1 OPTIMIZATION OF THE CONTRAST TISSUE RATIO IN ULTRA- SOUND CONTRAST IMAGING BY AN ADAPTIVE TRANSMIT FRE-

Conventionnal ultrasound contrast imaging systems use a fixed transmit frequency. However it is known that the insonified medium (microbubbles) is time-varying and therefore an adapted time-varying excitation is expected. We suggest an adaptive imaging technique which selects the optimal transmit frequency that maximizes the contrast tissue ratio (CTR). The method is proposed with the contrast imaging and the harmonic contrast imaging. Simulations were carried out for encapsulated microbubbles of 2µm-radius by considering the modified Rayleigh-Plesset equation for various pressure levels (80 kPa up to 420kPa). In vitro experiments have been carried out using using a 2.25 MHz transmitter transducer and using a programmable waveform. We show, through experiments, that our adaptive imaging technique increases the CTR of 2 dB compared to the standard method looking at the central frequency. By proposing a close loop system whose frequency adapts itself with the perfused media, throughout the examination, the optimization system adapt itself to the bubble population to seek the best trade-off between the bubble response and the transducer bandwidth. 1