 Conventional phase contrast imaging can be extended by applying flow-sensitive gradients in 3 orthogonal planes within a 3D volume throughout the cardiac cycle.Such 4D imaging encodes the velocity of flowing blood at each voxel in the volume enabling fluid dynamics to be visualised using specialist software.Applications are in complex congenital heart disease and for research into cardiovascular flow characteristics - however it is not in routine clinical use due to the complexity of post-processing and relatively long acquisition times.