Finite element modelling and image reconstruction in single and multi-frequency electrical impedance tomography.

Electrical impedance tomography (EIT) is an imaging technique that aims to reconstruct the internal conductivity distribution of a body, based on the electrical measurements taken on its periphery. While relatively new it has received attention as a possible new medical imaging technique which offers non-hazardous applications and low-cost instrumentation.To understand the full potential of this new mode of imaging, a numerical modelling method has been used in order to investigate the behaviour of an EIT system. Using this mode of analysis, it is possible to simulate many experiments that otherwise physically would be very time consuming and expensive. Such investigation will include the effect and quantification of various physical conditions which have effects on the obtained boundary voltages of an EIT system. An aim of EIT is the ability to reconstruct accurate images of internal conductivity distributions from the measured boundary voltages. Image reconstruction in EIT using the sensitivity algorithm is generally based on the assumption that the initial conductivity distribution of the body being imaged is uniform. The technique of image reconstruction using the sensitivity algorithm is described and reconstructed images are presented. Improvements in image quality and accuracy are demonstrated when accurate a-priori 'anatomical' information, in the form of a model of the distribution of conductivity within the region to be imaged, are used. In practice correct a-priori information is not available, for example, the conductivity values within the various anatomical regions will not be known. An iterative algorithm is presented which allows the conductivity parameters of the a-priori model to be calculated during image reconstruction. Multi-frequency EIT is a modified approach of the single frequency method by which tissue characterisation has been proposed by imaging the internal conductivity of region over a range of frequencies. However, due to instrumentation drawbacks, only the real parts of the boundary voltages are presently measured. These real only voltages have sofar (in practice) been used to reconstruct images of the changes in internal conductivity of a region with frequency. The penalty for ignoring the imaginary parts of the data are presented and results obtained show that to accurately image the internal conductivity of a region, not only complex data are needed, but also some a-priori information about the region may be necessary.