基于脉冲压缩与二相编码激励的超声检测提高信噪比研究

【作者】 刘波；

【导师】 郭建中；

【作者基本信息】 陕西师范大学 ， 信号与信息处理， 2011， 硕士

【摘要】 超声成像作为临床检查手段方便、有效、价格低廉,在医学诊断领域应用广泛。传统的单脉冲激励模式超声成像,受到生物组织对于超声效应受限程度的限制,脉冲幅值不能太高,因而检测深度和检测分辨率二者不可兼得。雷达系统中经常使用的大时宽带宽积信号,运用在超声检测和超声成像领域,可以较好地解决检测深度和检测分辨率这二者之间的矛盾,在保持检测分辨率的同时大大提高回波信号处理后的信噪比,增大了检测深度,提升了超声成像的质量,成为近年来医学超声诊断设备领域的研究热点。基于编码激励和脉冲压缩技术,针对常用超声检测换能器的等效频带特性,本文采用正弦波载频调制二相编码信号,进行超声激励的仿真和实验研究,设计激励信号的匹配滤波器和失配滤波器,从时域和频域研究超声换能器对激励信号的影响,针对信噪比增益和分辨率损失研究超声换能器对激励信号脉冲压缩结果的影响,对信噪比增益损失和主瓣展宽系数进行曲线拟合分析,探讨超声换能器与激励信号的最优匹配方式,优化调制参数后选择最佳激励信号进行多目标环境下的复杂信号回波的处理分析。本文的研究内容主要包括：(1)改进矩形方波基础码调制形成二相编码激励信号,选择使用正弦波载频调制的二相编码激励信号,分别设计了脉冲压缩所需要的匹配滤波器和失配滤波器,对回波信号进行了匹配处理和失配处理。(2)计算机控制任意波形发生器产生软件设计的二相编码激励信号,激励换能器,对接收到的超声回波信号分别通过匹配滤波和失配滤波进行脉冲压缩后,研究分析脉冲压缩处理后信号的信噪比增益。(3)针对超声换能器具有的等效带通滤波特性,采用单位码元载有不同正弦波载频周期个数的巴克码激励信号来激励换能器,在时域和频域对回波信号的特性进行仿真分析和实验验证,从信噪比增益损失和分辨率损失两方面研究换能器对理想状态下的脉冲压缩的性能影响,探索最优激励参数,优化调制参数并选用最佳激励信号。(4)针对多目标环境下的复杂回波,研究间距不同的四根靶线对应的回波信号,采用最优激励信号激励换能器,通过对其复杂的混叠回波信号进行脉冲压缩,恢复四根靶线的位置信息,与相应的单脉冲实验结果进行信噪比和分辨率的对照研究。本文的分析研究结果表明：(1)二相编码激励信号通过超声换能器后,信号频带宽度和能量都有所减小；与不考虑超声换能器影响的理想状态脉冲压缩性能相比较,脉冲压缩结果的信噪比增益有所下降,压缩后的脉冲主瓣系数有所展宽(分辨率下降),并综合信噪比增益损失和分辨率损失两个参数,针对超声换能器的特性优化调制参数,选用单位码元载有3个正弦波载频周期数的13位巴克码调制信号最佳,作为超声换能器的最佳激励信号。(2)本文采用正弦波调制的二相编码信号激励超声换能器,在多目标复杂环境下,选用最佳激励信号,对不同间距四根靶线的复杂回波混叠信号分别进行匹配滤波处理和失配滤波处理,脉冲压缩失配处理结果的主旁瓣比值接近-40dB。比较传统单脉冲激励,二相编码激励在保证系统分辨率的同时,信噪比有了很大的提高,可以检测到传统设备很难检测到的淹没在噪声中的微弱信号,该激励方案可以很好的应用在超声检测领域。更多还原

【Abstract】 Ultrasonic imaging is considered to be an convenient、effective、cheap diagnosing means,and it has been widely used in the domain of medical diagnosing.Due to the limitation of the tissue’s bioeffets,the amplitude of the single pulse used in traditional single pulse excitation mode is limited, the penetration depth and resolution can not be obtained in the same time.Through using the large time-bandwidth product signal which is usually applied in radar system,the paradox of the penetration depth and resolution can be well resolved in ultrasonic detection and ultrasonic imaging,the SNR(signal to noise ratio)of the echo can be enhanced significantly after pulse compression with the same resolution compared with the single pulse excitation,and the penetration depth is been increased, the quality of the traditional ultrasonic imaging is been elevated,and coded excitation has become a hot spot in the medical ultrasonic diagnosing research.In this study, considering the equivalent bandpass filterting property of the transducer, a series of bi-phase coding signals have been used to excite the transducer in simulation and experiment research, the matched filter and mismatched filter have been designed. The transducer effect on the exciting signals in time and frequency domain is analyzed,and curve fitting is used to analyze the SNR gain loss and the resolution loss caused by the trandcucer effect on the pulse compression result.The optimum exciting signal is chosen to experiment in the multi-target environment after the modulation parameter is optimized.This study covers the following aspects:(1) Different from the bi-phase coding signal modulated by base code in the traditional coded excitation imaging, this study uses the bi-phase coding signal modulated by the sine carrier to excite the transdcucer, and the matched filter and mismatched filter used for pulse compression is designed. Considering the bi-phase coding signal’s property, also the echo signal is processed by matched filter and mismatched filter.(2) The bi-phase coding signal used in experiment is produced by the arbitrary waveform editing software which can control the arbitrary waveform generator through personal computer to transmit the designed signal to excite the transducer. Then the received echo signal is processed through matched filter and mismatched filter,the SNR gain after pulse compression is analyzed. (3) Considering the equivalent bandpass filterting property of the transducer, the barker code signals with each chip modulated by different period number of sine carrier are used to excite the transducer, for simulation and experiment the echo is processed in time and frequency domain. Considering the SNR gain and resolution loss,the transducer effect on the pulse compression result is analyzed.The optimum exciting signal is chosen for the following research.(4) Considering the complex echo during the multi-target environment, the transducer is excited by the optimum signal, and the echo signal reflected by four target wires is processed through pulse compression technique. After pulse compression, four narrow pulse which has large amplitude is obtained, and the result of the SNR and resolution after pulse compression compared with the single pulse mode is analyzed.The results of this study shows that:(1) From the analysis result of the transducer effect on both the exciting signal itself and the pulse compression result,we can infer that the frequency spectrum and energy of the exciting signal decreases after passing the transducer.Compared with the ideal pulse compression result without considering the transducer effect,the property of the actual pulse compression result is degraded, its SNR gain and resolution both decrease.After the modulation parameter is optimized considering the transducer frequency spectrum property,the 13 bit barker code signal with each chip modulated by 3 period sine carrier is chosen to be the optimum signal to excite the transducer.(2) The optimum signal is chosen to excite the transducer during the multi-target environment, after using the matched filter and mismatched filter to compress the echo reflected by four target wires, we can obtain the compressed signal which has nearly-40dB PSL.The SNR of the echo can be enhanced significantly with the same resolution using bi-phase coded excitation and pulse compression technique compared with the traditional single pulse excitation mode,it can detect the weak signal submerged in the background noise,it proves that coded excitation and pulse compression technique can be well applied in the domain of ultrasonic imaging system.更多还原