Research And Design Of Multi-Frequency Electrical Impedance Tomography Hardware System

Electrical impedance tomography is based on the internal impedance change of biological tissues.The weak voltage signal of biological epidermal tissues is measured and processed by applying the exciting current signal to the electrodes.Then the impedance distribution in biological tissues is reacted by image reconstruction algorithms.It is a good imaging technology in the field of clinical medical diagnosis and biologic pathological examination.Because of its unique advantages such as small damage,dynamic detection and low cost,it has always been the research focus of many scientific groups in the world.In the multi-frequency electrical impedance tomography,the frequency of the excitation signal source can be dynamically adjusted,which effectively expands the application of the electrical impedance tomography technology and also can obtain more abundant impedance information of biological tissues.In this paper,each module of the multi-frequency electrical impedance tomography hardware system is designed and the hardware circuits are built with high performance integrated chips.The multi-frequency impedance tomography hardware system with FPGA + MCU as the core processor is realized.The working frequency range is between 10KHz~100KHz.The whole multi-frequency electrical impedance tomography hardware system can run smoothly and efficiently.The main contents of the work are as follows:(1)The multi-frequency signal source based on FPGA is realized.The signal source can generate the excitation signal and the same frequency reference signal needed in the demodulation technology.The phase difference between these two signals is alaways kept at 90°,which can effectively overcome the deficiency of the component parameter error in the phase reversal process.It can provide a good accuracy for the multi-frequency electrical impedance tomography hardware system.(2)The other components of the multi-frequency impedance tomography hardware system are designed,which includes the improved Howland current source circuit,the 16 multi-channels switch circuit,the voltage follower circuit,the high-precision differential amplifier circuit,the programmable gain amplification circuit,the band-pass filter circuit,the analog demodulation circuit,the 16 bit A/D conversion circuit and the power conversion circuit,and using FPGA + MCU as the core control processor.The two controllers work individually and do not interfere with each other,which makes the whole hardware system work fluently and can improve the speed of signal acquisition effectively.(3)The basic demodulation principles of the switching demodulation and multiplication demodulation in the existing demodulation technology are studied.The hardware circuits required for the demodulation of the switching and the multiplication demodulation are designed and implemented.The sine/cosine voltage signals generated by the multi-frequency signal source are used respectively as the reference signals to obtain the real part and the imaginary part of biological tissue impedance.(4)The performance of each module of the system and the consistency of the experimental physical models are tested.A certain concentration of physiological saline is poured into the experimental physical model,and the related voltage data obtained by the multi-frequency electrical impedance tomography hardware system are used to test the system channel consistency under the different injecting current signals.Finally,different experimental objects are placed inside the experimental physical model,then image reconstruction results are obtained by the reconstruction algorithms.