def get_id_lookup_from_demographics_file(demographics_df):
    """
    Extract a lookup (Redcap ID -> NCANDA SID) from a demographics DataFrame.

    Expects a demographics_df outputted by `process_demographics_file`.
    """
    return (demographics_df
            .reset_index()
            .set_index('study_id')
            .to_dict()
            .get('mri_xnat_sid'))