Variations in the Configuration of the Circle of Willis in an East Asian Population: A 3D Magnetic Resonance Angiography Study

International Journal of Anatomy and Physiology | Vol. 15, No. 9, September 2026 | pp. 185–196

DOI: 10.46882/2026/IJAP/000183

Article Type: Original Research Article

Title: Variations in the Configuration of the Circle of Willis in an East Asian Population: A 3D Magnetic Resonance Angiography Study

Names of Authors: Kenji Sato¹, Takashi Abe²

Authors’ Affiliations: ¹Department of Anatomy and Neurobiology, Kyoto University Graduate School of Medicine, Kyoto, Japan; ²Department of Radiology, Tokyo Medical and Dental University, Tokyo, Japan

Abstract:
The anatomical completeness of the Circle of Willis (CoW) determines collateral blood flow configuration during ischemic cerebrovascular events. This study assessed the prevalence of classic and variant CoW architectures in a large cohort using three-dimensional time-of-flight magnetic resonance angiography (3D-TOF-MRA). Brain MRA scans of 620 healthy adult subjects (aged 20 to 75 years) were retrospectively examined. A complete symmetrical CoW was observed in only 42.3% of the cases (n = 262). Variant configurations were identified in 57.7% of the subjects (n = 358). The most common variant in the anterior circulation was hypoplasia or absence of the anterior communicating artery (ACoA), which occurred in 11.8% of individuals. In the posterior circulation, a fetal-type posterior cerebral artery (fPCA)—where the PCA originates directly from the internal carotid artery—was observed unilaterally in 14.5% and bilaterally in 4.8% of scans. Hypoplasia of the pre-communicating (P1) segment of the PCA was strongly associated with fPCA presence (p < 0.001). Asymmetry and hypoplasia were significantly more prevalent in the posterior circulation (38.2%) than in the anterior circulation (19.5%). These findings show that a complete CoW is absent in more than half of this East Asian sub-population, highlighting the value of non-invasive MRA screening to evaluate vascular resilience before neurosurgical or interventional neuroradiological procedures.

Keywords: Circle of Willis, Anatomical Variations, Magnetic Resonance Angiography, Fetal Posterior Cerebral artery, Collateral Circulation

Manuscript Timeline: Received: July 01, 2026; Revised: August 12, 2026; Accepted: August 28, 2026; Published: September 11, 2026

Citation: Sato K., & Abe T. (2026). Variations in the Configuration of the Circle of Willis in an East Asian Population: A 3D Magnetic Resonance Angiography Study. International Journal of Anatomy and Physiology, 15(9), 185–196. DOI: 10.46882/2026/IJAP/000183

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