Longitudinal view of a radio-cephalic fistula using VFI, which represents the flow with many colored vectors frame. In this frame at the systolic peak it shows high velocity red vectors at the arterial side just before the fistula anastomosis, with recirculation and reverse flow (1), multidirectional low-velocity green vector against the venous wall on the venous side of the anastomosis (3) and faster vector streamline at the venous side of the fistula (2).