【作者】 刘志朋；

【导师】 王延群；

【作者基本信息】 北京协和医学院 ， 生物医学工程， 2013， 博士

【摘要】 磁声耦合效应成像是一种新型的旨在对早期病变组织进行诊断的功能成像方法。该方法利用磁声耦合效应,通过外加电磁场激励信号把反映组织电特性的信息转换为声信号进行检测,进而对声信号进行重建,实现组织电特性成像。该方法将输入的电磁能量转化为机械能,声源的产生和传播是关键问题。该方法的研究涉及到多学科交叉,仍处于探索阶段,还存在尚未解决的问题,目前仅实现了实验样本电导率边界信息的成像,尚未实现组织内部电导率信息的准确重建。本文为研究电磁激励下的电导率成像技术,通过理论模型仿真以及物理实验两条途径展开工作：首先,建立模拟实验样本声学、电学特性的长方体嵌套模型,建立基于洛伦兹力散度声源的电磁场、声场正逆问题仿真求解方法,在考虑声传感器特性基础上,求解声源信息和传播至样本表面的声压信号；反演重建声源分布和电导率分布；仿真比较磁脉冲激励和电流脉冲两种激励方式下产生声源的异同,结果表明磁脉冲激励下正问题声源除边界外有内部声源分布,两种方式下重建声源只有边界。其次,建立磁声耦合效应声信号检测实验系统,测试了实验系统的信噪比、分辨率等参数；为了比较激励源与声信号之间关系,设计实验,分别对直铜线、铜线圆环、导电橡胶样本进行电流脉冲激励,激励信号分别是0.7MHz、1MHz、1.3MHz的单周期正弦脉冲,采集记录样本激发的声压信号；采用频谱法分析声压信号频率特征,从信号与系统的角度分析输入输出幅频响应,分析声信号特征与激励信号的关系；采用相关估计法分析声压信号与激励源及声传感器响应之间的相关程度,建立激励源特征、声传感器响应与声信号之间关系,结果表明铜线与铜环激发声信号与激励源、声传感器响应相关程度较好；导电硅胶的信号与两者之间相关性非常小。在分析声信号特征基础上,建立声信号重建声源的反卷积投影算法,铜环、导电硅胶样本的重建声源结果表明：重建声源只有边界,在实验误差范围内重建边界与样本边界相符。该结果与仿真中电流激励的洛伦兹力散度声源只有边界声源的结果一致。受限于声传感器的带宽限制,检测到的声压信号在0.2-1.8MHz;此外,由于实验系统信噪比不够高,而且在反卷积过程中带入更多噪声,在声源重建中形成系统误差。下一步工作应重点提高检测系统信噪比,增大声传感器带宽,以及研制抑制噪声的反卷积算法,最终实现组织内部电导率成像。更多还原

【Abstract】 Imaging by magneto-acoustic coupling effect is a new functional imaging modality for diagnosis of malignant lesions in the early stage. In this way, the tissue is excited by magnetic field energy, and the magnetic energy is transformed to acoustic vibration based on magneto-acoustic coupling effect, whereas, acoustic signal carried the electric information of the tissue is detected by transducer. Then, reconstruction algorithm is conducted to the collected data, and electric conductivity of the tissue could be obtained. The key issue in this method is acoustic source as well as its mechanism of generation and propagation. Since this method involve and cross the research of theories multi-disciplinarily and many kinds of technology synthetically, many problems have not been solved, study still in stage of probing both in theory and practice. Up to now, only the electric conductive boundary of the experimental phantom is reconstructed with the experimental signal, conductive distribution inside the tissue couldn’t be reconstructed yet.In order to study the technology of magneto-acoustic tomography, simulation of theoretical model and physical experiments are performed. At first, model is established with cuboid shape which simulating the acoustic characteristics and electric parameter of the experimental sample. Secondly, considering the characteristics of acoustic transducer, the numerical simulation method is established to solve the direct-inverse problems in electro-magnetic field and acoustic field, according to acoustic source of Lorentz force divergence. After that, the distribution of acoustic source and electric conductivity are reconstructed. In the simulation, the differences are analyzed between the acoustic sources induced by magnetic pulse and that excited by electric pulse current. The analyzing results display that, there is acoustic source inside the boundary obtained in direct problem, which is induced by magnetic pulse, simultaneously, only electric conductive boundary of the model could be reconstructed either excited by magnetic pulse or electric pulse.Next, the experimental system is set up to detect the acoustic pressure signal, also, the SNR and solution of this system is tested practically. For the purpose of comparing the relationship between the exciting signal and acoustic signal, experiment is carried out to impose electric current pulse to samples, including direct copper wire and copper wire circle as well as conductive rubber with the concentric circles structure. In this experiment, while electric current pulse with frequency of 0.7MHz,1MHz and1.3MHz are applied to the samples, acoustic signals are recorded by the detecting system, respectively. After the signal collected, frequency-spectrum method is used to get the amplitude-frequency response of the input/output system, also, the relationship between the exciting signal and acoustic signal are studied. Moreover, correlation estimate method is adopted here to obtain the correlation degree between acoustic signal and transducer response as well as exciting signal, so as to explore the relationship between the acoustic signal and exiting signal together with transducer response. As a result, both of acoustic signals induced in copper wire and copper wire circle get high correlation degree with the exiting signal and transducer response, however, acoustic signal of conductive rubber get poor correlation degree with the other two signals.Last, according to the analyzing results of the acoustic signal, reverse convolution projection algorithm is given to reconstruct the distribution of acoustic source. Adopted the algorithm, the distribution of acoustic sources are obtained with the data of copper wire circle and conductive rubber, respectively. The results suggest that when considering the experimental system error, the reconstructed boundary of acoustic source is basically same as that of the samples. And hence, the results agree with the simulation, which validated that only the boundary could be reconstructed based on acoustic source of Lorentz force divergence.Nevertheless, limited by the bandwidth of transducer, acoustic pressure detected by the system is filtered by the transducer’s characteristics. So, the effect frequency segment of the received signal distribute from200KHz to1.8MHz. On the other hand, since the SNR of the detecting system is not high enough, more noise is involved in the process of reverse convolution, and produce system errors in reconstruction.Furthermore, key technology should be researched to promote SNR of the detecting system promptly. In the next work, how to widen the bandwidth of the transducer should be considered as an important issue. Additional, reverse convolution algorithm with suppressing noise function should be developed to get reconstruction image with high quality. Therefore, distribution of conductivity inside the tissue might be reconstructed in the near future.更多还原