I want to have an upcoming pet scan compared to the one 3 months earlier. do i request a comparative report or a comparative study? what exactly?

I don't: know any facility that would perform a PET scan without a physician's order, so you probably don't need to worry about how to word the request. If you already have the order and want to make sure the facility has the proper information, when you call to schedule the test, tell them that you had a prior PET scan(when and where) and that this is a follow up study.