Dept. of Electr. Eng., Nat. Taiwan Univ. of Sci. & Technol., Taipei, Taiwan.
IEEE Trans Ultrason Ferroelectr Freq Control. 2009 Oct;56(10):2198-206. doi: 10.1109/TUFFC.2009.1302.
Tissue background suppression is essential for harmonic detection of ultrasonic contrast microbubbles. To reduce the tissue harmonic amplitude for improvement of contrast-to-tissue ratio (CTR), the method of third harmonic (3f(0)) transmit phasing uses an additional 3f(0) transmit signal to provide mutual cancellation between the frequency-sum component and the frequency-difference component of tissue harmonic signal. Chirp excitation can further improve the SNR in harmonic imaging without requiring an excessive transmit pressure and thus reduce potential bubble destruction. However, for effective suppression of tissue harmonic background in 3f(0) transmit phasing, the 3f(0) chirp waveform has to be carefully designed for the generation of spectrally matched cancellation pairs over the entire second harmonic band. In this study, we proposed a chirp waveform suitable for the method of 3f(0) 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 3f(0) transmit phasing can markedly suppress the tissue harmonic amplitude for CTR improvement together with effective SNR increase of contrast harmonic signal. Our results indicate that, as compared with the conventional Gaussian pulse, the DBCS waveform can provide 6-dB improvement of SNR in 3f(0) transmit phasing with a CTR increase of 3 dB. Nevertheless, the limitation of available transmit bandwidth and the frequency-dependent attenuation can degrade the performance of the DBCS waveform in tissue suppression. The design of the DBCS waveform is also applicable to other dual-frequency imaging techniques that rely on the harmonic generation at the difference frequency.
组织背景抑制对于超声对比微泡的谐波检测至关重要。为了降低组织谐波幅度，以提高对比信噪比（contrast-to-tissue ratio，CTR），三谐波（3f(0)）发射相位法采用额外的 3f(0)发射信号，以实现组织谐波信号的频率和分量与频率差分量之间的相互抵消。啁啾激发可以在不要求过高发射压力的情况下进一步提高谐波成像中的 SNR，从而降低潜在的气泡破坏。然而，为了在 3f(0)发射相位法中有效抑制组织谐波背景，3f(0)啁啾波形必须精心设计，以便在整个二次谐波带中产生光谱匹配的抵消对。在本研究中，我们提出了一种适用于 3f(0)发射相位法的啁啾波形，即不同带宽啁啾信号（Different-Bandwidth Chirp Signal，DBCS）。利用 DBCS 波形，组织谐波信号的频率差分量成为类似于其频率和分量的啁啾信号。因此，DBCS 波形与 3f(0)发射相位法的结合可以显著抑制组织谐波幅度，提高 CTR，同时有效提高对比谐波信号的 SNR。我们的结果表明，与传统的高斯脉冲相比，DBCS 波形可以在 3f(0)发射相位法中提供 6dB 的 SNR 改善，同时 CTR 提高 3dB。然而，可用发射带宽的限制和频率相关衰减会降低 DBCS 波形在组织抑制中的性能。DBCS 波形的设计也适用于其他依赖于差频谐波产生的双频成像技术。
