Department of Electrical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan.
Ultrasonics. 2013 Mar;53(3):717-26. doi: 10.1016/j.ultras.2012.10.014. Epub 2012 Nov 9.
The presence of tissue harmonic generation during acoustic propagation is one major limitation in nonlinear detection of microbubble contrast agents. However, conventional solutions for tissue harmonic suppression are not applicable in dual-frequency (DF) harmonic imaging. In DF harmonic imaging, the second harmonic signal at second harmonic (2f(0)) frequency and the inter-modulation harmonic signal at fundamental (f(0)) frequency are simultaneously generated for imaging and both need to be suppressed to improve contrast-to-tissue ratio (CTR). In this study, a novel phase-coded pulse sequence is developed to accomplish DF tissue harmonic suppression.
Phase-coded pulse sequence utilizes multiple firings with equidistant transmit phase for harmonic cancellation in the sum of respective echoes. For the f(0) transmit component, the transmit phase comes from the equidistant set of {-2π/3, 0, 2π/3} to suppress the second harmonic signal at 2f(0) frequency. Moreover, in order to provide the inter-modulation harmonic suppression at f(0) frequency, the 2f(0) transmit phase has to be particularly manipulated for the corresponding f(0) transmit phase.
The proposed three-pulse sequence can remove not only the second-order harmonic signal but also other higher-order counterparts at both f(0) and 2f(0) frequencies. Measurements were performed at f(0) equal to 2.25 MHz and using hydrophone in water and contrast agents in tissue phantom. Experimental results indicate that the sequence reduces the tissue harmonic magnitude by about 20 dB along the entire axial depths and the corresponding CTR improves at both frequencies.
In DF harmonic imaging, the proposed phase-coded sequence can effectively remove the tissue harmonic background at both f(0) and 2f(0) frequencies for improvement of contrast detection.
在声波传播过程中产生的组织谐波是非线性检测微泡造影剂的一个主要限制因素。然而，用于组织谐波抑制的常规解决方案在双频（DF）谐波成象中并不适用。在 DF 谐波成象中，基波（f(0)）频率处的二次谐波信号和二次谐波（2f(0)）频率处的互调谐波信号同时产生，用于成像，两者都需要抑制以提高对比到组织比（CTR）。在本研究中，开发了一种新的相位编码脉冲序列来实现 DF 组织谐波抑制。
相位编码脉冲序列利用多个具有等距发射相位的发射来实现各自回波的和中的谐波抵消。对于 f(0)发射分量，发射相位来自于等距的 {-2π/3, 0, 2π/3} 集合，以抑制 2f(0) 频率处的二次谐波信号。此外，为了在 f(0) 频率处提供互调谐波抑制，必须特别操纵相应的 f(0) 发射相位来提供 2f(0) 发射相位。
所提出的三脉冲序列不仅可以去除 f(0)和 2f(0)频率处的二阶谐波信号，还可以去除其他高阶谐波信号。在 f(0)等于 2.25 MHz 的情况下，在水和组织模拟体中的水听器和造影剂中进行了测量。实验结果表明，该序列在整个轴向深度上使组织谐波幅度降低约 20 dB，并且在两个频率处的 CTR 都得到了提高。
在 DF 谐波成象中，所提出的相位编码序列可以有效地去除 f(0)和 2f(0)频率处的组织谐波背景，以提高对比度检测。
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