The contrast medical imaging is an active research domain in which several coding techniques of the transmit have been proposed to increase the contrast. In this paper, we focus on the continuous coding of the Ultrasound transmits, such as "chirp" sequences, and their insonation on the medium under exploration. We empirically show that there are particular values for the frequency parameters of the frequency coded excitation "chirp" which optimize the contrast in contrast enhanced ultrasound images. We also show that in the nonlinear response of micro-bubbles the modulation indices are no longer symmetrical.