Misaridis T X, Gammelmark K, Jørgensen C H, Lindberg N, Thomsen A H, Pedersen M H, Jensen J A
Department of Information Technology, Technical University of Denmark, Lyngby, Denmark.
Ultrasonics. 2000 Mar;38(1-8):183-9. doi: 10.1016/s0041-624x(99)00130-4.
Improvement in signal-to-noise ratio (SNR) and/or penetration depth can be achieved in medical ultrasound by using long coded waveforms, in a similar manner as in radars or sonars. However, the time-bandwidth product (TB) improvement, and thereby SNR improvement is considerably lower in medical ultrasound, due to the lower available bandwidth. There is still space for about 20 dB improvement in the SNR, which will yield a penetration depth up to 20 cm at 5 MHz [M. O'Donnell, IEEE Trans. Ultrason. Ferroelectr. Freq. Contr., 39(3) (1992) 341]. The limited TB additionally yields unacceptably high range sidelobes. However, the frequency weighting from the ultrasonic transducer's bandwidth, although suboptimal, can be beneficial in sidelobe reduction. The purpose of this study is an experimental evaluation of the above considerations in a coded excitation ultrasound system. A coded excitation system based on a modified commercial scanner is presented. A predistorted FM signal is proposed in order to keep the resulting range sidelobes at acceptably low levels. The effect of the transducer is taken into account in the design of the compression filter. Intensity levels have been considered and simulations on the expected improvement in SNR are also presented. Images of a wire phantom and clinical images have been taken with the coded system. The images show a significant improvement in penetration depth and they preserve both axial resolution and contrast.
通过使用长编码波形，医学超声可以实现信噪比（SNR）和/或穿透深度的提高，这与雷达或声纳中的方式类似。然而，由于可用带宽较低，医学超声中的时间带宽积（TB）改善以及由此带来的SNR改善要低得多。SNR仍有大约20 dB的提升空间，这将在5 MHz时产生高达20 cm的穿透深度[M. O'Donnell，《IEEE超声、铁电与频率控制学报》，39(3) (1992) 341]。有限的TB还会产生高得不可接受的距离旁瓣。然而，超声换能器带宽带来的频率加权虽然并非最优，但在降低旁瓣方面可能是有益的。本研究的目的是对编码激励超声系统中的上述考虑因素进行实验评估。提出了一种基于改进型商用扫描仪的编码激励系统。为了将产生的距离旁瓣保持在可接受的低水平，提出了一种预失真调频信号。在压缩滤波器的设计中考虑了换能器的影响。考虑了强度水平，并给出了关于SNR预期改善的模拟结果。使用编码系统拍摄了线阵模型图像和临床图像。这些图像显示穿透深度有显著改善，并且保持了轴向分辨率和对比度。
