The effect of third harmonic transmit phasing on contrast agent responses for CTR improvement

Abstract

Harmonic detection of ultrasonic contrast agents provides a limited contrast-to-tissue ratio (CTR) mainly because of background tissue harmonic signals. To suppress tissue harmonic amplitude, the method of third harmonic (3f(0)) transmit phasing can be utilized by phasing an additional 3f(0) transmit signal to achieve effective cancellation between the frequency-sum component and the frequency-difference component of the tissue harmonic signal. For contrast agents, on the other hand, our results indicate that the nonlinear responses from microbubbles behave very differently in 3f(0) transmit phasing. Specifically, the suppression phase for the contrast microbubble deviates from that for tissue. Moreover, the achievable suppression of the contrast harmonic signal is markedly reduced. Consequently, with the selection of 3f(0) transmit phase that leads to maximal tissue suppression, the CTR can be optimally improved in harmonic imaging. Nevertheless, the CTR improvement in 3f(0) transmit phasing can be compromised when the tissue suppression decreases in the presence of spectral leakage. Consequently, multi-pulse sequences such as pulse inversion are suggested for leakage removal in 3f(0) transmit phasing.