《用于脑部电呢阻抗断层成像的高稳定性恒流源的设计.doc》由会员分享，可在线阅读，更多相关《用于脑部电呢阻抗断层成像的高稳定性恒流源的设计.doc（11页珍藏版）》请在三一办公上搜索。
1、 用于脑部电阻抗断层成像的高稳定性恒流源的设计 作者： 史学涛，尤富生，霍旭阳，付峰，刘锐岗，董秀珍【关键词】 电阻抗断层成像；恒流源；分布电容；屏蔽驱动 Design of a current source with high stability for brain electrical impedance tomography system 【Abstr
2、act】 AIM: To improve the stability of the current source for brain electrical impedance tomography (EIT) data acquisition system. METHODS: A shield guard technique was employed to minimize the shunting effect of the stray capacitance of wires between electrodes and current source. At the same time,
3、an output current compensating method was adopted to compensate the current loss caused by the multiplexers stray capacitance. RESULTS: The simulation results showed that after those methods were used, the maximum relative current error on a 1.5 k load dropped from 7% to 0.4% in working frequency ra
4、nge, and the practical results also showed that the final maximum relative current error on this load was less than 1%. CONCLUSION: The shield guard method for wires and the output current compensating method for current source can effectively improve the stability of current sources output current.
5、 【Keywords】 electrical impedance tomography; current source; stray capacitance; shield guard 【摘要】 目的: 提高脑部电阻抗断层成像数据采集系统的恒流源的输出稳定度. 方法：采用屏蔽驱动的方法减弱电极导线分布电容的分流作用，同时采取输出电流补偿的方法对多路开关上的分布电容分流作用进行补偿结果：仿真结果表明通过这些措施后流经1.5 k负载的激励电流的幅度在工作频率范围内最大相对偏差可由原来的7%降至0.4%以下最终所实现的激励源的最
6、大相对偏差也小于1%结论：所采取的电极导线屏蔽驱动和输出电流补偿的方法可以达到有效提高恒流源输出电流稳定度的目的 【关键词】 电阻抗断层成像；恒流源；分布电容；屏蔽驱动 电阻断层抗成像（electrical impedance tomography，EIT）是一种通过体表弱电信号激励和体表电信号测量的方法获取目标区域（某一断面）内组织电阻抗分布信息，并以图像的方式反映出来的新型医学成像技术1与现有的成像技术相比，该技术具有结构简单、设备小巧、易于操作、无创无害等特点，在对机体生理或病理状态的功能成像
7、等方面有着诱人的应用前景，吸引着国内外众多的学者，是近年来生物医学工程领域的研究热点之一其中，高精度数据采集系统的研制是该技术的研究重点和难点之一 我们目前的研究目标是利用EIT技术研制一种能实现对人脑出血、脑水肿等疾病进行长时间图像监护的仪器由于有颅骨这样高电阻率组织的存在2，脑部EIT测量时，目标区域电阻率相对较高，同时又由于颅骨的影响，由颅内组织电阻率变化所导致的体表电信号的变化量也相对较弱，因而对相应的数据采集系统提出了更高的要求3本研究以提高数据采集系统中恒流源的输出稳定度为目标，分析影响系统工作时输出电流稳定性的主要因素，并以屏蔽驱动和输出补偿的
8、方式弱化这些因素的影响，达到提高输出稳定度的目的. 1原理 目前的EIT数据采集系统多采用电流激励、电压测量的工作模式测量时，以等间距贴于目标外周的16或32个电极中的1对（或多对）进行激励，注入一定频率的弱激励电流，同时测量其余电极对上的电压差4理想情况下，恒流源的输出阻抗无穷大，此时不论负载阻抗如何变化，流经负载的电流始终是恒定值，因而通过电压与电流间的比值可精确求解出测量电极间的传输阻抗但如图1所示，对于实际的EIT系统，虽然恒流源自身的输出阻抗可以达到数M以上的水平，但由于激励电流要通过多路开关和长达1 m以上的电
9、极导线才能进入目标区域，多路开关的导通电阻、公共端等效电容、等效输入电容以及电极导线的杂散电容等因素会对激励电流形成一定的影响为分析这些因素的影响，我们设恒流源输出阻抗为Zs，多路开关公共端等效电容为C1、任意一对相邻的输入端等效并联电容为C2、各通道的导通电阻为R，同时，假设负载阻抗为ZLoad，导线间电容为Cline在忽略导线电阻的情况下，可以得到恒流源工作时的等效电路(图2). 此时，令C=C2/Cline=C2+Cline，可得实际流过负载的电流Iload：式中，为激励电流的角频率 可以看出，Iload不仅会随负载阻抗而变化，同时还会随激
10、励信号频率的改变而改变由于我们的系统要求在1200 kHz的频率范围内提供相对可靠的阻抗信息，同时，前期的实验结果表明，脑EIT成像时，采用准对向驱动模式1情况下，相对恒流源而言，负载阻抗一般在0.51.5 k之间因而，我们根据现有电子元件的相关参数，取R=300 ，C1=30 pF，C2=2pF1，Cline=100pF，Zs=4 M，并将恒流源输出电流设为1 mA，以仿真的方式获得了如图3所示的ZLoad分别为0.5 k，1 k和1.5 k时，负载电流随频率变化的曲线可见，随着频率的上升，流经负载的电流急剧下降，特别是当负载为1.5 k，频率为200 kHz时，负载电流下降幅度可达7%以上
11、，严重影响了EIT系统的测量精度因而采取适当的措施降低这种影响是非常必要的. 2方法和结果 通过公式(1)可以看出，由于恒流源输出阻抗Zs远大于多路开关的导通电阻R和负载阻抗ZLoad，导致负载电流随频率和负载变化的主要原因是分布电容其中，导线分布电容Cline由导线的长度和布局决定，往往会随着环境的改变而变化为减小Cline对激励电流的分流作用，我们对电极导线进行了屏蔽驱动，具体方法是以同轴电缆的芯线连接电极与恒流源，同时还与一个增益为1的电压缓冲放大器的输入级相联，缓冲放大器的输出级与同轴电缆的屏蔽层相连工作时，由于芯
12、线与屏蔽层电压相等，激励电流不会分流，从而将Cline的影响减到可以忽略不记的程度. 多路开关的公共端和输入端的等效电容C2和C1由现有器件水平决定，无法降低但根据公式(1)，负载电流总是随频率的升高而降低，如果我们能使恒流源输出电流Is随频率的升高而升高，则会产生补偿作用，从而在一定范围内提高负载上的电流根据这一原理，我们实现了如图4所示的恒流源图中A1为美国Analog Devices公司的AD844型电流反馈型运算放大器当其同相输入端（In+）有一电压Vs时，放大器内部电路会在负相输入端（In-）建立一个与之相等的电压，同时通过内部电流镜的作用在输出
13、端建立一个与负相输入端大小相等的电流因而可得恒流源输出电流Is：因而，我们取负载阻抗为1 k，同时取Ri=4 k代入上式，可得Ccomp=12.5 pF此时，通过仿真可得如图5所示的负载阻抗分别为500 ，1 k和1.5 k时负载电流随频率的变化曲线从中可见：由于Ccomp的引入，当负载取为1 k时，负载电流基本不随频率而改变而当负载取为1.5 k时，负载电流的最大相对偏差也不到0.4%，相对未加补偿时7%的最大偏差而言，降低了10倍以上 根据以上结果，在实际电路的实现上，我们通过匹配，将补偿电容设置为13 pF，同时引入微调电位器，使Ri可在3.94.1
