During ADNI1, GO and 2 the Mayo ADIRL provided “preprocessed” MRI scans for the MPRAGE/IRFSPGR (3D T1) series for use by the processing sites and general research community. This preprocessing included gradient warp and multichannel head coil image inhomogeneity effects that were not corrected online by some (most) scanner models at that time.  Today however, 3D  gradient warping corrections are applied by all modern MRI scanners.  The vendors now also have built-in corrections for intensity inhomogeneity  (“Prescan Normalize”, “CLEAR” and “PURE”).  Now that all modern scanners include these corrections, there is no need to provide “preprocessed” images for ADNI3.

To view more details about the preprocessing step taken:

http://adni.loni.usc.edu/methods/mri-analysis/mri-pre-processing/