EEG in fMRI: High frequency "deltas" in EEG after gradient artifact removal
These seem to be simply gradient artifact remnants- Brain products analyzer didn't fully clean them.

Looking over the spectrogram of another EEG before the gradient artifact removal- it seems as if this noise is merely leftovers of the gradient artifact caused by the fMRI system (noticed that these spikes were greatly reduced by the brain vision analyzer gradient artifact cleaner, but in some cases- this transform left much noise in these frequencies).