def get_series_number(nrrd_file):
    """
    (str) -> str
    Return series number for the given nrrd_file.
    >>>nrrd_file = '/paulsen/MRx/PHD_032/0454/31774/ANONRAW/0454_31774_DWI-31_7.nrrd'
    >>>get_scan_type(nrrd_file)
    7
    """
    return nrrd_file.split("_")[-1].replace(".nrrd", "")