Improvement of the power response in contrast imaging with transmit frequency optimization

Abstract

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). Two algorithms have been proposed to find an US excitation for which the frequency has been optimal with microbubbles. Simulations were carried out for encapsulated microbubbles of 2 ¿m-radius by considering the modified Rayleigh-Plesset equation for a 2.25 MHz transmitted frequency and for various pressure levels (20 kPa up to 420 kPa). In vitro experiments have been carried out using a 2.25 MHz transducer and using a programmable waveform generator. Responses of a 1/2000 blood mimicking fluid-diluted solution of Sonovue¿ were measured by a 3.5 MHz transducer. We show through simulations that our adaptive imaging technique allows to reduce the transmit maximal pressure. As for in vitro experiments the CTR can reach 10 dB. By proposing a close loop system whose frequency adapts itself with the perfused media, throughout the examination, the optimization system adapt itself to the remaining bubbles population thus allowing an increase of the 30% examination duration.