I can't go too much into details, but suffice it to say that we're not using super conductors at all.

High T super conductors will be very interesting in the next decade, I think, but right now there are a few issues that I'm aware of:

1) The high T super conductors are almost all crystals, not ductile metals. This means it can be very difficult to form coils or other structures out of them.

2) High T super conductors generally can't withstand as high a magnetic field as the lower T super conductors. I'm not sure how practical a concern this is for MRI.

The benefit of High T super conductors is that you switch from using liquid helium as your cryogen to liquid nitrogen. This is a major simplification, but we're not all the way to room temp yet, unfortunately.