| Summary: | 碩士 === 國立臺灣科技大學 === 電機工程系 === 97 === The method of third harmonic (3f0) transmit phasing utilizes an additional 3f0 transmit signal to provide mutual cancellation and addition between the frequency-sum component and the frequency-difference component of tissue harmonic signal. Chirp excitation can further improve the signal-to-noise ratio (SNR) in harmonic imaging without requiring an excessive transmit pressure and thus reduce potential bubble destruction. However, for effective suppression and enhancement of tissue harmonic signal in 3f0 transmit phasing, the 3f0 chirp waveform has to be carefully designed for the generation of spectrally matched cancellation and addition pairs over the entire second harmonic band. In this study, we proposed a chirp waveform suitable for the method of 3f0 transmit phasing, the different-bandwidth chirp signal (DBCS). With the DBCS waveform, the frequency-difference component of tissue harmonic signal becomes a chirp signal similar to its frequency-sum counterpart. Thus, the combination of the DBCS waveform with the 3f0 transmit phasing can markedly suppress the tissue harmonic amplitude for contrast-to-tissue ratio (CTR) improvement together with effective SNR increase of contrast harmonic signal. Furthermore, the DBCS waveform also can enhance the tissue harmonic amplitude for SNR improvement of tissue harmonic. Our results indicate that, as compared to the conventional Gaussian pulse, the DBCS waveform can provide 6-dB improvement of contrast harmonic SNR in 3f0 transmit phasing with a CTR increase of 3-dB. In addition, the DBCS waveform can provide not only marked 8-dB improvement of harmonic SNR but also comparable pulse length to conventional transmit scheme. Nevertheless, the limitation of available transmit bandwidth and the frequency-dependent attenuation can degrade the performance of the DBCS waveform in tissue suppression and enhancement. The design of the DBCS waveform is also applicable to other multi-frequency imaging techniques which rely on the harmonic generation at the difference frequency.
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