Dunås T, et al. - Through a prospective study of 35 subjects, experts assessed and optimized three proposed segmentation methods and determined the exactness of in vivo 4D flow MRI (it allows for high-resolution velocity measurements with whole-brain coverage) blood flow rate evaluations in major cerebral arteries, with high-resolution 2D phase contrast magnetic resonance imaging (2D PCMRI) as a reference. Blood flow rates measured with 4D flow MRI, to the reference, in nine main cerebral arteries were differentiated. With a notable lower standard deviation for local, the least average flow variation was determined for 20% local or 10% global thresholding with the help of the thresholding methods. A large systematic underestimation of flow was observed for all clustering methods in comparison with 2D PCMRI. Hence, in contrast with a globally fixed threshold, a locally adapted threshold value presented a more stable outcome. Moreover, 4D flow with the proposed segmentation method was concluded to have the potential to become a beneficial reliable clinical tool for evaluation of blood flow in the major cerebral arteries.