IOP PUBLISHING PHYSIOLOGICAL MEASUREMENT
Physiol. Meas. 30 (2009) S201–S224 doi:10.1088/0967-3334/30/6/S14
A modelling study to inform specification and optimal
electrode placement for imaging of neuronal
depolarization during visual evoked responses by
electrical and magnetic detection impedance
tomography
O Gilad
1,2,5
,LHoresh
3
and D S Holder
1,4
1
Department of Medical Physics & Bioengineering, University College London, London, UK
2
The Abramson Center for Medical Physics, Tel-Aviv University, Israel
3
Scientific Computing, Mathematics and Computer Science, Emory University, Atlanta,
GA, USA
4
Department of Clinical Neurophysiology, University College London Hospitals, London, UK
E-mail: o.gilad@ucl.ac.uk, horesh@mathcs.emory.edu and d.holder@ucl.ac.uk
Received 12 December 2008 accepted for publication 25 February 2009
Published 2 June 2009
Online at stacks.iop.org/PM/30/S201
Abstract
Electrical impedance tomography (EIT) has the potential to achieve non-
invasive functional imaging of fast neuronal activity in the human brain due
to opening of ion channels during neuronal depolarization. Local changes of
resistance in the cerebral cortex are about 1%, but the size and location of
changes recorded on the scalp are unknown. The purpose of this work was
to develop an anatomically realistic finite element model of the adult human
head and use it to predict the amplitude and topography of changes on the
scalp, and so inform specification for an in vivo measuring system. A detailed
anatomically realistic finite element (FE) model of the head was produced from
high resolution MRI. Simulations were performed for impedance changes in
the visual cortex during evoked activity with recording of scalp potentials by
electrodes or magnetic flux density by magnetoencephalography (MEG) in
response to current injected with electrodes. The predicted changes were
validated by recordings in saline filled tanks and with boundary voltages
measured on the human scalp. Peak changes were 1.03 ± 0.75 µV (0.0039 ±
0.0034%) and 27± 13 fT (0.2± 0.5%) respectively, which yielded an estimated
peak signal-to-noise ratio of about 4 for in vivo averaging over 10 min and
1 mA current injection. The largest scalp changes were over the occipital
cortex. This modelling suggests, for the first time, that reproducible changes
could be recorded on the scalp in vivo in single channels, although a higher
SNR would be desirable for accurate image production. The findings suggest
5
Author to whom any correspondence should be addressed.
0967-3334/09/060201+24$30.00 ? 2009 Institute of Physics and Engineering in Medicine Printed in the UK S201

S202 O Gilad et al
that an in vivo study is warranted in order to determine signal size but methods
to improve SNR, such as prolonged averaging or other signal processing may
be needed for accurate image production.
Keywords: electrical impedance tomography, neural imaging, evoked
responses, brain modelling, finite elements
(Some figures in this article are in colour only in the electronic version)
1. Introduction
1.1. Possible use of EIT for imaging fast neural activity in the brain
Functional neuroimaging has improved greatly in the past two decades but the ‘holy grail’
would be to image neuronal activity non-invasively with a temporal and spatial resolution of
about 1 ms and 1 mm respectively. Currently, there has been great interest in such ‘neural
imaging’ with proposals for the use of MRI (Bodurka et al 1999, Bodurka and Bandettini 2002,
Kamei et al 1999, Kilner et al 2004, Kim and Ogawa 2002), infrared (Cohen 1973, Stepnoski
et al 1991) or inverse source modelling (Baillet et al 2001, Cohen 1968,Daleet al 2000,
Dale and Halgren 2001, Hamalainen 1992, Hamalainen et al 1993, Michel et al 2004) for this
purpose, but no method has yet been shown to be successful. Electrical impedance tomography
(EIT) is a novel medical imaging method which has the potential to achieve the desired
temporal resolution, by imaging the electrical impedance changes (Holder 1987) which occur
over milliseconds when neuronal ion channels open during activity and the cell membrane
resistivity decreases (Cole and Curtis 1939). This work arose from an attempt to record
such changes, for the first time, in human subjects during visual evoked potentials, with non-
invasive scalp electrodes or detection of magnetic flux density by magnetoencephalography
(MEG). Prior to attempting this study, we wanted to model the expected changes in order
to estimate if such a study appeared feasible and suggest optimal electrode placement. In
the event, we proceeded to the planned human study; these empirical findings are presented
elsewhere (Gilad et al 2009, Gilad and Holder 2009).
EIT provides information regarding the internal electrical properties inside a body based
on non-invasive voltage measurements on its boundary. Data acquisition is performed through
an array of electrodes which are attached to the surface of the imaged object. Sequences of
small insensible currents, typically of about 1 mA, are injected into the object through these
electrodes and the corresponding boundary electric potentials are measured over a predefined
set of electrodes. The process is repeated for numerous different configurations of applied
current. The internal admittivity or impedivity distribution can be inferred using this boundary
data. EIT was first proposed as a medical imaging method by Henderson and Webster (1978)
and was initially applied to chest imaging (Brown et al 1985, Brown and Seagar 1987,
Metherall et al 1996). Potential applications of EIT for imaging brain function and pathology
include detection and monitoring of cerebral ischaemia and haemorrhage (Gibson et al 2000,
Holder 1992a, Horesh et al 2005,McArdleet al 1988,McEwanet al 2006, Romsauerova
et al 2006), localization of epileptic foci (Bagshaw et al 2003, Fabrizi et al 2006b), normal
haemodynamic brain function (Tidswell et al 2001) and neuronal activity (Boone and Holder
1995, Holder 1987).