【作者】 傅娟；

【导师】 韦岗；

【作者基本信息】 华南理工大学 ， 通信与信息系统， 2014， 博士

【摘要】 在医学超声成像系统中，轴向分辨力、信噪比（探测深度）和对比度是衡量成像质量的主要指标。在传统短脉冲激励成像中，轴向分辨力和信噪比是一对难以调和的矛盾。而编码激励技术可以很好地解决这对矛盾。在发射端，对发射信号进行编码，在不提高发射信号的峰值功率下提高平均功率，从而提高系统的信噪比；在接收端，对回波信号进行脉冲压缩，恢复轴向分辨力。编码激励技术是提高超声成像质量的一种有效的方法，具有广阔的应用前景。在对现有理论和方法进行调研的基础上，本文提出了一些新的编码激励方法。其主要创新性成果有：1）为了提高轴向分辨力和确保对比度，提出了一种基于幅度加权的预失真线性调频编码激励方法。该方法将线性调频发射信号幅度加权技术和回波信号旁瓣抑制技术相结合，一方面补偿超声探头对发射信号的影响，使得回波信号的带宽不局限于探头，提高轴向分辨力；另一方面消除发射信号幅频特性的菲涅耳波纹，提高发射信号的带宽并采用失配滤波器进行脉冲压缩，实现旁瓣抑制，确保成像对比度。仿真结果表明，与恒包络的线性调频编码激励相比，该方法不仅可以提高轴向分辨力，而且确保对比度；在轴向分辨力和对比度两方面，取得较好的折中效果。FieldII仿真B超图像结果同样证实该方法提高轴向分辨力。2）传统Barker编码激励是采用单载频的正弦信号调制Barker编码的。本文中，提出了一种基于线性调频载波调制的Barker编码激励方法。对比正弦载波，线性调频载波的时间-带宽积可以大于1。因此，该方法通过提高载波带宽，可以提高轴向分辨力；另一方面，通过提高单位码片时间（即载波时间宽度），可以提高系统的信噪比。此外，系统的信噪比增益不仅能通过Barker码长获得，还可以通过载波的时间-带宽积获得。为了确保成像对比度，在脉冲压缩时，首先进行线性调频载波的脉冲压缩，然后进行Barker编码的脉冲压缩，实现旁瓣抑制。对于线性调频载波的脉冲压缩，当载波的时间-带宽积较小时，确保匹配滤波输出的主瓣内无旁瓣，此时只需要考虑匹配滤波器；当载波的时间-带宽积较大时，使得匹配滤波输出的主瓣内有旁瓣，此时需要考虑加切比雪夫窗的失配滤波器。对于Barker编码的脉冲压缩，采用基于峰值旁瓣水平最小的失配滤波器。仿真结果证明了所提出方法的有效性。3）提出了一种基于伪chirp载波调制的Barker编码激励方法。与传统的基于正弦载波调制的Barker编码激励相比，该方法采用伪chirp载波调制Barker编码，可以优化轴向分辨力和系统的信噪比。与基于线性调频载波调制的Barker编码激励相比，该方法的编码信号幅值是二进制形式的，平均功率大，从而提高系统的信噪比，同时保持相近的轴向分辨力和对比度，而且发射激励电路简单。仿真结果证明了所提出方法的有效性。4）提出了一种基于线性调频载波调制的Golay编码激励方法。与传统的基于正弦载波调制的Golay编码激励相比，该方法采用线性调频载波调制Golay互补序列对，可以优化轴向分辨力和系统的信噪比。与基于线性调频载波调制的Barker编码激励相比，该方法采用增加长度的Golay互补序列对，可以获得更高的信噪比及其增益，而且旁瓣幅度大大降低，因而增强成像对比度，同时保持相近的轴向分辨力。在脉冲压缩滤波器的设计中，对于Golay编码的脉冲压缩，只需要考虑匹配滤波器，脉冲压缩算法简单。仿真结果证明了所提出方法的有效性。5）提出了一种基于线性调频载波调制的二进制最优编码激励方法。与基于线性调频载波调制的Barker编码激励相比，该方法使用增加长度的二进制最优序列，可以获得更高的信噪比及其增益，同时保持相近的轴向分辨力。与基于线性调频载波调制的Golay编码激励相比，该方法只需要发射信号一次，脉冲压缩性能不易受组织运动的影响。在脉冲压缩滤波器的设计中，对于二进制最优序列的脉冲压缩，采用基于最小均方误差的逆滤波器，实现旁瓣抑制，确保成像对比度。仿真结果证明了所提出方法的有效性。更多还原

