EEG in fMRI: High frequency "deltas" in EEG after gradient artifact removal EEG was taken inside an fMRI, including ECG, EXG and eye tracking system.
Sampling rate was 5000 Hz. Sync box was used. 
After running Brain Product Analyzer gradient artifact removal (using markers from the fMRI for each volume, and after removal- down sampling to 250 Hz.)- one could still identify high frequency "ripples" (high frequency oscillations) which ruined the CB cleaning.
I planned to add some low pass or some other filter to get rid of these, and before performing the filter checked the spectrum of the noise and found the high frequency noises shown below.
Any idea what causes these? 

Thanks!
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).