如何在DIPY中跟踪不同的流线？

import numpy as np
import nibabel as nib
from dipy.data import read_stanford_labels
from dipy.tracking import utils
from dipy.tracking.local import LocalTracking
from dipy.tracking.streamline import Streamlines
from dipy.direction import DeterministicMaximumDirectionGetter

hardi_img, gtab, labels_img = read_stanford_labels()
data = hardi_img.get_data()
labels = labels_img.get_data()
affine = hardi_img.affine

directions = DeterministicMaximumDirectionGetter.from_shcoeff(data, sphere)

seeds = utils.seeds_from_mask(labels == 2, density=1, affine=affine)
stopping_criterion = utils.BinaryStoppingCriterion(labels == 3)

streamlines_generator = LocalTracking(directions, stopping_criterion, seeds, affine, step_size=0.5)
streamlines = Streamlines(streamlines_generator)