We investigated the influence of voxel size on the accuracy and precision of intracranial vessel wall thickness measurements on MR images. Circle of Willis specimens were scanned at ultra-high resolution (0.11mm). Downsampling these images showed that distinguishing thin (0.35-0.45mm), medium (0.65-0.75mm) and thick (0.95-1.05mm) vessel walls requires voxel sizes below 0.55-0.66mm, although thickness measurements showed considerable bias at those resolutions. Unbiased measurements required a voxel size of 0.2mm or less. A clinically used MRI protocol (0.8mm), could only correctly measure vessel walls thicker than 0.9mm. In summary, current intracranial vessel wall MRI protocols provide limited quantification of vessel wall thickness.