Design Of Three Dimensional Electrical Impedance Tomography System

Electrical impedance tomography(EIT) is a relatively new imaging modality developed in the recent30years, which is a potential alternatives received considerable attention based on non-ionizing sensing fields that can be built from inexpensive components and could therefore be routinely used. Eventually EIT could be a supplementary technology to some imaging techniques, since it is both safer and more pleasant for the patient.EIT has previously been restricted to the assumption that data originates from a two dimensional (2D) object. In practice, the object is generally a three dimensional (3D) structure, and thus the injected current will flow within the plane of measurement and also through the off-plane volume. This discrepancy leads to images containing significant artifact, impeding analysis and the diagnostic utility of EIT. Three Dimension Electrical Impedance Tomography (3D EIT) is developing based on the above opinion. In order to obtain more information of subject, the measurement could be accomplished by scanning in three dimensional.This paper mainly includes two parts:electrode optimization design of3D electrical impedance tomography and3D EIT hardware system design based on the FPGA.The electrode optimization design of3D electrical impedance tomography includes:1.3D EIT models are established up by COMSOL Multiphysics. Four kinds of electrodes attached uniformly circling to the physical experiment modals are studied:square (â… ), round (â…¡), concentric cylindrical combination electrode (â…¢), square-round combination electrode (â…£).2. Simulation one:Extended kinds of electrode due to combination electrode.Simulation experiments were carried out respectively based on QBD (Quasi-opposite drive between two layers)working pattern, and EIT images were reconstructed. Images were reconstructed from five layers of four targets which are in the same conductivity and same height. Then, by studying the targets as voltage dynamic range, the uniformity of sensitivity field distribution and the correlation coefficient of the images, the measured data and reconstructed images are evaluated under the proposed extended electrodes structure.Results show that combination electrode under external excitation and internal measurement mode is better than the others.3. Simulation two:Based on QBD working pattern, eight different duty ratios (DR) between0.2and0.7are studied comparatively. Jacobian matrix was gotten to reconstruct the EIT image. Images were reconstructed from five layers of five targets which are in the same conductivity but in different height. Then, by studying the targets as voltage dynamic range, the uniformity of sensitivity field distribution and the correlation coefficient of the images, the measured data and reconstructed images are evaluated under the refied four electrodes structure. Results show that all of them under DR0.4are better than other DR,and IV is better than others.The improvement of digital EIT hardware system based on the FPGA includes:1. Using Spartan3E series XC3S500E-4PQ208as the core, eight embedded microprocessor PicoBlaze realize constant current excitation source, analog switch channel switch, serial port communication and other logic control function. In order to improve the system anti-interference characteristics, use Altium Designer software designing and commissioning the4layer PCB board modularly.2. FPGA hardware is developed by VHDL language, and the PC interface is completed by Delphi software, which can realize real-time data storage and curve plotting.3. In experiments on the calibration board and30cm-diameter barrel, better SNR and consistency are achieved for the whole system performance, and the experimental results show that the system can realize3D electrical impedance data acquisition function.4. Putting organic glass rod in the physical experiment model, reconstructing by CGLS (conjugate gradient algorithm) after data collection based on QBD working pattern.In the end of the paper, some subsequent improvement schemes are put forward in terms of the electrode, the board, debugging methods.