Nowicki A, Trots I, Lewin P A, Secomski W, Tymkiewicz R
Institute of Fundamental Technological Research, Polish Academy of Sciences, Swietokrzyska 21, 00-049 Warsaw, Poland.
Ultrasonics. 2007 Dec;47(1-4):64-73. doi: 10.1016/j.ultras.2007.07.003. Epub 2007 Aug 6.
In contrast to previously published papers [A. Nowicki, Z. Klimonda, M. Lewandowski, J. Litniewski, P.A. Lewin, I. Trots, Comparison of sound fields generated by different coded excitations - Experimental results, Ultrasonics 44 (1) (2006) 121-129; J. Litniewski, A. Nowicki, Z. Klimonda, M. Lewandowski, Sound fields for coded excitations in water and tissue: experimental approach, Ultrasound Med. Biol. 33 (4) (2007) 601-607], which examined the factors influencing the spatial resolution of coded complementary Golay sequences (CGS), this paper investigates the effect of ultrasound imaging transducer's fractional bandwidth on the gain of the compressed echo signal for different spectral widths of the CGS. Two different bit lengths were considered, specifically one and two cycles. Three transducers having fractional bandwidth of 25%, 58% and 80% and operating at frequencies 6, 4.4 and 6 MHz, respectively were examined (one of the 6 MHz sources was focused and made of composite material). The experimental results have shown that by increasing the code length, i.e. decreasing the bandwidth, the compressed echo amplitude could be enhanced. The smaller the bandwidth was the larger was the gain; the pulse-echo sensitivity of the echo amplitude increased by 1.88, 1.62 and 1.47, for 25%, 58% and 80% bandwidths, respectively. These results indicate that two cycles bit length excitation is more suitable for use with bandwidth limited commercially available imaging transducers. Further, the time resolution is retained for transducers with two cycles excitation providing the fractional bandwidth is lower than approximately 90%. The results of this work also show that adjusting the code length allows signal-to-noise-ratio (SNR) to be enhanced while using limited (less that 80%) bandwidth imaging transducers. Also, for such bandwidth limited transducers two cycles excitation would not decrease the time resolution, obtained with "conventional" spike excitation. Hence, CGS excitation could be successfully implemented with the existing, relatively narrow band imaging transducers without the need to use usually more expensive wideband, composite ones.
与之前发表的论文[A. Nowicki, Z. Klimonda, M. Lewandowski, J. Litniewski, P.A. Lewin, I. Trots, 不同编码激励产生的声场比较——实验结果, 《超声学》44 (1) (2006) 121 - 129; J. Litniewski, A. Nowicki, Z. Klimonda, M. Lewandowski, 水中和组织中编码激励的声场：实验方法, 《超声医学与生物学》33 (4) (2007) 601 - 607]不同，之前的论文研究了影响编码互补戈莱序列（CGS）空间分辨率的因素，而本文研究了超声成像换能器的分数带宽对不同频谱宽度的CGS压缩回波信号增益的影响。考虑了两种不同的码长，具体为一个周期和两个周期。研究了三个分数带宽分别为25%、58%和80%且工作频率分别为6 MHz、4.4 MHz和6 MHz的换能器（其中一个6 MHz的源是聚焦的且由复合材料制成）。实验结果表明，通过增加码长，即减小带宽，可以提高压缩回波幅度。带宽越小，增益越大；对于25%、58%和80%的带宽，回波幅度的脉冲 - 回波灵敏度分别提高了1.88、1.62和1.47。这些结果表明，两个周期的码长激励更适合与带宽受限的商用成像换能器一起使用。此外，对于分数带宽低于约90%的换能器，采用两个周期激励时时间分辨率得以保留。本文的研究结果还表明，在使用有限（小于80%）带宽成像换能器时，调整码长可以提高信噪比（SNR）。而且，对于这种带宽受限的换能器，两个周期激励不会降低通过“传统”尖峰激励获得的时间分辨率。因此，CGS激励可以在现有的、相对窄带的成像换能器上成功实现，而无需使用通常更昂贵的宽带复合材料换能器。
