Electrical Impedance Tomography is a non-invasive, non-ionizing, non-destructive and painless imaging technology that can distinguish between cancerous and non-cancerous cells by reproducing tomographic images of the electrical impedance distribution within the body. The primary scope of this thesis is the study of hardware modules required for an EIT system. The key component in any EIT system is the excitation system. Impedance measurement can be performed by applying either a current or voltage through emitting electrodes and then measuring the resulting voltages or current on receiving electrodes.
In this research, both types of excitation systems are investigated and developed for the Sussex EIM system. Firstly, a current source (CS) excitation system is investigated and developed. The performance of the excitation system degrades due to the unwanted capacitance within the system. Hence two CS circuits: Enhance Howland Source (EHS) and EHS combined with a General impedance convertor (GIC: to minimise the unwanted capacitance) are evaluated. Another technique (guard-amplifier) has also been investigated and developed to minimise the effect of stray capacitance. The accuracy of both types of CS circuits are evaluated in terms of its output impedance along with other performance parameters for different loading conditions and the results are compared to show their performance. Both CS circuits were affected by the loading voltage problem. A bootstrapping technique is investigated and integrated with both CS circuits to overcome the loading voltage problem. The research shows that both CS circuits were unable to achieve a high frequency bandwidth (i.e. ≥10MHz) and were limited to 2-3MHz. Alternatively, a discrete components current source was also investigated and developed to achieve a high frequency bandwidth and other desirable performance parameters. The research also introduces a microcontroller module to control the multiplexing involved for different CS circuit configurations via serial port interface software running on a PC.
For breast cancer diagnosis, the interesting characteristics of breast tissues mostly lie above 1MHz, therefore a wideband excitation source covering high frequencies (i.e. ≥1-10MHz) is required. Hence, a second type of the excitation system is investigated. A constant voltage source (VS) circuit was developed for a wide frequency bandwidth with low output impedance. The research investigated three VS architectures and based on their initial bandwidth comparison, a differential VS system was developed to provide a wide frequency bandwidth (≥10MHz). The research presents the performance of the developed VS excitation system for different loading configurations reporting acceptable performance parameters. A voltage measurement system is also developed in this research work. Two different differential amplifier circuits were investigated and developed to measure precise differential voltage at a high frequency.
The research reports a performance comparison of possible types of excitation systems. Results are compared to establish their relationship to performance parameters: frequency bandwidth, output impedance, SNR and phase difference over a wide bandwidth (i.e. up to 10MHz). The objective of this study is to investigate which design is the most appropriate for constructing a wideband excitation system for the Sussex EIM system or any other EIT based biomedical application with wide a bandwidth requirement
