Hi, Philipp-

We have seen similar rapid variations in other cases of breathing apparatuses used in FMRI.

You should open the initial EPI in the GUI, and flip through the volumes (e.g., open a graph window and hit the right arrow key over it to surf through the data). Does the subject appear to be moving?

My guess is that these oscillating, estimated "micromovements"---I wouldn't call them white noise, because they appear too regular---are likely related to the breathing apparatus, perhaps affecting the magnetic field slightly or related to breathing movements. Unless the head is fixed with a styrofoam cap or something, the subject can still move, even though anaesthetized (e.g., their chest motion will follow breathing), and their breathing pattern can be different than normal. But again, my guess would be, based on the rapid frequency, that it is related to the machine a bit.

--pt