Avery, JP; (2015) Improving Electrical Impedance Tomography of brain function with a novel servo-controlled electrode helmet. Doctoral thesis , UCL (University College London).

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Abstract

Electrical Impedance Tomography (EIT) is a medical imaging technique which reconstructs the internal conductivity of an object from boundary measurements. EIT has the potential to provide a novel means of imaging in acute stroke, epilepsy or traumatic brain injury. Previous studies, whilst demonstrating the potential of the technique, have not been successful clinically.The work in this thesis aims to address fundamental limitations including measurement drift in electronic hardware, lack of an anatomically realistic tank phantom for rigorous testing, poor electrode-skin contact and mis-location of scalp electrodes. Chapter 1 provides an introduction of the principles of bioimpedance and EIT, as well as a review of previous clinical studies. Chapter 2 details the development of a novel anatomically realistic head phantom, simulating the human adult head with scalp electrodes, using a 3D printer and cylindrical holes to provide simulated conductivity. This replicated the varying spatial conductivity of the skull within 5 % of the true value. Two multifrequency EIT systems with parallel voltage recording were optimised for recording in the adult head with scalp electrodes, in chapter 3. Measurement drift was reduced by better case design and temperature control and data quality was improved with an updated interface to the current source and signal processing. The UCL ScouseTom system, performed best, with lower noise in all resistor and tank measurements, but the differences were masked during scalp recordings. Further, both systems produced similar results in the realistic adult head tank from chapter 2. Recent advances in EIT imaging coupled with the developments in chapters 2 and 3 provided opportunity to reassess the feasibility of monitoring epilepsy with EIT. Biologically representative perturbations was localised to within 8 mm in the head tank, with less than half the image error of previous studies. However, the key limitations of application time and measurement drift with scalp electrodes had yet to be addressed. Therefore the focus of the work in chapter 5 and chapter 6 was the design and testing of a novel self-adjusting electrode helmet. Skin-electrode impedance was continuously optimised by constant pressure, rotation and feedback control, and position sensors returned the co-ordinates of electrode tips. Finally, experiments with this helmet were undertaken to assess the feasibility of future clinical recordings.

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