Pulsatility of blood flow in human perivascular spaces may play an important role in clearing metabolic waste from the brain. However, flow measurement within small arterioles using MRI are hampered by the limited spatial resolutions of MRI. We report an approach that can accurately measure flow in sub-voxel tubular structures such as arterioles by combining time of flight and phase contrast MRI. The accuracy of our method were first demonstrated in a flow phantom study. Then, flows of penetrating arterioles were obtained in human subjects. Our results demonstrate the importance of correction of partial volume effects in measuring arteriole flow.