在對比劑諧波影像上，由於背景組織也有著顯著的組織諧波信號產生，故使得對比劑諧波影像的對比度(contrast-to-tissue ratio, CTR)受到限制。為抑制組織諧波信號，我們可以利用三倍頻發射相位法使得組織諧波信號的頻率和成分(frequency-sum component)與頻率差成份(frequency-difference component)有一相反的相位，以產生相消性的組織諧波強度抑制，而對於對比劑而言，我們的結果也指出在三倍頻發射相位法中其抑制相位與組織有異而且抑制程度也比組織明顯減少，該現象乃基於對比劑非常不同於組織的獨特非線性響應。換言之，將外加的三倍頻信號搭配一適當的相位與振幅大小便可以有效地抑制組織諧波強度但維持一定的對比劑諧波信號強度，此時便能有效提升CTR。不過我們也發現諧波溢漏(harmonic leakage)會對影像對比度的改善造成妨礙，這時就必須使用脈衝反向技術(pulse-inversion)來移除諧波溢漏的影響。

Harmonic detection of ultrasonic contrast agents provides limited contrast-to-tissue ratio (CTR) mainly because of background tissue harmonic signals. To suppress tissue harmonic amplitude, the method of third harmonic (3f0) transmit phasing can be utilized by phasing an additional 3f0 transmit signal to achieve effective cancellation between the frequency-sum component and the frequency-difference component of tissue harmonic signal. For contrast agents, however, our results indicate that the nonlinear responses from microbubbles behaves very differently in 3f0 transmit phasing. Specifically, the suppression phase for contrast microbubble deviates from that for tissue and 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 optimally improved in harmonic imaging. Nevertheless, the CTR improvement in 3f0 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 3f0 transmit phasing.

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