We use spatially-constrained Ward-clustering, KMeans, and Recursive Neighbor Agglomeration (ReNA) to create a set of parcels.
In a high dimensional regime, these methods can be interesting to create a 'compressed' representation of the data, replacing the data in the fMRI images by mean signals on the parcellation, which can subsequently be used for statistical analysis or machine learning.
Also, these methods can be used to learn functional connectomes and subsequently for classification tasks.
Which clustering method to use, an empirical comparison can be found in this paper
* Bertrand Thirion, Gael Varoquaux, Elvis Dohmatob, Jean-Baptiste Poline. `Which fMRI clustering gives good brain parcellations ? <https://doi.org/10.3389/fnins.2014.00167>`_ Frontiers in Neuroscience, 2014.
This parcellation may be useful in a supervised learning, see for instance
* Vincent Michel, Alexandre Gramfort, Gael Varoquaux, Evelyn Eger, Christine Keribin, Bertrand Thirion. `A supervised clustering approach for fMRI-based inference of brain states. <http://dx.doi.org/10.1016/j.patcog.2011.04.006>`_. Pattern Recognition, Elsevier, 2011.
The big picture discussion corresponding to this example can be found
in the documentation section
We download one subject of the movie watching dataset from Internet
from nilearn import datasets dataset = datasets.fetch_development_fmri(n_subjects=1) # print basic information on the dataset print('First subject functional nifti image (4D) is at: %s' % dataset.func) # 4D data
Transforming list of images to data matrix and build brain parcellations,
all can be done at once using
from nilearn.regions import Parcellations # Computing ward for the first time, will be long... This can be seen by # measuring using time import time start = time.time() # Agglomerative Clustering: ward # We build parameters of our own for this object. Parameters related to # masking, caching and defining number of clusters and specific parcellations # method. ward = Parcellations(method='ward', n_parcels=1000, standardize=False, smoothing_fwhm=2., memory='nilearn_cache', memory_level=1, verbose=1) # Call fit on functional dataset: single subject (less samples). ward.fit(dataset.func) print("Ward agglomeration 1000 clusters: %.2fs" % (time.time() - start)) # We compute now ward clustering with 2000 clusters and compare # time with 1000 clusters. To see the benefits of caching for second time. # We initialize class again with n_parcels=2000 this time. start = time.time() ward = Parcellations(method='ward', n_parcels=2000, standardize=False, smoothing_fwhm=2., memory='nilearn_cache', memory_level=1, verbose=1) ward.fit(dataset.func) print("Ward agglomeration 2000 clusters: %.2fs" % (time.time() - start))
First, we display the parcellations of the brain image stored in attribute
ward_labels_img = ward.labels_img_ # Now, ward_labels_img are Nifti1Image object, it can be saved to file # with the following code: ward_labels_img.to_filename('ward_parcellation.nii.gz') from nilearn import plotting from nilearn.image import mean_img, index_img first_plot = plotting.plot_roi(ward_labels_img, title="Ward parcellation", display_mode='xz') # Grab cut coordinates from this plot to use as a common for all plots cut_coords = first_plot.cut_coords
Second, we illustrate the effect that the clustering has on the signal. We show the original data, and the approximation provided by the clustering by averaging the signal on each parcel.
# Grab number of voxels from attribute mask image (mask_img_). import numpy as np from nilearn.image import get_data original_voxels = np.sum(get_data(ward.mask_img_)) # Compute mean over time on the functional image to use the mean # image for compressed representation comparisons mean_func_img = mean_img(dataset.func) # Compute common vmin and vmax vmin = np.min(get_data(mean_func_img)) vmax = np.max(get_data(mean_func_img)) plotting.plot_epi(mean_func_img, cut_coords=cut_coords, title='Original (%i voxels)' % original_voxels, vmax=vmax, vmin=vmin, display_mode='xz') # A reduced dataset can be created by taking the parcel-level average: # Note that Parcellation objects with any method have the opportunity to # use a `transform` call that modifies input features. Here it reduces their # dimension. Note that we `fit` before calling a `transform` so that average # signals can be created on the brain parcellations with fit call. fmri_reduced = ward.transform(dataset.func) # Display the corresponding data compressed using the parcellation using # parcels=2000. fmri_compressed = ward.inverse_transform(fmri_reduced) plotting.plot_epi(index_img(fmri_compressed, 0), cut_coords=cut_coords, title='Ward compressed representation (2000 parcels)', vmin=vmin, vmax=vmax, display_mode='xz') # As you can see below, this approximation is almost good, although there # are only 2000 parcels, instead of the original 60000 voxels
We use the same approach as with building parcellations using Ward clustering. But, in the range of a small number of clusters, it is most likely that we want to use standardization. Indeed with standardization and smoothing, the clusters will form as regions.
