| Ultrasonic detection of nonlinear contrast agent suffers from background tissue harmonic signals and thus provides limited contrast-to-tissue ratio (CTR). To suppress tissue harmonic amplitude, the method of third harmonic (3f0) transmit phasing is developed to cancel the conventional frequency-sum component of tissue harmonic signal with the frequency-difference counterpart by properly phasing an additional 3f0 transmit signal. On the other hand, this study reveals that the nonlinear responses from contrast bubbles behave very differently in 3f0 transmit phasing such that the suppression phase for bubble deviates from that for tissue. Moreover, the achievable suppression of contrast harmonic signal is markedly reduced. Consequently, with the selection of 3f0 transmit phase that leads to maximal tissue suppression, the CTR can be significantly improved in harmonic imaging. |