code: def surface_ras_shift(self):
          
          T1_path = self.dir / 'mri' / 'T1.mgz'
          assert T1_path.exists()
  
          T1 = nload(str(T1_path))
  
          return T1.header['Pxyz_c']
docs: Freesurfer uses two coordinate systems: one for volumes ("RAS") and
          one for surfaces ("tkReg", "tkRAS", and "Surface RAS").
          To get from surface to volume coordinates, add this numbers.
          To get from volume to surface coordinates, substract this numbers.