How can I cut exactly the first harmonic (H1) from a FFT of an image
The solution was to take Fourier transform 3D for each slice, then to chose only the 2nd component of the Transform to transform it back to the spatial space, and that's it.
The benefit of this is to detect if something is moving along the third axis(time in my case). 


for sl in range(img.shape[2]):
   #-----Fourier--H1-----------------------------------------
   # ff1[:, :, 1] H1 compnent 1, if 0 then DC
   ff1 = FFT.fftn(img[:,:,sl,:])
   fh = np.absolute(FFT.ifftn(ff1[:, :, 1])) 

   #-----Fourier--H1-----------------------------------------
```