Study On Coded Excitation Technique And Performance Optimization Of Medical Ultrasound Imaging

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.