# Calculate Path Length Map

We show how to calculate a Path Length Map for Anisotropic Radiation Therapy Contours given a set of streamlines and a region of interest (ROI). The Path Length Map is a volume in which each voxel’s value is the shortest distance along a streamline to a given region of interest (ROI). This map can be used to anisotropically modify radiation therapy treatment contours based on a tractography model of the local white matter anatomy, as described in [Jordan_2018_plm], by executing this tutorial with the gross tumor volume (GTV) as the ROI.

NOTE: The background value is set to -1 by default

from dipy.core.gradients import gradient_table
from dipy.data import get_fnames, default_sphere
from dipy.reconst.shm import CsaOdfModel
from dipy.direction import peaks_from_model
from dipy.tracking.stopping_criterion import ThresholdStoppingCriterion
from dipy.tracking import utils
from dipy.tracking.local_tracking import LocalTracking
from dipy.tracking.streamline import Streamlines
from dipy.viz import actor, window, colormap as cmap
from dipy.tracking.utils import path_length
import numpy as np
import matplotlib as mpl
from mpl_toolkits.axes_grid1 import AxesGrid


First, we need to generate some streamlines and visualize. For a more complete description of these steps, please refer to the example_probabilistic_fiber_tracking and the Visualization of ROI Surface Rendered with Streamlines Tutorials.

hardi_fname, hardi_bval_fname, hardi_bvec_fname = get_fnames('stanford_hardi')
label_fname = get_fnames('stanford_labels')

data, affine, hardi_img = load_nifti(hardi_fname, return_img=True)

white_matter = (labels == 1) | (labels == 2)

csa_model = CsaOdfModel(gtab, sh_order=6)
csa_peaks = peaks_from_model(csa_model, data, default_sphere,
relative_peak_threshold=.8,
min_separation_angle=45,

stopping_criterion = ThresholdStoppingCriterion(csa_peaks.gfa, .25)


We will use an anatomically-based corpus callosum ROI as our seed mask to demonstrate the method. In practice, this corpus callosum mask (labels == 2) should be replaced with the desired ROI mask (e.g. gross tumor volume (GTV), lesion mask, or electrode mask).

# Make a corpus callosum seed mask for tracking

# Make a streamline bundle model of the corpus callosum ROI connectivity
streamlines = LocalTracking(csa_peaks, stopping_criterion, seeds, affine,
step_size=2)
streamlines = Streamlines(streamlines)

# Visualize the streamlines and the Path Length Map base ROI
# (in this case also the seed ROI)

streamlines_actor = actor.line(streamlines, cmap.line_colors(streamlines))
surface_opacity = 0.5
surface_color = [0, 1, 1]
surface_color, surface_opacity)

scene = window.Scene()


Out:

/Users/koudoro/miniconda3/envs/dipy-env-37/lib/python3.7/site-packages/vtkmodules/util/numpy_support.py:66: DeprecationWarning: Converting np.character to a dtype is deprecated. The current result is np.dtype(np.str_) which is not strictly correct. Note that np.character is generally deprecated and 'S1' should be used.
if numpy_array_type == key or \
/Users/koudoro/miniconda3/envs/dipy-env-37/lib/python3.7/site-packages/vtkmodules/util/numpy_support.py:68: DeprecationWarning: Converting np.character to a dtype is deprecated. The current result is np.dtype(np.str_) which is not strictly correct. Note that np.character is generally deprecated and 'S1' should be used.
numpy_array_type == numpy.dtype(key):
/Users/koudoro/miniconda3/envs/dipy-env-37/lib/python3.7/site-packages/fury/utils.py:97: VisibleDeprecationWarning: Creating an ndarray from ragged nested sequences (which is a list-or-tuple of lists-or-tuples-or ndarrays with different lengths or shapes) is deprecated. If you meant to do this, you must specify 'dtype=object' when creating the ndarray
data = np.array(data)


