCAs) – Although major cerebral arteries, MCAs are not part of the anatomical circle.
Basilar artery – Formed by union of vertebral arteries; gives rise to PCAs.
Shape & Location:
- Lies in the interpeduncular cistern, encircling the optic chiasm, infundibulum, and mammillary bodies
- Best visualized on CT angiography, MR angiography, or digital subtraction angiography (DSA)
Functional Importance:
- Provides redundant blood supply to cerebral hemispheres
- Allows cross-flow during occlusion of a major vessel (e.g., ICA or vertebral artery)
Variants of the Circle of Willis:
Anatomical variations are very common, seen in over 50–60% of individuals. Understanding these is critical in stroke imaging and aneurysm evaluation.
Common Variants:
- Hypoplasia or aplasia of:
- A1 segment of ACA – most common (~20–25%)
- PComA – may be hypoplastic or absent
- Leads to asymmetry of the circle and impairs collateral flow
- Fetal origin of PCA:
- Seen in ~10–30% of people
- The PCA originates from the ICA via a dominant PComA, with hypoplastic or absent P1 segment
- Affects hemodynamics and interpretation of infarcts
- Duplicate or fenestrated vessels:
- e.g., Fenestration of the AComA or ICA
- May predispose to aneurysm formation
- Trifurcation of ACA:
- The ACA divides into three branches instead of two (bihemispheric ACA)
- Unpaired or azygos ACA:
- A single midline ACA supplies both hemispheres
Imaging of the Circle of Willis:
- CT Angiography (CTA):
- Fast, first-line imaging to evaluate vessel anatomy and aneurysms
- MR Angiography (MRA):
- Useful in patients contraindicated for iodinated contrast
- DSA (Digital Subtraction Angiography):
- Gold standard for detailed vascular anatomy, aneurysm detection, and endovascular planning
Subarachnoid Hemorrhage (SAH):
Definition:
Bleeding into the subarachnoid space, typically between the arachnoid and pia mater, leading to irritation of meninges and potential vasospasm.
Etiologies of SAH:
1. Aneurysmal rupture (≈85% of non-traumatic SAH):
- Most common cause
- Sites commonly involved:
- AComA (30–40%)
- PComA (25%)
- MCA bifurcation (20%)
- Basilar tip or PCA (10%)
2. Arteriovenous malformations (AVMs):
- Congenital vascular malformations that may rupture and bleed
3. Traumatic SAH:
- Usually localized over convexities or interhemispheric fissure
- Associated with contusions or skull fractures
4. Perimesencephalic non-aneurysmal SAH:
- Localized bleeding anterior to midbrain
- Negative angiogram; good prognosis
5. Mycotic or infectious aneurysms:
- Seen in septic emboli/endocarditis patients
6. Coagulopathy-related hemorrhage:
- Seen in anticoagulated patients, liver disease, thrombocytopenia
7. Vasculitis or RCVS (Reversible cerebral vasoconstriction syndrome):
- Inflammatory or spasm-related vessel rupture
Radiologic Findings in SAH:
Non-contrast CT (first-line):
- Hyperdense blood in:
- Basal cisterns (interpeduncular, suprasellar)
- Sylvian fissures
- Interhemispheric fissure
- Most sensitive in first 72 hours
- Sensitivity decreases over time
CT Angiography (CTA):
- Detects aneurysms or AVMs
- Helps plan endovascular or surgical treatment
MRI:
- FLAIR: Can show SAH after 3–4 days
- SWI: Detects small hemorrhages
- Useful in late presenters or CT-negative SAH
Conclusion:
The Circle of Willis forms the principal site for intracranial collateral circulation and aneurysm formation. Its anatomic variants are common and impact the risk of ischemia and SAH. Aneurysmal rupture is the most frequent cause of spontaneous SAH. Early and accurate imaging using CT, CTA, and DSA is essential for diagnosis and management.
