Zhao Feifei, Tong Ling, He Qiong, Luo Jianwen
Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing 100084, People's Republic of China.
Phys Med Biol. 2017 Feb 21;62(4):1565-1584. doi: 10.1088/1361-6560/aa58d7. Epub 2017 Jan 11.
Diverging wave (DW) based cardiac imaging has gained increasing interest in recent years given its capacity to achieve ultrahigh frame rate. However, the signal-to-noise ratio (SNR), contrast, and penetration depth of the resulting B-mode images are typically low as DWs spread energy over a large region. Coded excitation is known to be capable of increasing the SNR and penetration for ultrasound imaging. The aim of this study was therefore to test the feasibility of applying coded excitation in DW imaging to improve the corresponding SNR, contrast and penetration depth. To this end, two types of codes, i.e. a linear frequency modulated chirp code and a set of complementary Golay codes were tested in three different DW imaging schemes, i.e. 1 angle DW transmit without compounding, 3 and 5 angles DW transmits with coherent compounding. The performances (SNR, contrast ratio (CR), contrast-to-noise ratio (CNR), and penetration) of different imaging schemes were investigated by means of simulations and in vitro experiments. As for benchmark, corresponding DW imaging schemes with regular pulsed excitation as well as the conventional focused imaging scheme were also included. The results showed that the SNR was improved by about 10 dB using coded excitation while the penetration depth was increased by 2.5 cm and 1.8 cm using chirp code and Golay codes, respectively. The CNR and CR gains varied with the depth for different DW schemes using coded excitations. Specifically, for non-compounded DW imaging schemes, the gain in the CR was about 5 dB and 3 dB while the gain in the CNR was about 4.5 dB and 3.5 dB at larger depths using chirp code and Golay codes, respectively. For compounded imaging schemes, using coded excitation, the gain in the penetration and contrast were relatively smaller compared to non-compounded ones. Overall, these findings indicated the feasibility of coded excitation in improving the image quality of DW imaging. Preliminary in vivo cardiac images of a healthy volunteer were presented finally, and higher SNR and deeper penetration depth can be achieved by coded schemes.
近年来，基于发散波（DW）的心脏成像因其能够实现超高帧率而越来越受到关注。然而，由于发散波将能量分散在较大区域，所得到的B模式图像的信噪比（SNR）、对比度和穿透深度通常较低。已知编码激励能够提高超声成像的信噪比和穿透能力。因此，本研究的目的是测试在DW成像中应用编码激励以改善相应的信噪比、对比度和穿透深度的可行性。为此，在三种不同的DW成像方案中测试了两种类型的编码，即线性调频啁啾编码和一组互补格雷码，这三种方案分别是：1角度DW发射且不进行复合、3角度和5角度DW发射且进行相干复合。通过模拟和体外实验研究了不同成像方案的性能（信噪比、对比度比（CR）、对比噪声比（CNR）和穿透深度）。作为基准，还包括了采用常规脉冲激励的相应DW成像方案以及传统聚焦成像方案。结果表明，使用编码激励时，信噪比提高了约10 dB，而使用啁啾编码和格雷码时，穿透深度分别增加了2.5 cm和1.8 cm。对于使用编码激励的不同DW方案，CNR和CR增益随深度而变化。具体而言，对于非复合DW成像方案，在较大深度处，使用啁啾编码时CR增益约为5 dB，使用格雷码时约为3 dB；使用啁啾编码和格雷码时，CNR增益分别约为4.5 dB和3.5 dB。对于复合成像方案，与非复合方案相比，使用编码激励时穿透和对比度的增益相对较小。总体而言，这些发现表明编码激励在改善DW成像图像质量方面的可行性。最后展示了一名健康志愿者的初步体内心脏图像，通过编码方案可以实现更高的信噪比和更深的穿透深度。