【Abstract】 In medical ultrasound imaging system, axial resolution, signal-to-noise ratio (SNR orpenetration) and contrast resolution are crucial factors to consider in evalution of imagequality. The tradeoff between axial resolution and SNR is well-known in conventionalsingle-pulse ultrasound system. However, coded excitation technique can well circumvent thisproblem. Here the duration of the excitation signal can be increased while maintaining thepeak power. More acoustic energy can be transmitted, thereby increasing the SNR. Axialresolution is usually recovered by compressing the echo signal in the receiver. The codedexcitation technique has been suggested as an efficient means of improving the quality ofultrasound imaging, so it has a wide application prospect. Based on a deep study on therelated theory and existing algorithms, the dissertation has developed some new methods onthe coded excitation. The main contributions are as follows.1) In order to improve axial resolution and satisfy contrast resolution of medicalultrasound imaging, a predistorted linear frequency modulated (LFM) coded excitationmethod based on amplitude weighting is proposed. Combining the amplitude weightingtechnology of LFM transmit signal with the sidelobe reduction technology of echo signal, themethod on the one hand can compensate the influence of transducer impulse response on thetransmit signal, so the bandwidth of echo signal is not limited by the transducer and the axialresolution is improved. On the other hand, the method can remove the Fresnel ripples of thetransmit signal’s frequency response, increase its bandwidth and use mismatched filter withthe chebyshev window for pulse compression, so range sidelobes are suppressed to ensure thecontrast resolution. The simulation results reveal that, in contrast to constant-envelope LFMcoded excitation, the proposed method can improve axial resolution and satisfy contrastresolution of medical imaging, and achieve good compromise between axial resolution andcontrast resolution. FieldII Simulation results of B-mode image show that the axial resolutionof the proposed method is improved.2) Traditionally, a single-frequency sinusoidal signal is used as the carrier of Barkercoded excitation. A Barker coded excitation method with LFM carrier is proposed. Incomparison with sinusoidal carrier, LFM carrier has the time-bandwidth product on the orderof above one. Thus, the method can improve axial resolution by increasing the bandwidth ofLFM carrier. On the other hand, the method can improve SNR by increasing the chip duration of Barker code, which is as large as the duration of LFM carrier. Moreover, SNR gain can beachieved not only by the length of Barker code but also by the time-bandwidth product ofLFM carrier. In order to ensure contrast resolution of medical imaging, pulse compressionscheme is developed to suppress sidelobes. It consists of two pulse compression filters forboth LFM carrier and Barker code, successively. With regard to the pulse compression ofLFM carrier, when the time-bandwidth of LFM carrier is low enough to have no sidelobes inthe mainlobe of matched filtered output, mathched filter is only considered; when thetime-bandwidth of LFM carrier is high enough to have some sidelobes in the mainlobe ofmatched filtered output, mismathched filter with the chebyshev window is applied. In addition,mismatched filter based on minimum peak sidelobe level is applied for pulse compression ofBarker code. The simulation results demonstrate the effectiveness of the proposed method.3) A Barker coded excitation method with pseudo-chirp carrier is proposed. In contrast toconventional Barker coded excitation with sinusoidal carrier, the method can improve axialresolution and SNR. In contrast to the Barker coded excitation with LFM carrier, theexcitation signal in the proposed Barker coded excitation is binary, so the average power islarge, and the SNR is improved while maintaining almost same axial resolution and contrastresolution. In addition, the transmitter circuit is simple. The effectiveness of the proposedmethod is examined by simulations.4) A Golay coded excitation method with LFM carrier is proposed. In comparison withconventional Golay coded excitation with sinusoidal carrier, the method uses LFM carrier, soit can improve axial resouliton and SNR. In comprarison with the Barker coded excitationusing LFM carrier, the method uses long-length Golay complementary sequences, so it canboost SNR and its gain, and achieve low level of range sidelobes to enhance contrastresolution while maintaining nearly same axial resolution. Matched filter is only considereredfor pulse compression of Golay complementary sequences and the pulse compression schemeis simple. The simulation results demonstrate the effectiveness of the proposed method.5) An optimal binary coded excitation with LFM carrier is proposed. In contrast to theBarker coded excitation with LFM carrier, the method uses a long-length optimal binarysequence, so it can boost SNR and its gain while maintaining nearly same axial resolution. Incontrast to the Golay coded excitation with LFM carrier, the method has advantage of singletransmit, and has no problem with motion-dependant decoding error. Pulse compressionscheme is developed to ensure contrast resolution. Inverse filter based on minimum meansquare error is applied for pulse compression of optimal binary sequence to suppresssidelobes. The simulation results demonstrate the effectiveness of the proposed method.更多还原