雙頻 ( dual-frequency, DF ) 發射方法包含基頻與二倍頻兩個頻率成分，在雙頻組織諧波影像中 ( tissue harmonic imaging, THI )，可同時獲得激發波形之f0所產生的二次諧波位於2f0，與激發波形之f0、2f0耦合所產生的頻率差 ( frequency-difference ) 位於f0。為了提升訊雜比 ( signal-to-noise ratio, SNR ) 而結合啾聲編碼 ( chirp ) 於雙頻發射方法，在波形設計時需考慮所產生之諧波頻率差位於f0的頻寬。本研究提出兩種雙頻啾聲編碼發射方法 ( 即UD11與UU13 ) 能夠提供足夠的諧波信號頻寬。實驗結果顯示，UU13發射方法於脈衝壓縮時，易受到3f0的干擾而產生明顯的軸向旁瓣信號 ( range side-lobe )。即使UU13發射方法在f0的諧波封包與UD11差不多，但在2f0的諧波封包中，UD11發射方法能夠抑制較多的軸向旁瓣信號。組織仿體與線仿體的B-mode諧波影像也顯示，使用UD11發射方法能夠抑制較多的假影產生。相較於一般雙頻短脈衝發射方法，UD11能夠提升影像SNR約10 dB。綜合整體結果，UD11為雙頻啾聲編碼發射方法的最佳選擇。

Dual-frequency (DF) transmit waveform comprises of signals at both fundamental frequency (f0) and second harmonic frequency (2f0). With the DF transmit waveform, tissue harmonic imaging can be simultaneously performed not only using the conventional 2f0 second harmonic signal but also the f0 frequency-difference harmonic signal. Nonetheless, when chirp excitation is incorporated into the DF transmit waveform for improvement of harmonic SNR, particular waveform design is required to maintain the bandwidth of the f0 harmonic signal. In this study, two different chirp waveforms (i.e., UD11 and UU13) are proposed to provide the desired signal bandwidth. Experimental results indicate that the UU13 tends to suffer from high range side lobe level due to severe 3f0 interference. Consequently, the 2f0 harmonic envelope of the UD11 is consistently superior to that of the UU13 while the quality of the f0 harmonic envelope remains similar between the two DF transmit waveforms. B-mode harmonic images also show that the UD11 is less susceptible to range side lobe artifacts than the UU13. Compared to a short pulse, the UD11 waveform also improves the image SNR by about 10 dB. Therefore, it is concluded that the UD11 waveform is a better solution for chirp-encoded DF harmonic imaging.

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