14、k调整最终测量得的激励源在负载为1 k时工作频带内负载电流的最大相对偏差不大于0.2%，负载为1.5 k时负载电流的最大相对偏差也在1%以内，提高了负载电流的稳定度. 3讨论 EIT技术由于具有成本低、使用方便和对人体无创等优点受到了国内外众多的学者的青睐在EIT研究中，如何进一步提高测量系统的精度的问题是困扰研究人员的难点之一. 我们根据国外在脑EIT成像方面的研究基础5-6和课题组在前期研究的工作基础，在国内外率先提出将EIT技术应用于人颅脑出血、水肿等病的动态床旁图像监护，并通过近
15、期的研究基础也证实了这种方法的可行性7但由于头部高电阻率颅骨的存在，由颅内阻抗变化所导致的体表电信号被大大减弱，从而要求测量系统具有相对更高的测量精度而根据目前现有的器件水平，影响系统性能的主要因素之一是多路开关对恒流源的影响这种影响的主要表现是由于多路开关和电极导线等的分布电容的存在使得最终流入成像目标区域的负载电流不稳定，并随负载阻抗以及频率的变化而变化，从而会引入较大的非线性测量误差为减小这种误差，本研究一方面从减小电极导线分布电容的影响的角度出发，对电极导线进行了屏蔽驱动；另一方面从减小多路开关等效分布电容的影响的角度着手，采取对恒流源进行补偿的方法使实际流过负载的电流保持相对稳定通过
16、仿真结果可以看出：采用这些措施后，负载电流的稳定性提高了10倍以上在实际应用中虽然受器件等诸多因素的影响，负载电流的稳定度无法达到仿真的水平，但测量结果也表明电路的输出电流稳定度也有了明显的提高，从而提高了EIT测量结果的准确性 【参考文献】 1 史学涛，董秀珍，帅万钧,等. 适用于脑部电阻抗断层成像的单源驱动电流模式J. 第四军医大学学报， 2006, 27(3): 279-282. 2 Goncalves S, de Munck JC, Verbunt JPA, et al. In
17、vivo measurement of the brain and skull resistivities using an EITbased method and the combined analysis of SEF/SEP dataJ. IEEE Trans Biomed Eng, 2003, 50(9):1124-1128. 3 MurrietaLee JC, Pomfrett CJD, Beatty PCW, et al. EIT voltage changes on the human scalp due to brain stim
18、ulusJ. Proceedings of the 15th International Conference on ElectronicsC. Communications and Computers (CONIELECOMP 2005). 4 史学涛，尤富生，付峰，等. 同时工作于四种频率的多频电阻抗断层成像系统 J航天医学与医学工程，2006, 19(1): 47-50. 5 Yerworth RJ, Bayford RH, Cusick G， et al. Design and performance of the
19、 UCLH Mark 1b 64 channel electrical impedance tomography (EIT) system optimized for imaging brain functionJ. Physiol Meas, 2002, 23(2): 149-158. 6 Merwa R, Hollaus K, Scharfetter H. Detection of brain oedema using magnetic induction tomography: A feasibility study of the likely sen
20、sitivity and detectabilityJ. Physiol Meas, 2004, 25 (1): 1-8. 7 史学涛，董秀珍，霍旭阳,等. 电阻抗断层成像技术监护脑出血的可行性研究J. 航天医学与医学工程， 2007,20(1):待发表./Porganization system and working mode, the selection of a number of municipal strengthening group to build innovation and strengthen the organization of
21、the basic level organizations advanced model.(seven) the manual work of the Communist Youth League Organization published and League branch of manual work, to promote the use of two manuals in the citys organizations at various levels. The county committee, directly under the group (work) is under t
22、he jurisdiction of grassroots League and group (total) using branch ratio shall not be less than 70%.Three, to further strengthen the groups leadership and team building(a) to strengthen the leagues leadership at all levels of construction. Continue to do a good job of municipal Party Committee Orga
23、nization Department on the strengthening of the Communist Youth League cadres management opinions implementation. To promote the group counties (autonomous counties, cities) Commission and city directly under the group (workers) appointed to carry out the Five team building activities. Improve the L
24、eague Association Department working mechanism. Conscientiously fulfill the duties of CO, combined with the city group directly under (the) Committee of the general election and the adjustment, actively jointly with the unit where the party organization department good group leadership selection wit
25、h the work. Do a good job in municipal Party committee assistant committee leadership and leading cadres of the annual assessment work, adhere to and improve the assistant secretary of the Communist Youth League talk system group To weave individual conversation and collective conversation, to stren
