wow, you'd think people in the post-graduate radiology forum would be able to answer this question more than a bunch of pre-meds stressing over getting into med school. I'm thinking experience would matter in the OP's reason for asking for career advice.
sorry i just think that if you don't have anything constructive to say...don't speak for other people
Knowing beforehand will probably not give you an advantage in getting into medical school. However, knowing or leaning towards radiology in 1st and 2nd years will be beneficial since you can meet the radiology staff, do research, focus more on anatomy, etc. in these years and this will help in matching into a good radiology program.
Most med schools introduce students to CT/X-ray/some MRI anatomy early during anatomy, pathology, etc in the pre-clinical years. Then, in the clinical years you learn radiology by examining studies, going down to radiology to discuss results, etc on pretty much every rotation (except psych). Most schools will let you do a 2-4 week rotation in radiology as either an elective or required clerkship during 3rd year. Then you can do another one early in 4th year. It is actually rare for people to decide on radiology prior to applying to med school. Most decide in 3rd year or early 4th year.