You have to know the coordinate system of the matrix. See, eg, here https://surfer.nmr.mgh.harvard.edu/fswiki/CoordinateSystems
Once you know the coordinate system, I would use lta_convert to generate an appropriate file, eg,
lta_convert --inlta identity.nofile --src preop.mgz   --trg postop.mgz --outlta your.lta
Then edit the file in two places
1. The first line be something like
    type      = 0 # LINEAR_VOX_TO_VOX
    This must correspond to the coordinate system. By far the easiest thing will be a vox2vox
2. Replace the 4x4 matrix with your matrix
Then run
tkregisterfv --mov prepop.mgz --targ postop.mgz --reg your.lta
to see if it worked.
You can try just doing #2 with your matrix. Maybe it works

On 11/13/2023 4:47 PM, Peled, Noam,PhD wrote:

Dear group,

I have 2 images (preop t1 and postop CT), and a 4x4 registration matrix (as a text file).

Is there an easy way to create an lta or xfm file to use it with mri_vol2vol?

Thanks!

Noam
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