26、gthen the committees of all levels, the Standing Committee of the Standing Committee, the establishment of a sound system of central group learning, the system of investigation and study, the system of important issues and the reporting system.(two) to strengthen the work of education and training o
27、f cadres to conscientiously implement the Central Committee. On the implementation of grassroots cadres training project and opinions on carrying out the mission municipal CYL Cadres training project implementation opinions, full implementation of the project of the City Youth League cadres training
28、 education. Focus on the county grassroots cadres and workers focused on training, the new school system vocational training League cadres, for the Municipal Organization Department jointly held the third session of the Communist Youth League system training cadres in party. Focusing on the construc
29、tion of new socialist countryside, strengthening village cadres and rural construction team group member of the young team, the county organizations at various levels throughout the year is expected to train 100 00 rural grassroots cadres, plans to train 1000 village secretary of the village Party b
30、ranch.(three) actively promote grassroots cadres election work. Under the unified leadership of the party organization, to strengthen grassroots organizations direct work, standardize the election work rules and procedures, in the designated candidate, open competition to democratically elected bran
31、ch secretary at the same time, actively and steadily the development of Township, streets and other grassroots organizations in the direct election of the pilot work, on the basis of summarizing the experience and gradually expand the scope of the pilot, and gradually formed a set of measures and ca
32、dres election system.(four) to strengthen the cadres testing exercise. To study the development of XX city from 2006 to 2008 cadres testing exercise planning. To select outstanding cadres to the central mission in Beijing, Shanghai, Guangzhou and other central organs and sending work in developed ar
33、eas. With the Central Committee the implementation of the western region and ethnic minorities cadres training plan . To carry out the eighth batch of municipal organs attachment cadre management, selection, training work. The outstanding cadres to start the National Development Bank XX branch testi
34、ng exercise.(five) to strengthen the group construction. The main position to play the role of Youth League cadre education and training as the city, in accordance with the requirements of post training, further curriculum scientifically and rationally, adopt arrangement menu course, basic theory in
35、cludes group work, focus, hot and difficult issues, to enhance the content and practice; to strengthen the construction of teaching staff, the integration of teachers, establish a quality teacher training team, establish and implement course evaluation system; take delivery of training to the countr
36、yside, on-site training, and increase the rural grassroots cadres training efforts.Four, efforts to strengthen the work of young talent(a) to tenth XX young 54 medal in recognition of the publicity, and actively explore the XX youth service. 54 Medal of institutionalization, standardization and soci
37、alization mechanism. Give full play to the XX award in recognition of 54 youth medal Association, Youth Award winner in the 54 chapter, investigation activities. Who won the Youth Service medal, dedication to the society to create conditions, build the stage. During the 54, named in recognition of t
