Abstract: Many electrode Electrical
Impedance Tomography Spectroscopy (EITS) for Head Imaging
R.J.Yerworth1, R.H.Bayford1,2,B. Brown3, P. Mills3 M.Conway4, D.S.Holder1
1 Department of Clinical Neurophysiology, Middlesex Hospital, University College London, London W1T 3AA, UK
2 School of Health, Environment and Biological Sciences, Middlesex University, Archway Campus, London N19 5ND, UK.
3 Department of Medical Physics and Clinical Engineering, University of Sheffield, Floor I, Royal Hallamshire Hospital, Glossop Road, Sheffield, S10 2JF, UK
4 Department of Medical Physics and Bioengineering, Shropshire House, 11-20 Capper Street, University College London Hospitals NHS Trust, London WC1E 6JA, UK
Electrical Impedance Tomography Spectroscopy
(EITS), imaging with several frequencies simultaneously, offers major advantages
over single frequency EIT for head imaging, as it allows tissues to be differentiated
with respect to their frequency characteristics, not just their variation over
time. Our group is interested in developing EIT of brain function and would
like to develop and assess a system with many electrodes and flexible imaging
protocols in order to maximise spatial resolution. This paper describes the
development of an existing parallel 8-channel EITS system to include addressing
of 64 electrodes using a distributed multiplexer and novel stray capacitance
compensation features that have not previously been used in EIT.
The new system uses a single channel from the parallel system, which delivers 30 frequencies per image set in three complex waveforms. Cross-point switches enable 64 electrodes to be used, and are placed on the head in a distributed manner. Negative Impedance Converters (NICs) are employed to compensate for stray capacitance. Screened leads are used up to the cross-point switches with short unscreened leads from cross-point switch to multiplexer.
The use of cross-point switches and unscreened leads might have introduced capacitance effects which would reduce the frequency bandwidth, making it unsuitable for EITS. Since the capacitance of unscreened leads increases with length, the system minimises this by placing the multiplexers on the head. The use of NICs aims to negate most of the capacitance introduced by the cross-point switches.
This design results in a high quality portable 64 electrode EITS system with performance characteristics that compare favourably with the original 8-channel system.