Limin_Sun_Thesis_submit.pdf

CHAPTER 8 DISCUSSION
gradient artifacts with a moving weighted window. Unfortunately, these methods could not
avoid the residual gradient artifacts although adaptive noise cancellation (ANC) (Allen et al.,
2000), lowpass filter (Becker et al., 2005) and PCA (Niazy et al., 2005; Schmid et al., 2006)
have worked on reduction of residual gradient artifacts.
One purpose of the new gradient artifacts removing method presented here was to
minimize the residual gradient artifacts by optimizing the subtraction with respect to changes
in the artifact template (Becker et al., 2005 and Herrmann and Debener, 2008) that may occur
due to both non perfect technical stability of the EEG-MR-setting and subjects’ head
movements. Based on Becker’s MAS method, the MAMAS approach improved the
performance of artifact reduction by monitoring the change of electrodes position which
causes the gradient artifacts not to be equal from slice to slice. Besides this effect the
asynchronous timing of the EEG and MR measurement adds to these variations. Furthermore,
tiny long term trends were observed in the artifact waveforms. For a precise analysis of the
character and strength of these variations we conducted phantom measurements in order to
extract the pure artifact waveform without any subject related variability. In these
measurements a watermelon served as a phantom with the same type and number of
electrodes as used for the human measurements. The four different mechanically fixed
orientations lead to four different orientations of the electric loops (consisting of the water
melon surface, the electrodes and the cables) with respect to the gradient magnetic field. In
order to simulate a kind of worst case scenario the watermelon was rotated by two degrees per
step which is certainly at the upper limit of realistic movements. The amplitude differences
between the artifact-templates observed at different orientations were surprisingly high and
reached up to 10% of the absolute artifact amplitude thus underpinning the need for taking
these movements into account. However, even more surprising the shape of the artifact
template changed over short periods of only a few seconds although staying clearly below the
movement related fluctuations. The reported result demonstrates that gradient artifacts are
different from electrode positions and time to time. The underlying technical mechanisms
remain unclear. One may speculate if the warming of the gradient coils leads to very small
changes in the magnetic field giving rise to tiny yet not negligible variations of the artifact
shape. This observation explains the fact that among the evaluated weighting functions the
efficiency in removing the artifact was most pronounced in the Becker version which has the
strongest focus on those epochs positioned closest to the artifact under consideration. In
general two gradient artifacts are similar (yet still slightly varying) within short intervals but
are more and more different with increasing temporal distance. In summary, the gradient
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