Huang Sheng-Wen, Li Pai-Chi
Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan.
IEEE Trans Ultrason Ferroelectr Freq Control. 2007 May;54(5):947-56. doi: 10.1109/tuffc.2007.340.
This paper proposes an approach to designing binary codes suitable for high-frequency applications of coded excitation in medical ultrasound. For a high-frequency ultrasound system, transmitting well-designed binary codes with a low sampling ratio (i.e., the bit rate divided by the transducer center frequency) is a practical way to improve the signal-to-noise ratio (SNR) because the challenge of implementing arbitrary-waveform generators for transmitting nonbinary codes increases with the frequency and the switching speed of square-wave pulsers are limited. One conventional approach designs codes using a base sequence that modulates wideband sequences up to the transducer passband. Because a major portion of codes is excluded as a candidate, codes designed using this approach typically need long compression filters for restoring the axial resolution, and they do not improve the SNR efficiently. In contrast, the approach proposed here searches all the codes that match the transducer passband; hence, the resultant codes exhibit better performance. The technique was tested using a bit rate of 50 MHz and a sampling ratio of 2. For a transducer with an ideal Gaussian frequency response with a center frequency of 25 MHz and a -6 dB bandwidth of 15 MHz, the SNR for the same side-lobe extent was 1 to 6 dB higher for the codes designed using the proposed approach compared with those designed using the conventional approach. When a real transducer response with a center frequency of 26.4 MHz and a one-way -6 dB bandwidth of 20.7 MHz was considered, the codes designed using the proposed approach were superior by 0.5 to 5 dB. Therefore, our approach is better than the conventional approach for designing binary codes for high-frequency ultrasound, with the results indicating that the moderate bit rate of 50 MHz will suffice when the ultrasonic center frequency is 25 MHz.
本文提出了一种设计适用于医学超声编码激励高频应用的二进制编码的方法。对于高频超声系统，以低采样率（即比特率除以换能器中心频率）传输精心设计的二进制编码是提高信噪比（SNR）的一种实用方法，因为实现用于传输非二进制编码的任意波形发生器的挑战会随着频率增加，并且方波脉冲发生器的开关速度是有限的。一种传统方法是使用一个基础序列来设计编码，该基础序列将宽带序列调制到换能器通带。由于大部分编码被排除在候选范围之外，使用这种方法设计的编码通常需要长压缩滤波器来恢复轴向分辨率，并且它们不能有效地提高信噪比。相比之下，这里提出的方法会搜索所有与换能器通带匹配的编码；因此，所得编码表现出更好的性能。该技术在比特率为50 MHz和采样率为2的情况下进行了测试。对于一个中心频率为25 MHz、-6 dB带宽为15 MHz的具有理想高斯频率响应的换能器，与使用传统方法设计的编码相比，使用所提出方法设计的编码在相同旁瓣程度下的信噪比高1至6 dB。当考虑中心频率为26.4 MHz、单向-6 dB带宽为20.7 MHz的实际换能器响应时，使用所提出方法设计的编码优势为0.5至5 dB。因此，我们的方法在为高频超声设计二进制编码方面优于传统方法，结果表明当超声中心频率为25 MHz时，50 MHz的适度比特率就足够了。
