Biomedical Engineering Reference
In-Depth Information
Many of these EEG features have been empirically used for addressing clinical and
research questions; however, often their genesis is poorly understood. Simultaneous
EEG-fMRI offers the exciting possibility of imaging the brain regions associated
with these EEG phenomena, as well as allowing us to make inferences about the
large-scale neuronal pathways involved in generation of such EEG activity.
Additionally, EEG has the potential to alter the traditional methodology of gen-
erating fMRI images. As noted previously, the fMRI is based on a statistical compar-
ison between multiple trials of an idealized “resting” phase and “task” phase. The
statistical basis of this comparison relies on the behavioral responses of the individ-
ual; however, it is well known that behavioral changes do not necessarily generate
exactly repeatable neuronal responses, and vice versa. Mental states such as atten-
tion, fatigue, and so forth play an important role in the underlying neuronal dynam-
ics, a factor that current fMRI reconstruction methods largely ignore by relying
solely on behavioral cues and outcomes. Because the EEG is a direct reflection of
some of these mental states, its incorporation into the way fMRI images are gener-
ated could make them more reflective of the true neuronal dynamics.
The ultimate goal of combining EEG with fMRI is to exploit the complementary
information in these two separate datasets to better understand the functional
dynamics of the brain. This chapter aims to address the theoretical and practical
considerations for recording and analyzing simultaneous EEG-fMRI, as well as
some current and emerging applications.
12.2.2 Technical Challenges
The bore of the MRI scanner is a high-field-strength magnet, typically 1.0 Tesla to
4.0 Tesla. Additionally, as part of the fMRI imaging methodology, rapidly changing
magnetic gradient fields and RF pulses are applied during image acquisition. As a
result, recording EEGs inside the MRI scanner is especially challenging due to the
strong electromotive forces induced during these rapidly changing applied fields, as
well as moving conductor loops within the strong static magnetic field [44]. These
induced currents are orders of magnitude larger than the EEG itself. Not only do
they interfere with the EEG signal, they might cause severe injury to the subject due
to localized heating and burns [45].
The presence of EEG electrodes on the scalp might cause distortions in the MRI
images because of magnetic susceptibility resulting from the conductor elements of
the electrodes and chemical shift artifacts resulting from the saline gel in the elec-
trode-skin interface. However, sufficient technical advances have been made in the
past decade to overcome most of these issues, to the extent that simultaneous
EEG-fMRI can be safely recorded using specialized hardware, and the quality of
both are comparable to independently acquired fMRI and EEG.
12.2.2.1 Hardware Considerations
The quality of the acquired EEG and fMRI as well as patient safety can be substan-
tially improved by observing some general principles aimed toward avoidance of
current loops, minimization of movements (albeit very small ones) within the scan-
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