Study On Parts Of Key Techniques Of The Hardware System Of Electrical Impedance Tomography(eit) Image Monitoring

Electrical Impedance Tomography(EIT) is a new kind of imaging technology,which can image the distribution of resistivity inside an object. It can image biological tissues with non-invasive, continuous and functional images and it becomes a new imaging technology with great potential of medical imaging. According to technical characteristics of EIT, the study group on EIT of the Fourth Military Medical University (FMMU) first proposed the concept of EIT image monitoring in the international and developed the first EIT image monitoring system in the world in 2006 and started clinical trials of image monitoring using the EIT system. Clinical trials showed that the system could effectively reflect the resistivity changes within the human body and it has attractive prospect for clinical application. The advancement of the study on clinical monitoring trails made higher requirements for the performance of EIT hardware system which required advancement of some key techniques of the hardware system. Key techniques of the hardware system of EIT includes power technique, driving source technique, signal-detection technique, data-acquisition technique, interface technique and drive-measure controlling technique. In order to make EIT imaging monitoring system fit clinical application better, this paper studied EIT data acquisition system in the following aspects: driving-measuring mode , data transmission interface. This study aims at achieving higher performance of EIT hardware system. Moreover, in order to expand the monitoring occasions of EIT image monitoring system and reduce the volume and weight of the hardware system, this paper compared the performances of EIT monitoring system when powered by the three types of power sources. The specific work is as follows:1) Improvement of the drive-measure control mode of the front-circuitIn order to reduce the influence on the system accuracy by the electrode wires, the future EIT hardware system was designed to separate the pre-parts of drive and measure from the EIT host system and make connection with the electrode system in the neighborhood so that influence on the precision of measurement by the wire distribution parameter can be reduced as much as possible. In such a layout, the controlling wires are longer than that of the existing system. Therefore the existing method of direct control is no longer applicable. For the purpose of taking into account the requirements of total number of data bus and speed of data communication, this study improves the data exchange protocol based on serial peripheral interface (SPI) bus, whose top speeds can amount to 5Mbps. Response and handshake signals are introduced and a new more stable and reliable synchronous serial data interface bus is realized, which results in fast and interactive data exchange between the pre-circuit and EIT main control parts. Thus accuracy of data transmission is guaranteed.Based on the improved synchronous serial interface bus,this study adopted Complex Programmable Logic Device(CPLD) as the main controlling circuit of the pre-circuit.The key programs of data transfer of CPLD were compiled with Very high speed intergrated circuit Hardware Description Language(VHDL),and realized the transmission of control signal.This part of work laid the foundation for the front-circuit control of the new EIT system.2) Design of USB data transfer module for EIT systemIn order to gain more clinical application,the new EIT systems must have faster imaging speed which requires a faster data transfer interface.Based on the consideration of the performance of data transfer interface, the EIT monitor system adopted Universal Serial Bus(USB) interface as the data transfer interface. The main function of this interface in the EIT hardware system includes transferring the acquired data to the computer and transfering commands from the controlling computer to the EIT hardware system.The USB chip of the current EIT system only supported 8-bits data bus for EIT data transfer , which affected the speed of the data transfer.In order to increase the amount of data in one transfer and improve the speed of the data transfer, we selected an USB interface chip with 16-bits data bus.The USB interface of the current EIT system issued bulk transfer for the controlling data transfer,so there was some delay.This study used interrupt transfer for decreasing the delay. Based on the above considerations, a USB2.0 data transfer module was designed based on the existing USB1.1 interface.In this part,the may job included: overall structure of the USB interface was designed,and the circuit of the USB data transfer module was turned out, so that the USB chip could receive data from the main controller of EIT system via the special 16-bits DMA data bus.The firmware of Micro Control Unit(MCU) ,the driver of USB and test application were coded.The results of tests showed that the USB data transfer module achieved the following performances:it realized control transfer, outputting interrupt transfer and inputting bulk transfer and enabled the controlling computer initializing the USB data transfer module and getting its descriptors, enabled the controlling information transferred fom EIT software to EIT hardware system with the shortest delay; and enabled the EIT hardware system transferring the acquired data to the computer with a high speed.The highest speed achieved 675.2Kb/s.This rate could meet the requirements for reconstructing 2 images per second.This part of work laid the foundation for the new EIT system.3)The study on influence of power sources on performances of EIT system.The current EIT system adopted linear power source in order to meat high precise measuring.In order to expand the monitoring occasion such as in field or in some monitoring equipments ,battery was considered as the possible power source for EIT system.In order to reduce the weight and volume of EIT hardware system, the possibility of using switch power source in EIT system was considered.Main study methods were as follow:measuring the spectrum of the three power sources and their driving sources with Spectrum Analyzer(SA) , using the EIT system powered by each power source to image one object, and analyzing the accuracy of the acquired data and comparing the image quality among the three power sources.Through the above experiments,the follow results were obtained:(1)The spurious power level of battery was higher than that of linear power source,and the latter was higher than that of switch power source.The spurious power level order of the interferences in corresponding driving source was the same.(2) The accuracy of the data acquisition system were obtained after imaging one object. The results were as follows:In the full range of exciting frequency, the accuracies of the data acquisition system powered by battery were higher than that powered by linear power source,and the latter were higher than that powered by switch power source.In the low frequency band of exciting signal , the accuracies of the data acquisition system powered by the three power sources were in accordance with the above relationship, but only had a little difference. In the high frequency band of exciting signal ,the accuracies of the data acquisition system powered by battery were slightly higher than that powered by linera power source,and the latter were greater higher than that powered by switch power source.So this verified that DC power source affected the accuracy of data acquisition of the EIT hardware system.(3)The qualitity of the images reconstructed by EIT system power by each power source revealed that in the full exciting frequency band , the qulitity of the images reconstructed by EIT system powered by battery was higher than that powered by linear power source,and the latter was higher than that powered by switch power source, but the qulitity of the images reconstructed by EIT system powered by switch power source was close to that powered by the other two power sources.Based on the above results, it was concluded that with fewer spurs and lower spurious power level, battery power source could be used for EIT image monitoring system.The performance of EIT system powered by switch power source was close to that powered by the other power sources in the lower exciting frequency band , so this king of power source could be used in the EIT system whose exciting frequency was in this band.Through the study of the three key techniques in EIT image-monitoring system mentioned above,this study improved the performance of EIT system, expended the occasions of image monitoring, and laid a foundation for the development of the new EIT system,and provided more effective technical supports for clinical image monitoring of EIT.