Design And Implementation Of Multi-Frequency EIT System Based On FPGA

Electrical Impedance Tomography(EIT)is a relatively new medical imaging modality to reconstruct image of the impedance distribution of body's section by injecting small and safe currents and measuring the resulting voltages placed on the body surface with merits of non-invasion,non-radition and low cost.Because single-frequency EIT uses a fixed frequency as exciting frequency,it can only extract the impedance information at a fixed frequeny and can not extrat impedance information at different frequencies.In order to obtain more impedance information at differernt frequencies simultaneously,multi-frequency EIT measurement technologies come forth.Some systematical investigates of Multi-frequency EIT technologies are carried out here.The main contents are as follows:(1)The hardware circuit of multi-frequency current source for EIT is designed and realized,this multi-frequency current source can produce exciting current through the mixture of sinewave with multiple frequencies and amplitudes,and frequency of this current source can be easily regulated.In this multi-frequency current source circuit,DDS module is constructed by using hard language in FPGA,a DAC is usd to convert the digital signal into analog voltage signal,the voltage signal is converated to current signal by VCCS circuit,and the feedback circuit is specifically designed to improve out impedance.(2)Regarding to demodulation mehtods for multifrequency EIT,a FPGA-based multi-frequecny quadrature sequence demodulation is implemented.The process of quadrature sequence demodulation is to multiply the demodulated signal which is obtained by sampling with in-phase and quadrature signals as reference signals which are produced by FPGA and then to sum up.The in-phase and quadrature parts are obtained,and the real part and imaginary part of conductivity are converted to amplitude and phase by using CORDIC algorithm.This demodulator method can extract impedance information at differernt frequencies simultaneously with high speed and high precision.(3)The hardware circuit design of multi-frequency EIT system is carried out.This system comprises exciting current source for multi-frequency EIT,switch multiplexer of excitation and measurement,signal processing and A/D sampling,multi-frequecny quadrature sequence demodulation,and data-buffer and USB communication.This multi-frequency EIT system has characteristics of flexible structure,scalability,high precision of measurement and high speed.(4)The software system formulti-frequency EIT hardware system is implemented,including FPGA program,USB firmware programand PicoBlaze program.The procedure is verified by debugging and simulation.(5)Performances of multi-frequency EIT system,especially current source and demodulation method are verified by simulation and actual measurement.The experiment results show that output impedance of current source is high,which can meet requirement of EIT system,and the system works stably and reliably.