def get_num_label_vols(md):
    """
    Get the number of volumes used for labelling - e.g. 2 for tag-control pair data
    """
    iaf = md.get('iaf', 'tc')
    if iaf in ('tc', 'ct'):
        return 2
    elif iaf == 'diff':
        return 1
    elif iaf == 'mp':
        return md.get('nphases', 8)
    elif iaf == 've':
        return md.get('nenc', 8)
