code: def _substract_hemispheres(active, reference, active_sigma, reference_sigma, voxel_spacing):
      
      active_kernel = _create_structure_array(active_sigma, voxel_spacing)
      active_smoothed = gaussian_filter(active, sigma = active_kernel)
  
      reference_kernel = _create_structure_array(reference_sigma, voxel_spacing)
      reference_smoothed = gaussian_filter(reference, sigma = reference_kernel)
  
      return active_smoothed - reference_smoothed
docs: Helper function for `_extract_hemispheric_difference`.
      Smoothes both images and then substracts the reference from the active image.