Design Of Multi - Frequency Electrical Impedance Imaging System And Optimization Of Imaging Algorithm

Electrical Impedance Tomography (EIT) has been investigated extensively during the past decades as a visualization and measurement technique. Due to the advantages of being non-radiant and non-intrusive, rapid response, simple structure and low cost, EIT has wide foreground for application in the field of clinical monitoring and industrial measurement.The problem of EIT image reconstruction is a nonlinear and ill-posed problem. The reconstructed images always have artifacts because of the noise in the measure system. Hence the traditional image reconstruction method cannot avoid the artifacts which in reconstructed image by the noise.To improve the quality of image reconstruction, an images reconstruction algorithm of EIT based on expectation maximization (EM) method which is a statistical method is proposed. The EIT mathematical model is transformed into the non-negatively constrained likelihood minimization problem by the EM method. And this problem is solved using gradient projection-reduced Newton (GPRN) iteration method. Compared with the conjugate gradient (CG) and the Tikhonov method, both of the simulation and experimental results show that EIT can get better reconstruction results by the EM method. As a result, the EM method can get non-negative solution. Besides, the EM method can avoid the artifacts in the reconstructed images effectively and improve the quality of the reconstructed images.Most of the EIT systems to data have used single frequency sinusoidal excitation. Since media distribution of the object field is complex and the optimum frequency response varies with the type of media, which composes the object field, the information obtained from single frequency EIT system is limited. As a result, the impedance distribution of the object field cannot be reconstructed very well.To solve the problem, a multi-frequency EIT system is designed based on FPGA. A matrix-structure mux module, which consists of reed relay, is used for channel selection. The system has two parts, i.e. hardware part and software part. The hardware part is controlled by FPGA. The digital multiplier is used for the phase sensitive demodulate, the output of the digital multiplier is processed by the IIR filter. The software part is designed under Lab View environment. The control of hardware and image reconstruction are realized in the software part.The experimental results show that, the SNR of the multi-frequency EIT system is improved and the consistency of the channels is guaranteed. Furthermore, the frequency ranges of excitation current and measured voltage are expanded. The information of real part and imaginary part under multi-frequency current excitation are obtained. As a result, the reconstructed images of multi-frequency EIT system confirm the real situation of impedance distribution in the object field.