PhD Thesis: Electrical Impedance Tomography of Human Brain Function

Abstract

This thesis shows for the first time that Electrical Impedance Tomography (EIT) is able to detect and image changes in impedance during brain activity in humans. The work described is both experimental and theoretical.

In the first instance, the sources of artefacts in EIT images were investigated. Although an impedance change could be correctly localised, both increases and decreases in impedance were present. The characteristics of the impedance change were examined by measuring the impedance change directly.

Following this, a reconstruction technique was developed, designed for imaging the human head in 3D. The head was modelled as a homogeneous sphere and images reconstructed from this were tested using simulated data and data obtained in saline-filled tanks.

This algorithm was then used to generate images from EIT data recorded during evoked responses in human volunteers. Although images reconstructed from saline-filled tanks were able to localise an impedance change accurately, images reconstructed from human data localised the impedance change correctly in around a half of the experiments.

The possibility that the incorrect localisation was due to modelling errors was investigated by incorporating a finite element model of a real human head into the reconstruction process. The results from this method were similar to those obtained using the spherical model of the head.

Details of the thesis are given below.
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Preliminaries

- Title page, table of contents, acknowledgements, etc.

Introduction

- Background. EIT instrumentation, image reconstruction, applications of EIT, some physiology and the potential use of EIT in brain imaging.

A Reconstruction algorithm for a sphere

- Development of a 3D linearised reconstruction technique for brain imaging, assuming the head can be modelled as a homogeneous sphere. Discusses optimum electrode placement, an analytical calculation of the sensitivity matrix and matrix inversion by singular value decomposition. The technique is validated on simulated data and data acquired from saline-filled tanks.

Impedance imaging of human evoked responses

- Describes the human imaging protocol and modifications to the image reconstruction technique. Some results, including images, are presented, and there is a detailed discussion of possible sources of error.

Inclusion of real head model

- A realistic finite element model of the head was obtained and used to calculate the sensitivity matrix. Little improvement in image quality was seen in human imaging. Possible reasons for this are discussed.

Conclusion

- Progress made, future work and possible future prospects for impedance imaging of the brain.

Appendices

- Methods of imaging the brain, Solution to Poisson's Equation for a sphere and 2D finite element modelling of the neonatal head.

References