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三维电阻抗成像数值算法和技术基础研究
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摘要
电阻抗成像(Electrical Impedance Tomography, EIT)是一种新型的医学成像技术,通过向被测场域施加电激励并测量其表面的电响应来重构其内部的电导率分布。与现有医学影像技术相比,电阻抗成像技术具有功能性成像、设备简单、成本低廉、体积小、对人体无创无害等优点。在临床医学领域,如脑部疾病、消化道疾病、肺部疾病、肿瘤诊断等方面具有良好的应用前景。目前电阻抗成像的研究主要集中在二维成像,现有的三维系统一般采用多层闭合电极的结构,主要应用在肺部、胃部疾病的诊断。而乳腺癌、腹腔内渗血等疾病的诊断,只需要研究皮肤下浅层区域的电导率分布,且乳房等器官不适合使用多层的闭合电极。本文拟在开放式电阻抗成像的基础上,研究三维电阻抗成像激励测量模式、重构算法、系统实现和物理实验等问题,可以应用在乳腺癌普查、前期诊断和腹腔内出血的检测。论文的主要研究内容包括以下几个方面:
     1)分析研究国内外现有电阻抗成像系统,针对临床应用提出平板电极加背电极的电阻抗成像结构,该结构可以使电流场分布更加均匀,并能增加探测深度。
     2)三维电阻抗成像正问题研究。在研究三维电阻抗成像电极模型的基础上,由麦克斯韦方程组推导全电极模型的有限元方程,在Matlab环境下编写有限元程序,并采用商业电磁场仿真软件Comsol对比程序计算精度。结果表明程序计算结果与Comsol结果误差小于4%,可以作为逆问题计算的基础。采用仿真手段对比研究现有的电阻抗成像激励测量模式,提出区分度和表面投影成像两种评估方法,结果表明背电极电流激励模式区分度高,投影图像效果好,为三维系统设计提供依据。
     3)三维电阻抗成像逆问题研究。采用基于牛顿拉弗逊算法的Tikhonov和NOSER正则化算法,首次针对平板电极结构系统开展三维场域的重构计算。提出类似拉普拉斯算子的Tikhonov正则化矩阵选取方法,并使用L曲线确定正则化参数。建立三维圆柱模型对单目标情况进行仿真,结果表明,两种正则化算法存在各自的优缺点。在此基础上提出混合这两种算法的混合正则化算法,通过在同一模型中的仿真,表明混合算法进一步降低条件数,改善逆问题性态,使重建过程趋于稳定,提高目标分辨率。由于三维场域逆问题计算量巨大,计算时间长,借鉴开放式电阻抗成像的方式,提出对二维图像进行线性插值的三维插值成像的思想,通过快速的二维成像以及线性插值,可以得到两层二维图像之间的电导率分布情况。
     4)测量系统研制。在深入探讨电阻抗成像激励源、信号测量关键问题基础上,介绍了三维多频电阻抗成像测量硬件系统各个模块设计思路以及实现方式,设计制作出实验样机,并对系统各项性能展开测试。
     5)实验研究。为验证样机的性能和重构算法有效性,开展了物理模型实验,包括水槽时差成像实验、琼脂模型时差成像实验、琼脂模型频差成像实验。设计模拟腹腔内出血的血块实验,通过改变血块体积模拟不同出血量。实验结果表明,三维电阻抗成像系统能够重构出深度4cm、大小1cm的目标,分辨不同体积的血块,图像的空间分辨率较高。
     最后对研究工作进行了总结,指出目前存在的问题及改进方向。
Electrical Impedance Tomography(EIT) is a new medical imaging modality thatimages by computing electrical properties inside the human body. In the clinicalmedical field it has a good application prospect, such as gastrointestinal disease, lungdisease, cancer diagnosis and brain disease. Compared to the current medical imagingtechnology, EIT has its own advantages, for example, functional imaging, simpleequipment, low-cost and non-invasion. So far the study for EIT has focused on the2-Dmodel. The electrode system of current3D EIT is multilayer closed electrode which isused in the lung, stomach disease diagnosis. While in the breast cancer andintra-abdominal ooze blood disease diagnosis, the conductivity distribution under theskin of the shallow area need to be study. This paper have study the3D EIT include themeasurement system, the stimulatied model, the reconstruction algorithm and physicalexperiment. This system can be used in breast cancer diagnosis and abdominal bleedingdetection. The main contents of the dissertation includethe following parts:
     1) The current breast cancer detection equipments, including the domestic andforeign existing EIT systems, were studied and analysed. The structure of plateelectrodes and the back electrode was proposed for clinical application, which makesthe current distribution more uniform, as well as increases the detecting depth.
     2) As for the forward problem of3-D EIT, the finite element equations of completeelectrode model were deduced based on the3-D electrode modeling and themathematical model in the electromagnetic problem. Finite element programs werecompiled in Matlab, and commercial electromagnetic field simulation software Comsolwas applied to compare the accuracy of the programs. The results show that the errorbetween Matlab and Comsol is less than4%, which indicate that Matlab programs canbe used as the basis of inverse calculation. The simulations of finite element programswas used to contrast several existing stimulation patterns, then two methods, thedistinguishability and surface projection image methods, were put forward to evaluatethe stimulation pattern. The results show that the back electrode pattern has obviousadvantages in differentiation degree and the projective quality, which provides providereference for the system design.
     3) Baesd on the characteristics of the inverse problems of EIT, the Tikhonovregularization algorithm and Newton's One-step Error Reconstructor (NOSER) were used to reconstructed the3D field firstly. The optimal method which is called L-curvemethod was used to select the better Tikhonov regularization parameter. And then theregularization matrix which is similar to the Laplace operator was also given. Then, a3-D cylinder model is set for simulation. The results show that the single regularizationalgorithm has its own merits and demerits, and therefore a hybrid algorithm waspresented based on Tikhonov regularization and the NOSER.Finally, a series ofsimulations were carried out to compare these three algorithms.The encouraging resultsindicate that this proposed algorithm can reduce the number of condition, make thereconstruction process stable and improve the spstial resolution. Owing to the greatamount of calculation in3D inverse problem, based on the2D image reconstruction foropen EIT,3D interpolation imaging algorithm was put forward, and linear interpolationalgorithm was used to get the images between two2D layers.
     4) The hardware system was developed. Based on the deep research on the keyproblem of EIT measurement and constant flow source, the principle and realization of3D multi-frequency measurement system were introduced. The experiment prototypewas built and the basic performances had been tested.
     5) To test the effectiveness of the performance of the prototype and the combinedreconstruction algorithm, the physical model experiment were carried out, includingtank dynamic imaging experiment, agar dynamic model experiment and the frequencydifference imaging experiments. A simulation experiment of abdominal bleeding wasdesigned, by changing the clot volume to simulate the different blood loss. Theexperimental results show that this3D imaging system can detect the1-cm diametertarget in4-cm-deep and distinguish different volume of blood clots. The spatialresolution of the image and the contrast is better.
     The final part is the summary description. This paper summarized the majorresearch achievements, as well as the existing problems and deficiencies. Meanwhile,the further researches on this topic were previewed.
引文
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