應用DSP晶片於生物電阻抗量測與成像之研究

Study of Bioelectrical Impedance Measurement and Imaging based on DSP

電阻抗斷層造影(Electrical impedance tomography, EIT)，因人體組織細胞之不同的導電率特性，獲得其橫截面電阻抗影像之系統。故本論文將建立一套可調頻率及相位信號源之多頻電阻抗量測與成像系統。系統由直接數位合成(Digital Synthesis，DDS)產生所需之頻率與相位之1kHz~100kHz正弦波做為參考與解調端之信號源。及藉由數位類比轉換器(Digital to Analog Converter, DAC)控制電流大小。並由電壓控制電流源(Voltage Control Current Source，VCCS)，產生穩定之電流源，經待測物後實現數位解調以量取訊號。DDS電路輸出經VCCS輸出，頻率由1kHz到100kHz，其THD平均為0.91%。以DSP進行數位解調時，在頻率1kHz至100kHz時，進行解調與精密電阻之間的相關係數皆達r2=0.9988。當頻率於1kHz、10kHz及100kHz下進行解調，電阻分別從100Ω，330Ω，510Ω，電容從1n~0.47μF下解調，在不同頻率對應電阻與電容並聯電阻抗之虛部電抗值[Im(Z)]相關係數值也皆大於0.9849。解調值與電阻抗之實部電抗值[Re(Z)]的相關係數，其數值也達0.9878以上。而實際量測相位的線性關係，在測試範圍的電抗值與不同頻率下進行量測，其相關係數皆達0.9096以上。影像重建目前使用逆投影(Back-projection)法作為重建成像法。分別在1kHz，10kHz，60Hz，100kHz情況下，槽中央分別置放銅柱或塑膠柱，得其0°與90°電阻抗影像。系統亦提供GUI介面，便於使用者操作。本系統已獲得重建後橫截面之影像，實現初步之電阻抗影像系統。

Electrical impedance tomography is an imaging system employed to obtain the cross section of electrical impedance of a body via the measurement of different conductivity properties of human tissue cells. Therefore, in this paper, we attempt to establish a milt-frequency electrical impedance measurement and imaging system with adjustable frequency and phase of the signal source. The output signals after VCCS have an average THD of 0.91%. After digital demodulation for frequencies ranging from 1kHz to 100kHz, the correlation coefficient squared (r2) between the demodulator and precision resistors are greater than 0.9988. For the test target corresponding resistance from 100Ω, 330Ω, 510Ω, and capacitor from 1n to 0.47μF in parallel at different frequencies at 1kHz, 10kHz and 100kHz respectively, the r2 between the demodulator measured and the imaginary part of the reactance [Im(Z)] are greater than 0.9849. In the same condition, the r2 between the demodulator measured and the real value of the reactance [Re(Z)] are greater than 0.9878. For the phase of the impedance, the r2 are greater than 0.9096. For image reconstruction experimentation, we use the back-projection method as a reconstruction algorithm. When the signals including 1kHz, 10kHz, 60Hz, and 100kHz respectively are fed into the simulation tank with plastic and copper cylindrical objects are placed centrally in the tank, we obtain the electrical impedance cross-section images for signal phase of 0 ° and 90 °. The system also provides a GUI interface for easy user operation. We have obtained the cross-section of the image and preliminary established the electrical impedance imaging system.