Department of Electrical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan.
IEEE Trans Ultrason Ferroelectr Freq Control. 2011 Feb;58(2):379-88. doi: 10.1109/TUFFC.2011.1815.
Sub-harmonic imaging techniques have been shown to provide a higher contrast-to-tissue ratio (CTR) at the cost of relatively low signal intensity from ultrasound contrast agents (UCAs). In this study, we propose a method of dual-frequency excitation to further enhance the CTR of subharmonic imaging. A dual-frequency excitation pulse is an amplitude-modulated waveform which consists of two sinusoids with frequencies of f₁ (e.g., 9 MHz) and f₂ (e.g., 6 MHz) and the resulting envelope component at (f₁ - f₂) (e.g., 3 MHz) can serve as a driving force to excite the nonlinear response of UCAs. In this study, the f₂, at twice of the resonance frequency of UCAs, is adopted to efficiently generate a sub-harmonic component at half of the f₂ frequency, and f₁ is included to enhance the high-order nonlinear response of UCAs at the sub-harmonic frequency. The second- and third-order nonlinear components resulting from the envelope component would spectrally overlap at the sub-harmonic frequency when f₁ and f₂ are properly selected. We further optimize the generation of the sub-harmonic component by tuning the phase terms between second- and third-order nonlinear components. The results show that, with dual-frequency excitation, the CTR at sub-harmonic frequency improves compared with the conventional tone-burst method. Moreover, the CTR changes periodically with the relative phase of the separate frequency component in the dual-frequency excitation, leading to a difference of as much as 9.1 dB between the maximal and minimal CTR at 300 kPa acoustic pressure. The echo produced from the envelope component appears to be specific for UCAs, and thus the proposed method has the potential to improve both SNR and CTR in sub-harmonic imaging. Nevertheless, the dual-frequency waveform may suffer from frequency-dependent attenuation that degrades the generation of the envelope component. The deviation of the microbubble's resonance characteristics from the selection of dual-frequency transmission may also decrease the CTR improvement.
次谐波成像是一种超声成像技术，通过降低超声对比剂（UCA）的信号强度来提高对比噪声比（CNR）。在本研究中，我们提出了一种双频激励方法，以进一步提高次谐波成像的 CNR。双频激励脉冲是一种调幅波形，由两个频率分别为 f₁（例如 9MHz）和 f₂（例如 6MHz）的正弦波组成，其包络分量在（f₁-f₂）（例如 3MHz）处可以作为驱动 UCA 非线性响应的力。在本研究中，采用频率为 UCA 共振频率两倍的 f₂，以有效地在 f₂频率的一半处产生次谐波分量，同时包含 f₁以增强 UCA 在次谐波频率处的高阶非线性响应。当 f₁和 f₂被适当选择时，包络分量产生的二阶和三阶非线性分量会在次谐波频率处光谱重叠。我们通过调整二阶和三阶非线性分量之间的相位项进一步优化次谐波分量的产生。结果表明，与传统的单频激励方法相比，双频激励可以提高次谐波频率处的 CNR。此外，CNR 随双频激励中独立频率分量的相对相位周期性变化，在 300kPa 声压下，最大和最小 CNR 之间的差异可达 9.1dB。包络分量产生的回波似乎是 UCA 特有的，因此，所提出的方法有可能提高次谐波成像中的 SNR 和 CNR。然而，双频波形可能会受到频率相关衰减的影响，从而降低包络分量的产生。微泡的共振特性与双频传输的选择之间的偏差也可能降低 CNR 的提高。