If you set interactive to True (below), the scene will pop up in an interactive window.

interactive = False
if interactive:
window.show(scene)

window.record(scene, n_frames=1, out_path='plm_roi_sls.png',
size=(800, 800))

#    **A top view of corpus callosum streamlines with the blue transparent ROI
#    in the center**.
#
#
# Now we calculate the Path Length Map using the corpus callosum streamline
# bundle and corpus callosum ROI.
#
# NOTE: the mask used to seed the tracking does not have to be the Path
# Length Map base ROI, as we do here, but it often makes sense for them to be
# the same ROI if we want a map of the whole brain's distance back to our ROI.
# (e.g. we could test a hypothesis about the motor system by making a
# streamline bundle model of the cortico-spinal track (CST) and input a lesion
# mask as our Path Length Map base ROI to restrict the analysis to the CST)

# calculate the WMPL
wmpl = path_length(streamlines, affine, path_length_map_base_roi)

# save the WMPL as a nifti
save_nifti('example_cc_path_length_map.nii.gz', wmpl.astype(np.float32),
affine)

# get the T1 to show anatomical context of the WMPL
t1_fname = get_fnames('stanford_t1')

fig = mpl.pyplot.figure()
ax = AxesGrid(fig, 111,
nrows_ncols=(1, 3),
cbar_location="right",
cbar_mode="single",
cbar_size="10%",


We will mask our WMPL to ignore values less than zero because negative numbers indicate no path back to the ROI was found in the provided streamlines

wmpl_show = np.ma.masked_where(wmpl < 0, wmpl)

slx, sly, slz = [60, 50, 35]
ax[0].matshow(np.rot90(t1_data[:, slx, :]), cmap=mpl.cm.bone)
im = ax[0].matshow(np.rot90(wmpl_show[:, slx, :]),
cmap=mpl.cm.cool, vmin=0, vmax=80)

ax[1].matshow(np.rot90(t1_data[:, sly, :]), cmap=mpl.cm.bone)
im = ax[1].matshow(np.rot90(wmpl_show[:, sly, :]), cmap=mpl.cm.cool,
vmin=0, vmax=80)

ax[2].matshow(np.rot90(t1_data[:, slz, :]), cmap=mpl.cm.bone)
im = ax[2].matshow(np.rot90(wmpl_show[:, slz, :]),
cmap=mpl.cm.cool, vmin=0, vmax=80)

ax.cbar_axes[0].colorbar(im)
for lax in ax:
lax.set_xticks([])
lax.set_yticks([])
fig.savefig("Path_Length_Map.png")


Out:

/Users/koudoro/Software/dipy/doc/examples/streamlines_analysis/path_length_map.py:161: MatplotlibDeprecationWarning: Since 3.2, mpl_toolkits's own colorbar implementation is deprecated; it will be removed two minor releases later.  Set the 'mpl_toolkits.legacy_colorbar' rcParam to False to use Matplotlib's default colorbar implementation and suppress this deprecation warning.
ax.cbar_axes[0].colorbar(im)
/Users/koudoro/miniconda3/envs/dipy-env-37/lib/python3.7/site-packages/mpl_toolkits/axes_grid1/axes_grid.py:46: MatplotlibDeprecationWarning:
The mpl_toolkits.axes_grid1.colorbar module was deprecated in Matplotlib 3.2 and will be removed two minor releases later. Use matplotlib.colorbar instead.
from .colorbar import Colorbar


Path Length Map showing the shortest distance, along a streamline, from the corpus callosum ROI with the background set to -1.

## References

Jordan_2018_plm

Jordan K. et al., “An Open-Source Tool for Anisotropic

Radiation Therapy Planning in Neuro-oncology Using DW-MRI Tractography”, PREPRINT (biorxiv), 2018.

Total running time of the script: ( 0 minutes 18.905 seconds)

Gallery generated by Sphinx-Gallery