Investigations in electrical impedance tomography
This thesis presents an investigation of various designs and implementation aspects of
multi-frequency Electrical Impedance Tomography (EIT) systems for medical applications.
EIT presents a relatively new imaging modality that involves the measurement of the complex
impedance of a body through voltage measurements around the body's surface, when
it is subjected to electrical excitation. The primary region of interest for measurement
involves excitation in the frequency range from several kHz to about a few MHz.
EIT system design objectives were defined which are the starting point of a detailed error
analysis of an EIT system. The analysis undertaken introduced new aspects in terms of the
multiplexers' on-resistance, the CMV error analysis and the investigation of the feedthrough
errors incorporating the frequency dependence of the skin-electrode interface. A specification
of a novel multi-frequency EIT system has been derived through careful consideration
of the design objectives based on the results of the error analysis. The merits and drawbacks
of different types of stimulus signal for bio-impedance measurements are reviewed
and a novel multi-frequency signal for the in vivo measurement of biological impedances
has been introduced.
An active electrode was built for differential voltage measurement which combines a
superior CMRR performance, compared to previously reported implementations, with high
input impedance. The implemented circuit has been designed to allow further miniaturisation
by means of hybrid semiconductor technologies. Prototypes of several digital subsystem
components of the specified EIT system were designed and validated the concept of the novel
multi-frequency EIT system.
For testing and calibrating the developed front-end electronics, a novel EIT phantom
systems is presented, which employs active impedance elements. Utilising active impedance
elements enables computer control of the actual impedance values which simplifies and
automates the measurement of phantom impedances over a wide range.