Hi, why is it that when I try to synchronize two MPR windows containing different series from the same study it works perfectly, whereas when I try to synchronize two MPR windows with series belonging to different studies (for example, a follow-up exam and its previous one), the synchronization is impossible or ineffective?

Because with different exams Radiant cannot know which pixels are in the same position.
If you make one exam with different sequences the localisation information gets stored into the dicom file.
If you only change the coil, move the coil or make a control exam, the localisation information for the sequences is not the same anymore - thus Radiant cannot know where the same point in the images is.
This could be overcome by Image Fusion of the different exams (basically by finding anatomically identical structures the program can generate a new localisation system for everything), but that requires more sophisticated calculations which for now Radiant does not provide.