The conversions between  tkrRas and scannerRAS are as following in Freesurfer:

tkrRAS = tkrvox2ras * inv(vox2ras) * scannerRAS

scannerRAS = vox2ras * inv(tkrvox2ras) * tkrRAS

Best,

Yujing

From: freesurfer-bounces@nmr.mgh.harvard.edu <freesurfer-bounces@nmr.mgh.harvard.edu> On Behalf Of Eric Tsang
Sent: Thursday, December 14, 2023 5:51 PM
To: freesurfer@nmr.mgh.harvard.edu
Subject: [Freesurfer] Convert Scanner RAS to Vertex RAS

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Dear All,

    I have great deal of trouble converting scanner RAS to vertex RAS. I have tried a couple of formula (in the following) which had failed to produce accurate coordinate.

[tkR tkA tkrS] = [ScannerRAS 1] / (vox2ras * inv(tkrvox2ras))

[tkR tkA tkrS] = [ScannerRAS 1] * inv(vox2ras0)

Both of these formula produced distorted coordinates. Please help!

Thanks,

Tim