Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu, Taiwan, Republic of China.
Phys Med Biol. 2011 May 7;56(9):2767-78. doi: 10.1088/0031-9155/56/9/009. Epub 2011 Apr 5.
The method of dual-frequency (DF) difference excitation is capable of generating a low-frequency envelope component as the driving force of commercial contrast microbubbles by using a high-frequency pulse. Although the DF difference excitation method provides good lateral resolution in high-frequency contrast imaging, it suffers from degraded axial resolution because a longer-than-usual envelope component is required to induce the oscillation of microbubbles. In this study, a coded excitation technique (i.e. chirp waveform) is combined with the DF difference excitation method (also referred to as the DF chirp excitation method) to improve the axial resolution of contrast imaging while maintaining the impinging insonation energy. B-mode images were constructed to compare the performance of the DF chirp excitation method with the conventional tone-burst pulse method. Results indicate that the proposed DF chirp excitation method can provide better axial resolution after pulse compression. Moreover, as compared to the tone-burst pulse with the same pulse duration, the pulse compression results in a higher signal-to-noise ratio because of the temporal concentration of the received energy. Nevertheless, images with the DF chirp excitation method demonstrated noticeable image artefacts resulting from the range sidelobes. The DF chirp excitation method also produced obvious tissue harmonic generation that could degrade the contrast-to-tissue ratio at higher acoustic pressures.
双频（DF）差频激发方法能够利用高频脉冲产生低频包络分量作为商业对比微泡的驱动力。虽然 DF 差频激发方法在高频对比成像中提供了良好的横向分辨率，但由于需要更长的包络分量来诱导微泡的振荡，因此轴向分辨率会降低。在这项研究中，编码激发技术（即啁啾波形）与 DF 差频激发方法（也称为 DF 啁啾激发方法）相结合，在保持脉冲入射能量的同时提高了对比成像的轴向分辨率。构建了 B 模式图像，以比较 DF 啁啾激发方法与传统的单音脉冲方法的性能。结果表明，经过脉冲压缩后，所提出的 DF 啁啾激发方法可以提供更好的轴向分辨率。此外，与具有相同脉冲持续时间的单音脉冲相比，由于接收能量的时间集中，脉冲压缩会产生更高的信噪比。然而，DF 啁啾激发方法产生的图像具有明显的图像伪影，这是由于距离旁瓣引起的。DF 啁啾激发方法还产生了明显的组织谐波产生，这可能会降低在较高声压下的对比到组织比。
