Your interpretation is not correct. RAS always means that R is pos x, A is pos y, and S is pos z.
That is the voxel-to-RAS transform. Here voxel means column-row-slice. So this matrix takes a col, row, slice and converts it into a RAS. So that first negative 1 means that as the column number increases the R decreases (ie, becomes more leftward)
Look at our wiki http://surfer.nmr.mgh.harvard.edu/fswiki/CoordinateSystems for more info
doug
On 06/23/2014 01:44 PM, Arman Eshaghi wrote:
Hi,
I'm trying to understand the conversion from RAS (freesurfer surface coordinate) to volume space (x, y, and z.. According to Graham Wideman documentation here http://www.grahamwideman.com/gw/brain/fs/coords/fscoords.htm (if it is not outdated), Right is negative x, *Anterior is positive z*, and *Superior is positive y*.
Just to double check, usign mri_info on a sample T1.mgz in one subject I get this ras to voxel transform:
-1 0 0 126.944 0 0 -1 135.16 0 1 0 110.6 0 0 0 1
Assuming we have correctly set the (0, 0, 0), and we can extract R A and S (respectively) coordinates using C++ or Python code it means: Right is negative x, *Anterior is negative z*, and *Superior is positive y* (conflicting coordinates are in vold). Am I missing anything here, or coordinate systems have changed from the probably outdated documentation? Thanks in advance for any input on this rather confusing point.
All the best, Arman
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