Re: [Freesurfer] MNI to RAS surface coordinates

Here are instructions to go from a surface RAS to the mni305 RAS, so just invert them. doug

I have an RAS point on the surface (tkrR tkrA tkrS) ("Vertex RAS" from tksurfer) and want to compute the MNI305 RAS that corresponds to this point:

MNI305RAS = TalXFM*Norig*inv(Torig)*[tkrR tkrA tkrS 1]' TalXFM: subject/orig/transforms/talairach.xfm Norig: mri_info --vox2ras orig.mgz Torig: mri_info --vox2ras-tkr orig.mgz

Test: click on a point in tksurfer. Use "Vertex RAS" to compute MNI305RAS. Compare MNI305RAS to "Vertex MNI Talairach". Also, hit Save Point in tksurfer. In tkmedit, Goto Saved Point, compare MNI305RAS to "MNI Coordinates". Note: you may need to use the "Volume RAS" from tkmedit in the computation above to get an exact match.

On 06/06/2012 09:43 AM, Yolanda Vives wrote:

Dear Freesurfer experts,

How can I transform from MNI coordinates to RAS surface coordinates?

Regards, Yolanda

2012/6/5 Yolanda Vives <yvives@pic.es mailto:yvives@pic.es>

Dear Freesurfer experts,

I have performed a VBM analysis with SPM, using some VOIs. Now I
would like to compare the volumetric results obtained with SPM
with those of cortical thickness obtained with Freesurfer in the
same VOIs (approximately).

For this purpose, I would like to transform the MNI coordinates of
my SPM VOIs into RAS surface coordinates of fsaverage, to draw a
label of more or less the same size of the SPM VOI and to propage
it to all the subjects in order to obtain an average cortical
thickness of this label (VOI) for each subject.

1) Would this procedure be correct?

2) How could I transform my MNI coordinates into RAS surface
coordinates?

Thank you in advance,
Yolanda

-- Yolanda Vives PIC (Port d'Informació Científica) Campus UAB, Edificio D E-08193 Bellaterra, Barcelona Telf. +34 93 586 8232 http://www.pic.es Avis - Aviso - Legal Notice: http://www.ifae.es/legal.html

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