EEG Recording During fMRI Experiments:
Image Quality
K. Krakow,1 P.J. Allen,2 M.R. Symms,1 L. Lemieux,1* O. Josephs,3
and D.R. Fish1,2
1Epilepsy Research Group, Department of Clinical Neurology, Institute of Neurology, University
College London, Queen Square, London, and National Society for Epilepsy,
Chalfont St. Peter, Bucks, UK
2Department of Clinical Neurophysiology, The National Hospital for Neurology and Neurosurgery,
Queen Square, London, UK
3Wellcome Department of Cognitive Neurology, Functional Imaging Laboratory, Institute of
Neurology, University College London, Queen Square, London, UK
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Abstract: Electroencephalographic (EEG) monitoring during functional magnetic resonance imaging
(fMRI) experiments is increasingly applied for studying physiological and pathological brain function.
However, the quality of the fMRI data can be significantly compromised by the EEG recording due to the
magnetic susceptibility of the EEG electrode assemblies and electromagnetic noise emitted by the EEG
recording equipment. We therefore investigated the effect of individual components of the EEG recording
equipment on the quality of echo planar images. The artifact associated with each component was
measured and compared to the minimum scalp-cortex distance measured in normal controls. The image
noise originating from the EEG recording equipment was identified as coherent noise and could be
eliminated by appropriate shielding of the EEG equipment. It was concluded that concurrent EEG and
fMRI could be performed without compromising the image quality significantly if suitable equipment is
used. The methods described and the results of this study should be useful to other researchers as a
framework for testing of their own equipment and for the selection of appropriate equipment for EEG
recording inside a MR scanner. Hum. Brain Mapping 10:10–15, 2000. © 2000 Wiley-Liss, Inc.
Key words: fMRI; EEG; electrophysiological recording; image quality; echo planar imaging; artifact;
materials; epilepsy; multimodal imaging
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INTRODUCTION
The recording of electroencephalograms (EEG) inside
the MR scanner has gained increasing interest. It is used
to correlate functional magnetic resonance imaging
(fMRI) acquisitions with spontaneous EEG events (e.g.,
epileptiform discharges in patients with epilepsy and
physiological EEG events such as oscillatory rhythms) or
evoked potentials and thus help to identify the genera-
tors of these events [Huang-Hellinger et al., 1995; Kra-
kow et al., 1999; Seeck et al., 1998; Warach et al., 1996]. A
further application is monitoring the state of arousal or
sleep during fMRI experiments [Portas et al., 1999].
EEG recording inside the magnetic fields of a MR
scanner is associated with significant technical prob-
lems. Safety issues [Lemieux et al., 1997] and EEG
Contract grant sponsor: Medical Research Council, UK.
Correspondence to: Dr. Louis Lemieux, MRI Unit, National Society
for Epilepsy, Chalfont St. Peter, Buckinghamshire SL9 0RJ, UK.
E-mail: l.lemieux@ion.ucl.ac.uk
Received for publication 11 May 1999; accepted 3 February 2000
r Human Brain Mapping 10:10–15(2000) r
© 2000 Wiley-Liss, Inc.

quality aspects [Allen et al., 1998] have already been
addressed, providing the basis for high quality and
safe EEG recordings inside the MR scanner. However,
the effect of the EEG recording equipment on the MRI
quality, particularly echo planar imaging (EPI), has
not yet been addressed in detail. We therefore inves-
tigated two effects of the EEG recording, which can
compromise MRI data: (1) Local signal drop out and
geometric distortion due to magnetic susceptibility
differences and the presence of eddy currents in EEG
electrode assemblies [Joseph et al., 1996]; (2) Degrada-
tion of the image signal-to-noise ratio because of elec-
tromagnetic noise emitted by the EEG recording
equipment.
The main purpose of this study was to evaluate
these effects on a representative sample of EEG re-
cording components to identify the most appropriate
components for our setting, and to provide a general
framework that can be useful to other researchers
interested in evaluating their own equipment for EEG
recording inside a MR scanner.
MATERIALS AND METHODS
All imaging was performed on a 1.5 T Horizon
EchoSpeed MRI scanner (General Electric, Milwaukee,
USA) unless stated otherwise.
Measurement of the scalp-cortex distance
T1-weighted inversion-recovery prepared volume
acquisitions as used in our standard scanning protocol
(Fast IRSPGR: TI/TR/TE/flip 5 450/15/4.2/20; 124
1.5-mm thick coronal slices; 256 3 192 matrix, 24 3 18
cm FOV) of ten healthy volunteers were acquired
(four males, six females, median age 36.0, range 17–50
years). The distance between the surface of the scalp
and the cortex was measured at locations correspond-
ing to the position of the FP1, F3, F7, C3, T3, P3, T5
electrodes of the 10/20 system, using our image dis-
play and analysis software, MRreg [Moran et al., 1999].
The following three experiments were carried out
using a gradient-echo EPI sequence similar to the one
we have used in clinical fMRI experiments [Krakow et
al., 1999]: TR/TE 5 3000/40, bandwidth 100 kHz, 24
cm FOV, flip angle 90, acquisition matrix 96 3 96,
reconstruction matrix 128 3 128. Fat saturation was
explicitly selected to prevent the scanner from using
the spectral spatial pulse. Twenty contiguous 5-mm
slices were acquired in an interleaved fashion.
Quantification of the local signal drop out and
geometric distortions on a phantom
The following components of EEG electrode assem-
blies were assessed: electrodes, conductive electrode
gel and paste, electrode adhesive, current-limiting re-
sistors, insulating sleeve enclosing the resistor, and
wire. Details of the origin and composition of the
components are given in Table I. Each component was
attached individually on the surface of a 10-cm glass
sphere filled with distilled water. Electrodes, resistors,
and wires were tested both with their long axis paral-
lel (placed on top of the phantom, axial sections) and
perpendicular (frontal side of phantom, coronal sec-
tions) to the B0 magnetic field. Adhesive and gel were
only tested on top of the phantom for practical rea-
sons. All objects were scanned twice in each position
on different occasions. Measurements were also made
with the whole electrode assembly attached to the
phantom. The maximum perpendicular depth of arti-
facts was measured in the images using MRreg.
Quantification of artifacts in vivo for the
components with acceptable artifacts as measured
in the phantom experiments
Electrode assemblies (consisting of an electrode, resis-
tor, resistor insulation, wire, and 0.1 ml of electrode
adhesive and gel) made of the components which gave
acceptable results in the first experiment where placed
on the right side of the scalp of a volunteer at the 10/20
electrode positions used in our clinical studies (FP2, F8,
T4, T6, O2) [Krakow et al., 1999]. These were compared
to electrode assemblies made up of nonoptimized com-
ponents which have been used previously for intra-MR
EEG (Ag/AgCl electrode, carbon current-limiting resis-
tor, silicone-insulated copper lead) [Krakow et al., 1998],
which were placed at the equivalent positions on the left
side of the head. Images were obtained using a high
resolution EPI (sequence parameters as above, except
matrix size: 256 3 256). The depth of the artifact was
measured using MRreg.
Quantification of the image noise caused by the
electromagnetic fields generated by the EEG
recording equipment
The EEG recording system consisted of electrode
assemblies, placed in the headcoil beneath the phan-
tom and connected to a non-ferrous headbox (devel-
oped in-house) located at the entrance to the bore of
the magnet (headcoil-headbox distance 5 125 cm).
The headbox was connected to an unscreened battery-
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