def get_acquisition_mode(acqu):
    """
    Retrieves the acquisition mode list from the header file. 
    known values:
    REAL,
    COMPLEX,
    TPPI,
    COMPLEX_TPPI,
    PHASE_MODULATION,
    ECHO_ANTIECHO
    """
    if 'ACQUISITION_MODE' in acqu:
        return acqu['ACQUISITION_MODE'][1]
    elif 'Phase Mod' in acqu:
        return acqu['Phase Mod']
    return None
