1. A system for contrast agent imaging with ultrasound, the system comprising: a beamformer operable to obtain ultrasound data representing a cubic fundamental response of contrast agents; a detector operable to estimate velocity and energy values as a function of the ultrasound data; a memory storing a display map indexed by the velocity and energy values for outputting a display value; and a display operable to display an image as a function of the display value; wherein the beamformer is operable to obtain the ultrasound data representative of said cubic fundamental response of contrast agents in response to using one of different possible sequences of pulses corresponding to different possible pulse repetition intervals, and wherein the display is responsive to said sequence of pulses and to said pulse repetition interval to display a velocity scale annotation as a function of the pulse repetition interval of the used one of the different possible sequences.