def Sensitivity3D(a, b):
    """
    Returns the sensitivity (true positive rate) between the predictions a and labels b
    :param a: {numpy array} 3D volume
    :param b: {numpy array} 3D volume
    :return:
    """

    ba = (a > 0).astype(int)
    bb = (b > 0).astype(int)

    true_positives  = (ba + bb == 2).sum()
    total_positives = bb.sum()

    return true_positives / total_positives