Hi Dylan

Sorry, did anyone ever answer this? The .lta files contain the transform but also information on the coordinate systems that they are in. I think that the standard talairach one is voxel to voxel, so it would indeed change with the image fov. There are tools for converting ltas to ras-to-ras, which I think is what you want (although you will need to make sure you get the correct ras space), but perhaps someone else can comment on this since I don't remember the details?

Cheers Bruce

-----Original Message----- From: freesurfer-bounces@nmr.mgh.harvard.edu freesurfer-bounces@nmr.mgh.harvard.edu On Behalf Of Tisdall, Dylan Sent: Thursday, December 24, 2020 11:41 AM To: freesurfer@nmr.mgh.harvard.edu Subject: [Freesurfer] talairach.lta coordinate spaces

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Hi list,

I'm trying to understand whether the talairach.lta transform is from the image-axes RAS to talairach or the scanner-axes RAS to talairach. Basically, if I rotate the imaging FOV and scan the same subject, should I expect that to change talairach.lta or not? My confusion stems from the discrepancy between what the scanner terms RAS coordinates (aligned to the gradient axes) vs. what might be plausibly called RAS in the image coordinates (aligned to the FOV axes).

The underlying goal here is to try to evaluate how gradient-axis-aligned someone's head was when they were loaded in the scanner.

Thanks, Dylan

--- Dylan Tisdall, PhD Research Assistant Professor Department of Radiology Perelman School of Medicine University of Pennsylvania

Room D406 Richards Medical Labs 3700 Hamilton Walk Philadelphia, PA 19104

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