Swollen foreskin?

Sounds like a mild infection, possibly a yeast infection. The best thing you can do is keep it very clean, washing well, and using a mild soap, but only mild to prevent further skin irritation or drying, as soap does that. The contrast agent wouldn't cause this, as more skin that that would be swollen, the foreskin is the same skin as the rest of your body after all. Best I can suggest is to wash well, and wash your hands before touching it. Best of luck, and I do hope your MRI results turn out well, that always can be a daunting thing.