Combining EEG and MRI Techniques - Quantitative EEG Analysis Methods and Clinical Applications - page 340

the presence of ferromagnetic materials in the area being imaged). However, with
the use of specialized electrodes as mentioned in Section 12.2.2.1 (especially thin
disk electrodes), these susceptibility artifacts in the MRI images are restricted to a
few millimeters in depth, less than the thickness of the scalp and skull. Therefore,
they do not interfere with the quality of an image of the cerebral cortex (and other
deeper structures) [46, 47]. The use of carbon fiber leads further prevents distortions
in the MR images [48]. The other source of artifacts in the MR images could poten-
tially arise from “chemical shift” effects induced by the electrode gel. These artifacts
are typically seen at the interface of fat and water in tissues and appear as dark or
bright bands at the edges. The use of oil-based electrode gels should therefore be
avoided. Figure 12.12 shows fMRI images of a patient (acquired with simultaneous
EEG). Note that the electrode artifacts on the scalp do not affect the quality of the
image of the cerebral cortex.
12.2.2.5 Effect of MRI Environment on Neural Activity
It could be argued that the presence of a strong magnetic field could affect brain sig-
nals. Also, the environment of the MRI scanner, including the high-decibel noise
during imaging, as well as the vibrations in the bore of the magnet, might have suffi-
cient psychological effect to alter the EEG, as compared to similar recording sessions
outside the scanner. The majority of scientific evidence suggests that there is no
effect of the high-field-strength magnet on various EEG phenomena such as P300
EEG
electrodes
Figure 12.12 Simultaneous EEG-fMRI data recorded from a subject performing left elbow flexions
and extensions during poststroke rehabilitation (Johns Hopkins University). Note that the magnetic
susceptibility artifacts underneath the EEG electrodes are restricted to the scalp and skull and do not
distort the fMRI. (Courtesy of Johns Hopkins University.)