M.Tech. (CS), Semester III, Course B50
Functional Brain Signal
Processing: EEG & fMRI
Lesson 18
Kaushik Majumdar
Indian Statistical Institute
Bangalore Center
kmajumdar@isibang.ac.in
Equipment and Acquisition
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EEG electrodes to reduce eddy current by
adding 5 to 10 kohm for 1.5 T fMRI scanner
and/or using specialized materials for making
them.
Reducing artifacts in EEG signals due to the
induced current.
Reducing heat due to induced current, which
may cause burn on the skin of the subject.
Equipment and Acquisition (cont)
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Every fMRI scanner is equipped with a very
sensitive RF detector. If EEG equipment
produces any RF wave that would
contaminate the fMRI signal. To avoid this
amplifier/digitizer can be run on battery.
Water based electrode gels proofed to
produce negligible chemical shift artifacts on
the scalp surface.
Equipment and Acquisition (cont)
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High sampling rate (1 to 5 kH) for EEG is
required for good modeling of MR artifacts.
Helium pump and other unnecessary
apparatus should be kept switched off during
acquisition to avoid noise due to those
equipments.
Digitized signal to be carried out of the
shielded room through optical fibers to
preserve isolation of the MR scanner.
Equipment and Acquisition (cont)
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To reduce residual artifacts in EEG due to
MR scanning following must be adhered to:
(1) MR volume acquisition must be stable in
time (always equal time lag in between any
two successive volume acquisition).
(2) MR device and EEG amplifier will have to
be synchronized.
EEG and fMRI Data Fusion
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A brain region may have a large metabolic
load without measurable EEG generation.
This can happen in deep brain region from
where EEG will not reach to the electrodes
on the scalp. EEG activity tangential to the
scalp or opposing and self-canceling sources
in the sulci may have substantial metabolic
demand without significant EEG
manifestation.
EEG and fMRI Data Fusion (cont)
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Inhibitory neuronal (for example GABA-ergic
stellate cells, which do not generate
measurable EEG due to concentric
configuration) activities on fMRI are
unpredictable.
A relatively small fraction of synchronously
firing neurons may generate a measurable
EEG on the scalp without a strong enough
hemodynamic activity.
Linden et al., 1999
EEG and fMRI Data Fusion (cont)
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An increase in neuronal
synchrony associated
with a decrease in
synaptic transmission
will be associated with
higher EEG amplitudes
and a negative fMRI
signal.
BOLD response to T
= target and N = nontarget.
Linden et al., 1999
Evoked Potential During the Same
Task of the Preceding Slide
Event related potential during visual oddball task. Black line for targets and grey for
non-targets.
Benar et al., 2002
BOLD Response to Interictal
Epileptic Discharges
Arrows indicate interictal
epileptic discharges.
Liu et al., 1998
fMRI Localization of EEG/MEG
Sources
Simultaneous EEG-fMRI
significantly improves source
localization of EEG over the
classical dipole source estimation.
A. K. Liu et al., PNAS, 95(5): 8945
– 8950, 1998.
Hanson et al., 2007
Converting EEG to RF Range to be
Detected by MR Scanner
References
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P. Ritter and A. Villringer, Simultaneous
EEG-fMRI, Neurosci. Behav. Rev., 30: 823 –
838, 2006.
http://fsl.fmrib.ox.ac.uk/eeglab/fmribplugin/
Artifact removal toolbox based on MATLAB.
For the detail of algorithms see R. K. Niazy et
al., Removal of FMRI environment artifacts
from EEG data using optimal basis set,
NeuroImage, 28: 720 – 737, 2005.
THANK YOU
This lecture is available at http://www.isibang.ac.in/~kaushik