Quantitative MRI Evaluation of Hemispheric Asymmetry in Circle of Willis Artery Diameters

Previous studies have identified hemispheric asymmetries in cerebral blood flow and volume, favoring the left hemisphere. Accordingly, we hypothesized that arteries on the left side of the circle of Willis (CoW) are larger than on the right. We compared artery diameters between the hemispheres.

Cranial time-of-flight magnetic resonance angiography scans of 1052 participants from a population-based cohort were assessed. Diameters of major CoW arteries (> 1.2 mm) were measured using a semiautomatic tool (mean ± standard deviation) and compared between the left and right hemisphere using a paired-samples t-test. As the posterior communicating arteries (Pcom) are often small and non-normally distributed, they were measured manually, categorized as "present" (≥ 1 mm) or "aplastic/hypoplastic" (< 1 mm), and compared using odds ratios (OR) with 95% confidence intervals (CI).

The A2 segment of the anterior cerebral artery was smaller on the left than on the right (1.97 ± 0.21 mm vs. 2.01 ± 0.22 mm; p < 0.001), while the vertebral artery (2.36 ± 0.44 mm vs. 2.25 ± 0.41 mm; p < 0.001) and P1 segment of the posterior cerebral artery (2.01 ± 0.28 mm vs. 1.98 ± 0.29 mm; p = 0.001) were larger on the left. The Pcom was less frequently present on the left (26.7%) than on the right (33.4%; OR 0.73, 95% CI 0.60-0.88). No left-right differences were found for the A1 segment, M1 segment of the middle cerebral artery, and internal carotid artery.

We found that some vessels were larger in the left hemisphere, whereas others were smaller. Future studies should investigate underlying mechanisms driving these specific asymmetries.