code: def demean(in_file, in_mask, only_mask=False, newpath=None):
      
      import os
      import numpy as np
      import nibabel as nb
      from nipype.utils.filemanip import fname_presuffix
  
      out_file = fname_presuffix(in_file, suffix='_demeaned',
                                 newpath=os.getcwd())
      nii = nb.load(in_file)
      msk = nb.load(in_mask).get_data()
      data = nii.get_data()
      if only_mask:
          data[msk > 0] -= np.median(data[msk > 0])
      else:
          data -= np.median(data[msk > 0])
      nb.Nifti1Image(data, nii.affine, nii.header).to_filename(
          out_file)
      return out_file
docs: Demean ``in_file`` within the mask defined by ``in_mask``