The Investigation and Implementation of electrical Impedance Tomography Hardware System
Electrical impedance tomography (EIT) is a medical imaging technology that provides a tomographic representation of the distribution of electrical impedance within the body. As the electrical impedance varies for different body tissues, it is possible to characterize tissues from the images and to detect physiological events. EIT systems have been developed from applying a single signal frequency to a range of frequencies. Imaging at multiple frequencies significantly improves the ability to characterize and differentiate heterogeneity within the region of interest. Applications of EIT are limited by its poor resolution as a consequence of limited number of electrodes and lack of independently published measurements. In a practical EIT system design the parallel structure is normally adopted as it provides a real time monitoring structure. However, there is a difficulty in expanding to a 2-dimensitional or 3-dimensitional high resolution imaging system, as the number of electrodes increase. In this thesis, a serial structure spectrum EIT system has been investigated and developed. Modelling of the electrical circuit has shown that the system bandwidth is degraded primarily by the signal transmission in the coaxial cable and multiplexer. To remove the capacitive effect of these components, a distribute system concept has been developed. The concept uses active electrodes in which a current source and a front end amplifier are embedded in the electrode which makes direct contact with the tissue being measured. The active electrode is based on the Howland current source. The required high output impedance of Howland current source can be realised by matching the two resistor arms. However, from the electrical equivalent circuit analysis the actual output impedance of this circuit was found to be degraded by the op-amp' s limited open loop gain, especially at higher frequencies. To solve the problem, the author describes in detail a novel method of compensating for the above effects. Subsequent circuit tests showed significant improvement after the compensation. Further, to improve the small signal noise ratio a programmable gain amplifier to adapt the frame data measurement was developed. These developments have led to the feasibility of active electrodes. The thesis describes in detail the development, of the MK2 EIT system which is presented as the output of this research.
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