1. A method for ultrasonic imaging with contrast agents comprising: transmitting ultrasonic pulses into a body infused with an ultrasonic contrast agent and receiving ultrasonic echo signals following a pulse transmission; forming coherent echo signals; differentiating coherent echo signals received from two pulse transmissions; processing the differentiated signals by persistence processing which causes a desired characteristic to persist in an image; and displaying an ultrasonic image which utilizes the persistence processed differentiated signals.