[Freesurfer] How to get the difference between orig.mgz and original image

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Hi experts, I selected hippocampus and amygdala as ROIs to estimated the reliability of a well-controlled multi-site study. But, amygdala volume (generated by FreeSurfer) presented a relatively poor ICC scores between sites. The major sources of between-site variation might be related to field inhomogeneity and other issues idiosyncratic to individual each scanners. Due to this reason, i want to check my data to confirm the field inhomogeneity located in amydala area or not. How can i get the difference between the original image and the images after motion and NU intensity? I am looking forward to your suggests.

Cheers, Jiachen