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%matplotlib inline

Morph volumetric source estimate

This example demonstrates how to morph an individual subject's :class:mne.VolSourceEstimate to a common reference space. We achieve this using :class:mne.SourceMorph. Pre-computed data will be morphed based on an affine transformation and a nonlinear registration method known as Symmetric Diffeomorphic Registration (SDR) by Avants et al. [1]_.

Transformation is estimated from the subject's anatomical T1 weighted MRI (brain) to FreeSurfer's 'fsaverage' T1 weighted MRI (brain) <https://surfer.nmr.mgh.harvard.edu/fswiki/FsAverage>__.

Afterwards the transformation will be applied to the volumetric source estimate. The result will be plotted, showing the fsaverage T1 weighted anatomical MRI, overlaid with the morphed volumetric source estimate.

References

.. [1] Avants, B. B., Epstein, C. L., Grossman, M., & Gee, J. C. (2009). Symmetric Diffeomorphic Image Registration with Cross- Correlation: Evaluating Automated Labeling of Elderly and Neurodegenerative Brain, 12(1), 26-41.



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# Author: Tommy Clausner <tommy.clausner@gmail.com>
#
# License: BSD (3-clause)
import os

import nibabel as nib
import mne
from mne.datasets import sample
from mne.minimum_norm import apply_inverse, read_inverse_operator
from nilearn.plotting import plot_glass_brain

print(__doc__)

Setup paths



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sample_dir_raw = sample.data_path()
sample_dir = os.path.join(sample_dir_raw, 'MEG', 'sample')
subjects_dir = os.path.join(sample_dir_raw, 'subjects')

fname_evoked = os.path.join(sample_dir, 'sample_audvis-ave.fif')
fname_inv = os.path.join(sample_dir, 'sample_audvis-meg-vol-7-meg-inv.fif')

fname_t1_fsaverage = os.path.join(subjects_dir, 'fsaverage', 'mri',
                                  'brain.mgz')

Compute example data. For reference see sphx_glr_auto_examples_inverse_plot_compute_mne_inverse_volume.py

Load data:



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evoked = mne.read_evokeds(fname_evoked, condition=0, baseline=(None, 0))
inverse_operator = read_inverse_operator(fname_inv)

# Apply inverse operator
stc = apply_inverse(evoked, inverse_operator, 1.0 / 3.0 ** 2, "dSPM")

# To save time
stc.crop(0.09, 0.09)

Get a SourceMorph object for VolSourceEstimate

subject_from can typically be inferred from :class:src <mne.SourceSpaces>, and subject_to is set to 'fsaverage' by default. subjects_dir can be None when set in the environment. In that case SourceMorph can be initialized taking src as only argument. See :class:mne.SourceMorph for more details.

The default parameter setting for spacing will cause the reference volumes to be resliced before computing the transform. A value of '5' would cause the function to reslice to an isotropic voxel size of 5 mm. The higher this value the less accurate but faster the computation will be.

A standard usage for volumetric data reads:



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morph = mne.compute_source_morph(inverse_operator['src'],
                                 subject_from='sample', subject_to='fsaverage',
                                 subjects_dir=subjects_dir)

Apply morph to VolSourceEstimate

The morph can be applied to the source estimate data, by giving it as the first argument to the :meth:morph.apply() <mne.SourceMorph.apply> method:



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stc_fsaverage = morph.apply(stc)

Convert morphed VolSourceEstimate into NIfTI

We can convert our morphed source estimate into a NIfTI volume using :meth:morph.apply(..., output='nifti1') <mne.SourceMorph.apply>.



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# Create mri-resolution volume of results
img_fsaverage = morph.apply(stc, mri_resolution=2, output='nifti1')

Plot results



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# Load fsaverage anatomical image
t1_fsaverage = nib.load(fname_t1_fsaverage)

# Plot glass brain (change to plot_anat to display an overlaid anatomical T1)
display = plot_glass_brain(t1_fsaverage,
                           title='subject results to fsaverage',
                           draw_cross=False,
                           annotate=True)

# Add functional data as overlay
display.add_overlay(img_fsaverage, alpha=0.75)

Reading and writing SourceMorph from and to disk

An instance of SourceMorph can be saved, by calling :meth:morph.save <mne.SourceMorph.save>.

This methods allows for specification of a filename under which the morph will be save in ".h5" format. If no file extension is provided, "-morph.h5" will be appended to the respective defined filename::

>>> morph.save('my-file-name')

Reading a saved source morph can be achieved by using :func:mne.read_source_morph::

>>> morph = mne.read_source_morph('my-file-name-morph.h5')

Once the environment is set up correctly, no information such as subject_from or subjects_dir must be provided, since it can be inferred from the data and used morph to 'fsaverage' by default. SourceMorph can further be used without creating an instance and assigning it to a variable. Instead :func:mne.compute_source_morph and :meth:mne.SourceMorph.apply can be easily chained into a handy one-liner. Taking this together the shortest possible way to morph data directly would be:



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stc_fsaverage_new = mne.compute_source_morph(
    inverse_operator['src'], subject_from='sample',
    subjects_dir=subjects_dir).apply(stc)