Cerebrovascular Anatomy

Cerebrovascular anatomy describes the arteries and veins of the brain as they appear on imaging: the anterior and posterior arterial circulations, the anastomotic circle of Willis that links them, and the deep and superficial cerebral veins and dural venous sinuses that drain the brain. The emphasis is on identifying these vessels and their frequent variants on angiography, CT angiography, and MR angiography and venography.

Definition

Cerebrovascular anatomy is the descriptive and topographic anatomy of the intracranial arteries and veins, including the anterior and posterior arterial circulations, the circle of Willis, and the cerebral venous and dural sinus drainage, together with their recognized variants as resolved on angiographic and cross-sectional imaging.

Scope

This topic covers the intracranial arterial supply (internal carotid and vertebrobasilar systems and their major branches), the circle of Willis and its common configurational variants, and the cerebral venous drainage through the deep veins, cortical veins, and dural sinuses. It treats cerebrovascular anatomy as depicted on imaging and is not a guide to diagnosing or treating stroke or other cerebrovascular disease.

Core questions

• How do the anterior (carotid) and posterior (vertebrobasilar) circulations supply the brain?
• How does the circle of Willis connect these circulations, and how often is it incomplete or variant?
• How is cerebral venous drainage organized through deep veins, cortical veins, and dural sinuses?
• How do CT angiography, MR angiography, and MR/CT venography depict these vessels?

Key concepts

• Anterior (internal carotid) and posterior (vertebrobasilar) circulations
• Circle of Willis as an anastomotic ring
• Anterior, middle, and posterior cerebral arteries and their segments
• Configurational variants of the circle of Willis (for example hypoplastic or absent segments, fetal posterior cerebral artery)
• Deep cerebral veins and dural venous sinuses
• Time-of-flight MR angiography and MR/CT venography

Mechanisms

The brain is supplied by two paired inflow systems: the internal carotid arteries anteriorly and the vertebral arteries (joining to form the basilar artery) posteriorly. Their terminal branches are linked at the base of the brain by the circle of Willis, an anastomotic ring formed by the anterior and posterior communicating arteries together with segments of the anterior, middle, and posterior cerebral arteries; this ring provides potential collateral pathways but is frequently incomplete or asymmetric. Venous blood drains from the brain through superficial cortical veins and a deep venous system into the dural venous sinuses, which converge toward the internal jugular veins. On imaging, arteries are displayed by time-of-flight or contrast MR angiography and CT angiography, while the venous system and dural sinuses are shown by MR or CT venography; recognizing the named segments and their common variants is the basis of consistent identification (dimmick-2009; krabbe-hartkamp-1998; canedo-antelo-2019).

Clinical relevance

Accurate identification of the cerebral arteries, the completeness of the circle of Willis, and the venous and dural sinus anatomy supports radiological description and procedural planning, and helps distinguish normal variants from pathology, for example differentiating a hypoplastic sinus or vessel segment from an occlusion. This entry describes how cerebrovascular anatomy is recognized and named on imaging and does not provide diagnostic criteria or treatment guidance.

Epidemiology

Variation in the circle of Willis is common: imaging series report that a complete, symmetric ring is present in only a minority of individuals, with hypoplastic or absent communicating segments and fetal-type posterior cerebral artery configurations among the frequent variants encountered on MR and CT angiography (krabbe-hartkamp-1998; dimmick-2009).

Evidence & guidelines

Descriptive cerebrovascular imaging anatomy rests on illustrated narrative reviews and imaging series that catalogue the normal arterial and venous anatomy and the prevalence of variants on MR and CT angiography and venography (dimmick-2009; krabbe-hartkamp-1998; canedo-antelo-2019).

History

The basal anastomotic ring of cerebral arteries was described by Thomas Willis in the seventeenth century and bears his name. Its anatomy was later imaged in the living patient through catheter cerebral angiography and, from the late twentieth century, noninvasively through time-of-flight MR angiography and CT angiography, which enabled large series documenting the prevalence of circle-of-Willis variants and the normal cerebral venous anatomy (krabbe-hartkamp-1998; dimmick-2009).

Seminal works

• dimmick-2009
• krabbe-hartkamp-1998
• canedo-antelo-2019

Frequently asked questions

Is the circle of Willis usually complete?

No. Imaging studies show that a fully formed, symmetric circle of Willis is found in only a minority of people; hypoplastic or absent communicating segments and other configurational variants are common and are normal findings rather than disease.

How is cerebral venous anatomy imaged?

The deep and superficial cerebral veins and the dural venous sinuses are typically displayed with MR venography or CT venography, which are timed to the venous phase so that the venous drainage pathways are opacified.