An enema likely won't be required nor will be helpful, they are going to inject a dye into you so they can get prostate pictures with and without contrast so they can see if your prostate cancer has gone beyond the prostate bed.

MRI's a a cakewalk, don't worry about that. You lie down for 30 minutes while a machine runs over you.

3 sets of kegels every day is FANTASTIC! I'm glad to hear you started them early! Keep them up, but make sure you learn to force their release because that's more important. To make sure (and to learn the sensation of the release) take a deep slow big belly breath and that will force them to relax.