Research On Two-dimensional Electrical Impedance Tomography System

Electrical Impedance Tomography(EIT)is based on the characteristics that different biological tissues and organs have different electrical impedance and the impedance changes when tissues and organs undergo physiological activities or lesions occur.Place a ring of electrodes on the body surface and apply an exciting current,then measure the voltage data on other electrodes.The computer uses the data to visualize the electrical impedance distribution in the body through imaging software to get functional images which react physiology or pathology of the tissue and organ.This achieves the purpose of non-destructive testing and provides basis for health assessment or disease diagnosis.In view of the shortcomings of current EIT systems,such as high-cost and large volume,a low-cost and portable two-dimensional EIT system is designed and constructed in this paper.The system consists of a host computer and a lower computer.The lower computer applies the excitation current and measures the voltage signal through 16 electrodes arranged on the body surface of the measured object.The upper computer imaging software that based on the finite element method of the MATLAB platform uses the data measured by the lower computer to realize the image reconstruction of the impedance distribution in the measured object.For ensuring certain measurement accuracy,the system sacrifices part of the measurement speed in exchange for the low cost and small volume of the system.The core of the lower computer is the excitation current generating circuit and the voltage signal measuring circuit.In this paper,a highly integrated excitation current source based on direct digital synthesizer(DDS)and a low-cost voltage signal measurement circuit based on the built-in analog-to-digital converter(A/D)of ARM microcontroller are designed.The excitation current source uses an enhanced Howland architecture voltage controlled current source.The voltage signal measurement circuit realizes program-controlled amplification of the high dynamic range voltage signal by cascaded two-stage programmable gain amplifier.The A/D uses the oversampling technology to improve the resolution and signal-to-noise ratio of the voltage signal measurement effectively.Since the system has not yet reached the level that can be directly used for anthropometric measurement,16 electrodes are installed on the bucket as the experimental environment and 16 electrodes are evenly arranged on the same plane inside the bucket.The bucket contains physiological saline in order to simulate the human environment.Then,objects with different impedances and different shapes are placed in the bucket for imaging experiments.The upper computer imaging software uses the measured data to reconstruct the image of the impedance distribution in the measured object,and realizes non-destructive testing.The experimental results show that the system can distinguish the basic position and basic shape of different objects.So it can meet the requirements of general non-real-time and non-destructive testing applications.At the end of this paper,the experimental results are analyzed and the ideas for improvement are put forward.