| Detection of ultrasound contrast agents (UCAs) based on their nonlinear characteristics are popular for imaging blood perfusion. For microcirculation imaging, a higher frequency system (> 10 MHz) is required to obtain higher spatial resolution. However, high-frequency nonlinear techniques usually suffer from insufficient signal-to-noise ratio (SNR) due to the intense attenuation and the limited resonance frequency of commercial UCAs. In this study, the proposed dual-frequency excitation method involves the simultaneous transmission of two high-frequency sinusoids to produce an envelope signal at the difference frequency, which provides the low-frequency driving force for UCAs nonlinear oscillation. The low-frequency envelope results in significant improvement in contrast-to-tissue ratio (CTR) while the high-frequency carrier signal provides the fine image resolution. Moreover, the destruction efficiency with high-frequency ultrasound also improves. The destruction threshold of UCAs with dual-frequency excitation is generally reduced as compared to the conventional tone burst. |