There is acknowledged variability in the Circle of Willis in the general population, yet the structure and function relationship of the cerebrovasculature is poorly understood. We aimed to demonstrate the feasibility of combining high resolution imaging techniques and computational fluid dynamics (CFD) to describe cerebrovascular structure and function in vivo. We examined the null hypothesis that monozygotic (MZ) twins (18-30yrs) would exhibit similar CoW structure and function. Six twin pairs underwent 3T magnetic resonance angiography of the head and neck and B-mode Doppler ultrasound for velocity and diameter recordings in the vertebral and internal carotid arteries under three conditions (rest, hypercapnia and exercise). Artery diameter, length, tortuosity and bifurcation angle were assessed in regions of interest of the CoW. CFD simulated hemodynamics to determine the cardiac-cycle time averaged wall shear stress (TAWSS), oscillatory shear index (OSI) and relative residence time (RRT). We observed low and insignificant intra-class correlations (ICC) between twins in all regions for diameter (ICC range 0.000-0.657, P>0.05), two of four regions for length (ICC range 0.355-0.368, P>0.05), all regions for tortuosity (ICC range 0.270-0.505, P>0.05), and all bifurcation angles (ICC range 0.000-0.547, P> 0.05). Similarly, no significant correlations were apparent for cerebral blood flow or CFD-derived measures of TAWSS, OSI and RRT, at rest or in response to hypercapnia or exercise. Therefore differences exist in CoW structure and associated shear stress in response to physiological stimulation. These data suggest that the structure, function and health of cerebrovascular arteries are not primarily genetically dependent.

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