[Freesurfer] mri_segment

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Dear Freesurfer experts, I would like to inquire about the flags "-whi" and "-ghi" in the command "mri_segement". According to FS wiki, the command "mri_normalize" sets white matter voxels around ~110 and gray matter around ~70. - The default white matter high limit in the command mri_segemnt is 125, what is the highest value to consider white matter voxels in an image? Is it 125? - What are the situations where we need to set the gray and white matter voxels at a high limit of 100 and 125 respectively in the command mri_segment? Does the brightness of the image or motion play a role in the decision of choosing high limits values?

Thank you, James