Add support for vox2ras-tkr reading in .mgz files

[larsoner]

In order to use orig.mgz files to transform surface vertex locations to MNI coordinates, two pieces of information must be extracted from orig.mgz. The first is the vox2ras transformation, accessible via $ mri_info --vox2ras orig.mgz using Freesurfer or in nibabel:

import nibabel as nib
img = nib.load('orig.mgz')
img.get_affine()
# or
img.get_header().get_vox2ras()

However, the second necessary piece of information stored in orig.mgz is the vox2ras-tkr matrix, accessible via $ mri_info --vox2ras-tkr orig.mgz. There is currently no way (that I know of) to get this information in nibabel. It would be great if this info could be added so that Freesurfer command-line dependencies could be removed (e.g., mne-tools/mne-python#524).

For reference in the associated transform, see the MNI305RAS conversion:
http://surfer.nmr.mgh.harvard.edu/fswiki/CoordinateSystems

[agramfort]

if you have some matlab code that reads this you should be able to patch :

https://github.com/nipy/nibabel/blob/master/nibabel/freesurfer/mghformat.py

to add this feature.

[larsoner]

Ref for myself to look here later:
http://surfer.nmr.mgh.harvard.edu/fswiki/CoordinateSystems?action=AttachFile&do=get&target=fscoordinates.pdf