MRI does have some caveats - it's not available to people with ferromagnetic material embedded within them, and not suitable for those who are claustrophobic. Also CT scans tend to be more accurate for bone, and are quicker to perform so less susceptible to the subject moving about.
There's also the issue that, at present, MRI scans are considerably more expensive than CT.
And a MRI needs an enclosure, I am not aware of small portable MRI systems that would allow you to image a single leg, or breast. X-Rays are more dangerous but are more versatile in some cases.
COTS for maybe a decade now. Look at sports medicine equipment suppliers. I'm unaware of any breast oriented products but they probably exist.
Google for Esaote (my coworker got scanned a couple times by their product) and "extremity MRI"
As you'd imagine the economics are non-trivial. The machine is smaller and cheaper that a full size MRI (google claims its fraction of a million, like the cost of a house) but obviously something that can only scan knees and ankles and elbows is a pretty tough sell unless you're a sports medicine practice in which case its a pretty easy sell...
On the other hand if a "whole body sized" MRI costs $2500 per scan, then you've only got to scan maybe 100 knees to pay the capital cost of a dedicated extremity MRI. And how long would that take at a busy sports medicine practice, like a week?
I would not describe the machine as portable. Being a giant magnet you probably need a special safe room to operate it, so I don't envision paramedic/EMT ever having a MRI in the ambulance or whatever.
WRT discussion in the comments about claustrophobia I don't mind the small spaces so much as being chained down for an hour or so, that drives me nuts. My MiL went thru some tests for her gallbladder that took hours, I'd be clawing at the walls after ten minutes. Also if I could get nitrous for my wisdom teeth I see no obvious reason a MRI patient couldn't be gassed but they "never" seem to do that, which I find weird. For $2500 for a full body scan you could at least give me a couple shots of vodka.
There's also the issue that, at present, MRI scans are considerably more expensive than CT.