Research On Wideband Electrical Impedance Tomography Based On Spectrum Analysis

Based on the electrical properties of the biological tissue,electrical impedance tomography(EIT)excites the sensitive field using the safe current and measures the boundary voltage simultaneously,then the distribution is reconstructed with image reconstruction algorithm.EIT technology has the advantages of no radiation,low cost and simple structure,and it is expected to detect trauma and lesion.The multi-frequency EIT reconstructs the images of the measured object using the spectrum characteristics without the homogeneous field information,but the bandwidth and frequency resolution are mutually constrained and the frequencies at which the images are reconstructed are not chosen flexibly.In this paper,a spectrum based wideband electrical impedance tomography(SWEIT)method and its hardware system were proposed.The chirp signal was used to obtain the impedance spectrum of the target in the bandwidth of 1MHz,then the multi-frequency imaging points and the reference frequencies were configured with the resolution of 10 kHz.The images were reconstructed in the frequency band where impedance and phase characteristics changed significantly,aiming to improve the bandwidth,frequency resolution and image contrast synchronously.The specific research work includes:(1)Based on FPGA,the DDS technology and the Howland circuit,the hardware system was designed to be qualified to excite the 1MHz chirp signal that was attenuated 43.75 dB at 1.7MHz and the multi-frequency signal with the spurious free dynamic range(SFDR)of at least 40 dBc.The output impedance of the current excitation source was 117k? at 1MHz,and the SWEIT met the medical electrical standard IEC 60601-1.(2)The dual-channel voltage measurement module was designed to acquire the reference and the boundary signal simultaneously,and the unknown impedance and phase shift were calculated according to the real-time current magnitude.Serial data was collected by multiplexing excitation-measurement module.The system had good channel consistency,and an average SNR of 55.9 dB.(3)The "two-excitation" method was proposed in this paper.Firstly,the amplitude-frequency and phase-frequency characteristics of the target were obtained to determine the sensitive bandwidth respectively,using the chirp signal.Then,the frequency points in the bandwidth were chosen to synthesize the multi-frequency signal to excite the measured field.Then the images were completed using the multi-frequency response information.(4)Based on the 0.1% weight/volume salt solution,using the 16-electrode array,experiments were carried out with carrot cylinder and nylon rod as the targets,and the amplitude and phase shift images were reconstructed to verify the system performance and method.Spectrum analysis had an absolute deviation of less than 2% and a phase shift relative deviation of less than 5%,which can be used to determine the electrical sensitive bandwidth.The average correlation coefficient of the reconstructed image was 0.8,and the average root mean square error was 12%.The position of the target was clear,and the tendency of the multi-frequency images was consistent with the electrical characteristics.