There is no ranking. There are a handful of departments (say, eight?) which are generally considered to be the "top"; a quick search of threads here and on AuntMinnie will give you a sense of which departments are the consensus for that group.
There's another large group of university departments (like, 20?) which are considered excellent, but which most would not class with that very top group.
Obviously, there's some correlation with the reputation of the associated medical school, but with some quirks. Some "top" med schools have "merely excellent" radiology departments, while some "merely excellent" med schools do have "top" rads departments.
These groupings describe the reputation of the different departments, which is related to (but not the same as) the quality of training you will receive in the program there (which depends on many other factors). This "presitge" factor is much more related to opportunities to do cutting-edge research and to make contacts in academic radiology. If you're not interested in research and academics, there isn't much useful information in the groupings for you. (Whether there's information for any residency applicant is also open to debate.)