We present a new method, the Phase Optical Flow, which provides an automatic determination of abdominal regional movements along the cardiac cycle and allows a real time determination of the optimal cardiac trigger time to be used in quantitative liver DWI. A phase based motion amplification was applied to real-time BTFE images acquired at 20 images/s. Then optical flow was used to derive the velocity vector field. The optimal cardiac time window was defined as the 100ms-period with minimal absolute vertical velocity. Validation was provided by liver DWI obtained at several cardiac trigger times.