The canvas portion of this window contains the arithmetic records, the record is composed of seven fields and unless otherwise noted, each field is represented by an entry widget. When deselected, all other fields will be shaded, hence selected operations will be easily recognized. The Oper field is represented by a menubutton and offers a choice of eleven functions. These functions are displayed in an orderly fashion, i.e., you can think of them as a command chain, where the input to the function is the output from the previous one. So rsmp takes as input the reconstructed volumes and blur takes as input the resampled volumes and so on.
Functions avg , avgmri and avgnn are on the same level, and so are stat and stats .
All of these functions are meant to work with PET data mostly, with the exception of avgmri , which is custom made for MRI data since it doesn't blur nor does it normalize the images. You could still use it on PET data. Remember that when we talk about normalization, we mean custom normalization for PET, not to say that MRI data or any other imaging modality shouldn't be normalized. This should change in the future and ad hoc normalization will be used depending on the imaging modality. Use it to average MRI data.