Dear Asieh,
In my experience you get better rigid-body registrations when relying on templates with a better GM-WM contrast, e.g. non-linear versions of the ICBM templates. See http://www.bic.mni.mcgill.ca/ServicesAtlases/ICBM152NLin2009 for those from 2009, possibly also use a skull-stripped version of that (for that purpose, mask mni_icbm152_t1_tal_nlin_asym_09c.nii with mni_icbm152_t1_tal_nlin_asym_09c_mask.nii via Imcalc).
In case origin/orientation of the your original data are completely off (e.g. origin very far away from the anterior commissure, for example in one of the corners, image rotated by 180° relative to the templates), which might be the case for files from data repositories due to them having certain conventions (no idea about ADNI), it might be necessary in an initial step to reorient the files manually so that they very roughly match the templates. As long as the displacement is consistent across images it should be sufficient to go with the same parameters for all the files though, thus load one image, reset orientations, and then select all the other files as well.
Note that "Coreg" really only does a rigid-body registration, for non-rigid ones you will have to use the "Normalise" or "Segment" modules.
Best
Helmut
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