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Thank you Daniel!
I actually don't think the 3dUnifize being necessary as "feature normalization" and "batch normalization" are typically used in most machine learning algorithms.
I guess that partial coverage may not be a big issue either, because sparse features are often seen in literature. I may be able to give it a shot if there is a convenient way to extract local costs from a buch of ROIs (filling zero for those without coverage) in the standard space.