Exploitation of multi-frequency excitation for contrast agent imaging

We propose in this study a contrast agent detection method based on a multi-frequency excitation. In fact, the frequency dependency of microbubbles linear scattering is known to differ significantly from the one of tissue. The method was evaluated using an open scanner with analog transmitters. In vitro experiments were performed using a 3.5 MHz single element in transmit. Gaussian pulses at 2.5 MHz, 4 MHz and 6 MHz were transmitted simultaneously. Echoes from a perfect reflector were compared to those from SonoVue^® microbubbles. The amplitude of each transmitted frequency component was chosen such as the scattered power from tissue was equal for the 3 frequencies. SonoVue^® responses were the highest at 2.5 MHz. They decayed with 8 dB and 14 dB at 4 MHz and 6 MHz respectively. Images were also recorded using a 128-element linear probe centered at 3.5 MHz. A flow phantom containing a 4 mm diameter tube was used in which 1/2000 diluted solution of SonoVue^® was circulating. The method provides a good contrast to tissue ratio enhancement. These results demonstrate that multi-frequency excitation scheme can be exploited further to discriminate between the response of contrast agent and tissue.