def streamline_measure(estimated_target_tract_idx, target_tract_idx):
    """Ratio of overlap between two tracts (in number of streamlines).
    """
    a = set(estimated_target_tract_idx.tolist())
    b = set(target_tract_idx.tolist())
    dsc = 2.0 * len(a.intersection(b)) / float(len(a) + len(b))
    return dsc