Background A promising technique to measure blood flow is timeresolved three-dimensional phase contrast MRI (PCMRI). In small structures as the Circle of Willis (CoW), the resolution of the measurement needs to be high (< 1 mm). Image quality may be compromised when SNR decreases with increasing resolution which leads to blood flow direction uncertainty and flow quantification inaccuracies. To increase SNR, PC-MRI can be conducted at higher field strengths. In this study timeresolved 3D PC-MRI is performed in the CoW of five volunteers at 3T and 7T to investigate the advantages of increased SNR.