Just a note, as there are several versions of that script, e.g. this one http://www.nemotos.net/?p=17 or this one https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=SPM;d1f675f1.0810 , they do NOT center on the AC. It's a coregistration onto the avg152.nii included in SPM, which is in MNI space and thus has a slightly shifted origin relative to the AC.
When manually reorienting files we nonetheless often rely on the AC as it's an easily identifiable anatomical structure. This also means a bias relative to the MNI templates (which one could additionally account for), but the bias is very similar for different brains, allowing for a similarly good/bad starting point, and the displacement is small relative to those one might observe without any correction.
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