def pis_map(diffn_img, index_low_b_val, index_high_b_val):
    """
    Produces the physically implausible signal (PIS) map [1].

    Parameters
    ----------
    diffn_img

    index_low_b_val : int
        index into the DWI identifying the image with lower b-value.
        Usually 0 referring to the b=0 (non-DW) image

    index_high_b_val : int
        index into the DWI identifying the image with higher b-value

    Returns
    -------
    pis_map : ndarray
        Binary 3d image identifying voxels to be PIS.

    References
    -----------
    1. D. Perrone et al. / NeuroImage 120 (2015) 441–455

    """

    pis = diffn_img[:, :, :, index_low_b_val] < diffn_img[:, :, :, index_high_b_val]

    return pis