20-Year Prospect
The medical profession isn't something you need to worry about, career-longevity-wise. As a whole, physicians are expected to grow in demand by fourteen percent over the coming decade, and radiologists will see an even higher increase in jobs (source).
Radiologists are on the good side of new medical technologies: they're trying to figure out what's wrong and do it in less invasive ways than had previously been possible. It's safe to say there will still be a high demand for that down the line.
Speaking of advances, there will be new technologies developing all the time, and it'll be important to keep a good handle on the changing equipment. Of course, that won't be a problem for a medical whiz like yourself, but you ought to understand beforehand that the MRI machines you see in your textbooks might be outdated by the time you get around to using them in a real-world setting.
But again, just because the equipment changes, it doesn't mean the jobs are going anywhere. Just keep your nose in those books and get ready to pull in your astronomically large paychecks.