38、he tenth XX young 54 medal. The association of XX editing of young talent , the exchange of information and communication platform for the XX of outstanding young talents.(two) the establishment of municipal Party committee of young talent information database. In accordance with the feasible, effec
39、tive, classification management, implement step by step principle, the establishment of municipal Party committee of youth talent database; guiding organizations at various levels generally established the young talent database, construction of city, county (directly under the Communist Youth League
40、), grassroots organizations three young talent information database the relevant information collection system, and gradually establish the citys young people, the Communist Youth League youth talent information network, provide information platform for young talent resource development.Five, take e
41、ffective measures to strengthen the mission management(a) to strengthen the mission management work. Strengthen the dues collection and management, continue to adhere to the dues collection regularly reporting system, the annual comprehensive award in recognition of the tour and the tour one vote ve
42、to system audit system. Do statistical work in the group. The group service award in recognition of the advanced units, standardization, promoting group management work institutionalized.(two) to strengthen the organization of research and information exchange efforts. Grassroots carry out research,
43、 efforts to form a number of theoretical results. Make full use of the network carrier, to explore the establishment of electronic group service system, promote the work of Communist Youth League information.Attachment 4:XX municipal Party Committee Propaganda Department of the Communist Youth Leagu
44、e work pointsIn 2006, the Communist Youth League propaganda ideological work XX to Deng Xiaoping theory and the important thought of Three Represents as guidance, study and implement the partys sixteen and the spirit of the the fifth Plenary Session of the 16th CPC Central Committee implementation o
45、f the eight plenary session of the two committee, the citys cultural work conference, the Propaganda Department of the meeting and the the fourth Plenary Session of the 15th CPC Central Committee permanent mission spirit, closely around 11th Five-Year XX the goal of full implementation of Scientific
46、 Outlook on Development, emancipating the mind, seeking truth from facts, keep pace with the times, close to reality, close to life, close to the young, persevere grasp orientation, grasping deepening, grasping construction innovation, further improve the groups propaganda and ideological work abili
47、ty, unity To guide the majority of members of the youth to achieve the 11th Five-Year planning tasks, the overall construction of a well-off society, building a socialist harmonious society to make new contributions.One, adhere to the Three Represents important thought and Scientific Outlook on Deve
48、lopment education to guide the youth, to further strengthen the ideological and political education of youth(a) armed with youth science theory. Unremittingly with Deng Xiaoping theory and the important thought of Three Represents armed members of youth education, to promote the study and implement to expand the depth and breadth. To study and implement the important thought of Three Represents with the establishment and implementation of Scientific Outlook on Development combine with the implementation of the requirements of constructing socialist harmonious society with