# class/functions can be used here as they are already imported above. # This object uses method='kmeans' for KMeans clustering with 10mm smoothing # and standardization ON start = time.time() kmeans = Parcellations(method='kmeans', n_parcels=50, standardize=True, smoothing_fwhm=10., memory='nilearn_cache', memory_level=1, verbose=1) # Call fit on functional dataset: single subject (less samples) kmeans.fit(dataset.func) print("KMeans clusters: %.2fs" % (time.time() - start))
Grab parcellations of brain image stored in attribute
kmeans_labels_img = kmeans.labels_img_ plotting.plot_roi(kmeans_labels_img, mean_func_img, title="KMeans parcellation", display_mode='xz') # kmeans_labels_img is a Nifti1Image object, it can be saved to file with # the following code: kmeans_labels_img.to_filename('kmeans_parcellation.nii.gz')
One interesting algorithmic property of ReNA (see References) is that it is very fast for a large number of parcels (notably faster than Ward). As before, the parcellation is done with a Parcellations object. The spatial constraints are implemented inside the Parcellations object.
More about ReNA clustering algorithm in the original paper
* A. Hoyos-Idrobo, G. Varoquaux, J. Kahn and B. Thirion, "Recursive Nearest Agglomeration (ReNA): Fast Clustering for Approximation of Structured Signals," in IEEE Transactions on Pattern Analysis and Machine Intelligence, vol. 41, no. 3, pp. 669-681, 1 March 2019. https://hal.archives-ouvertes.fr/hal-01366651/
start = time.time() rena = Parcellations(method='rena', n_parcels=5000, standardize=False, smoothing_fwhm=2., scaling=True) rena.fit_transform(dataset.func) print("ReNA 5000 clusters: %.2fs" % (time.time() - start))
First, we display the parcellations of the brain image stored in attribute
rena_labels_img = rena.labels_img_ # Now, rena_labels_img are Nifti1Image object, it can be saved to file # with the following code: rena_labels_img.to_filename('rena_parcellation.nii.gz') plotting.plot_roi(ward_labels_img, title="ReNA parcellation", display_mode='xz', cut_coords=cut_coords)
We illustrate the effect that the clustering has on the signal. We show the original data, and the approximation provided by the clustering by averaging the signal on each parcel.
We can then compare the results with the compressed representation obtained with Ward.
# Display the original data plotting.plot_epi(mean_func_img, cut_coords=cut_coords, title='Original (%i voxels)' % original_voxels, vmax=vmax, vmin=vmin, display_mode='xz') # A reduced data can be created by taking the parcel-level average: # Note that, as many scikit-learn objects, the ReNA object exposes # a transform method that modifies input features. Here it reduces their # dimension. # However, the data are in one single large 4D image, we need to use # index_img to do the split easily: fmri_reduced_rena = rena.transform(dataset.func) # Display the corresponding data compression using the parcellation compressed_img_rena = rena.inverse_transform(fmri_reduced_rena) plotting.plot_epi(index_img(compressed_img_rena, 0), cut_coords=cut_coords, title='ReNA compressed representation (5000 parcels)', vmin=vmin, vmax=vmax, display_mode='xz')
Even if the compressed signal is relatively close to the original signal, we can notice that Ward Clustering gives a slightly more accurate compressed representation. However, as said in the previous section, the computation time is reduced which could still make ReNA more relevant than Ward in some cases.