I was an MRI assistant this summer.
1) MRI's are always focused on one particular part of your body. Usually it's the heart, the brain, or the site of an injury.
2) Abcesses, scars might show. But doctors don't care because they doing an MRI to investigate an injury or confirm serious medical problems, not see what illegal shit you're doing.
3) They don't actually care what illegal shit you're doing. We had 1 heroin addict and 1 crack addict come in for MRI's. They admitted to this. Hospitals aren't looking to bust you for anything.
The worst that happens is we'll laugh at you behind closed doors. For instance one lady seemed to have abnormally large boobs for her size. The MRI was of shoulder/torso. Well guess what showed, HUGE implants. Her actual breasts would have been less than an A cup.