Multi-frequency Excitation Source And Quadrature Demodulation Technique In Digitized EIT System

Electrical Impedance Tomography (EIT) is a newly developed medical imaging technology after the appearance of morphology and structure imaging. EIT has three outstanding advantages of non-invasive, functional imaging and medical image monitoring.The physiological and pathological condition of human body, expressed in the form of electrical properties, is correlated to biological variances that happed at the level of molecule and cell. It is a significant task for EIT to effectively extract imaginary part of tissue impedance and give early or prospective results that happed in molecule.Aiming at implementing a digitalized, multi-frequency and complex impedance EIT system, primary study is made in multi-frequency excitation source and digital quadrature demodulation technique demodulator, on the basis of investigating and analyzing the worldwide development, the technical conditions and key progress of EIT. Design, construction and verification test is made to complete a multi-frequency excitation source based on FPGA and to complete a digital quadrature demodulator. Primary research results include the following aspects:1 Based on the requirement of multi-frequency measurement, a scheme based on DDS and FPGA to generate sinusoidal excitation signal is studied. Correlated circuit is designed and satisfactory multi-frequency sinusoidal signal is obtained.2 Based on an improved Howland circuit, a voltage controlled current source module is designed and an excitation source with high output impedance is constructed. Test results show that the output impedance keeps no less than 190 kQ when the frequency of signal ranges is 1 MHz.3 On the basis of analysis and study of the properties of analog and digital demodulation, a digital quadrature demodulation scheme is proposed. The demodulating process is closely studied. Finally, verification is made to test the demodulator's performance and feasible methods are pointed out to reduce the measurement error.The main work in this paper sets up a technical foundation for building a digital EIT system. It also creates conditions for EIT that focuses on special application object.