Study On The Open Electrical Impedance Tomography With Scanning Electrode

The organism itself is a complex system, and the characteristics of biologicaltissues are studied by the methods of optics, electricity, and magnetic. Becausedifferent organizations have different electrical impedance characteristics, and thedifference of the electrical impedance characteristics is large for the sameorganization in different status and frequency. So the electrical impedancemeasurement method is an important method for research on the electrical propertiesof the biological tissues. The electrical impedance tomography is a noninvasivefunctional imaging method, which reconstructs the distribution of the object interiorconductivity or resistivity by injecting the safe constant current or voltage into theimaging domain boundary through injection electrodes and measuring boundaryvoltage or current signal from non-injection electrodes, and choosing the static ordynamic electrical impedance tomography method according to the application. Dueto the limitation of the measurement information, the performance of the hardwaremeasurement system and the reconstruction algorithm, the resolution of theconventional electrical impedance tomography is difficult to be improved, thus it isonly as an auxiliary method applied to early screening of the disease and real-timemonitoring of the medical image. However, the open electrical impedancetomography can improve the drawbacks of the enclosed electrical impedancetomography such as electrode positioning and model matching. But due to thelimitation of the amount of measurement information, the open electrical impedanceimaging technology can’t meet the requirements of image quality for clinicalmedicine.The voltage measurement method by the scanning electrode is proposed in theopen electrical impedance tomography, which can reduce the influence of the fixedpaste electrode on the electrical impedance imaging quality, and increase the amountof the measurement information. Based on the advantage of the scanning electrode,the imaging quality and the investigation depth are improved in the open electricalimpedance tomography. And in order to in further improve the imaging quality for theopen electrical impedance tomography, the measurement system, the current injectionand the voltage measurement patterns are designed, several reconstruction algorithms are proposed, all of which can promote the development of the electrical impedancetomography, and lay a foundation for the open electrical impedance tomographyapplied in the clinical.１．In order to perform the detection and monitoring the electrical impedancedistribution in the superficial layer, the open electrical impedance tomography isemployed with the performance of strong adaptation and portable electrode location.Firstly, in order to obtain more measurement information in the limited measurementboundary, and reduce the influence of the fixed electrode contact area on the electricalimpedance imaging quality, the voltage measurement method by the single scanningelectrode driven by the motor is proposed. Secondly, the metal electrodes as thecurrent injection electrodes with small contact area are placed on the both sides of themeasurement boundary to improve the measurement precision and portability.2．In view of the proposed open electrical impedance tomography with thescanning electrode, the portable hardware measurement system with the highperformance is designed. In particular, the differential current source circuit isproposed, the performance of which is superior to the traditional Howland currentsource circuit. Through calculation and measurement, the proposed differentialcurrent source circuit can improve the output impedance, expand the loading capacity,and dramatically enhance the common mode rejection ratio of the measurementsystem. And for the existing stray inductance and inductance, distributed inductanceand inductance in the actual circuit, the compensation methods are put forward toincrease the stability especially in the case of high frequency.3．In view of the proposed open electrical impedance tomography with thescanning electrode, in order to simplify the measurement and calculation, the currentinjection and the voltage measurement patterns are optimally designed. Firstly, thecross current injection pattern with high imaging investigation depth is proposed.Secondly, for the two-dimension and three dimension open electrical impedancetomography, the number of the current injection electrode and the current injection aswell as the voltage measurement point are optimally studied. After the aboveoptimization design, the complexity of the measurement and the real-timeperformance are improved on the premise of guarantee the imaging quality.4．In order to in further improve the open electrical impedance imaging quality,several reconstruction algorithms are proposed. Firstly, based on the reconstructionobjectives with different depth, the Tikhonov regularization algorithm with the weighted parameter is proposed, which can be improve the imaging quality to someextent. Then the combined regularization algorithm based on Tikhonov and variationregularization algorithm is proposed, The results though the calculation and actual saltwater tank phantom measurement indicate that the combined regularization algorithmwith stable convergence performance can improve the reconstruction quality withsharp contrasts and is more robust to noise in comparison to traditional Tikhonovregularization and variation method solely. Lastly, in actual measurement because thepresent of the model error, measurement error, the calculation error and the electrodeerror, the electrical impedance imaging quality and the resolution can hardly meet therequirement. Therefore, the self-adaption optimization algorithm based on thecombined regularization algorithm is proposed, which is on the basis of thereconstructed image to improve the reconstruction robustness and spatial resolution.