Why can’t liquid nitrogen SC used for creating MRI?

Superconductors also have points where they stop superconducting because of the strength of the magnetic field or amount of current. YBCO and similar have these points below the useful level for MRIs.

There's research around finding ways to use them but nothing that is currently viable.

Wait, Commonwealth Fusion Systems is already building 20T REBCO magnets, can't those meet the requirements for MRIs?

You're right - for whatever reason I had convinced myself YBCO had issues with both high current and high field. It's just high current - anything outside of a single grain has low current density.

Doing some more research to help make up for my spreading of incorrect info, I did stumble across this - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5472374/ - it seems like it's a variety of factors that make most HST not really commercially viable for MRI machines at current.

Mitsubishi has even made a small scale MRI machine with HTS - https://www.medicaldesignandoutsourcing.com/worlds-first-3-t... - though I can't find any newer details around it, despite the timeline provided in the article.

Philips has this press release about researching high temperature MRI from 2022:

https://www.usa.philips.com/a-w/about/news/archive/standard/...

I guess the need is somewhat alleviated by using MRI which seal their helium:

https://www.usa.philips.com/healthcare/resources/landing/the...

It's quite likely that if you cool this thing at liquid nitrogen temperature, its performance will improve a lot.

Not the revolution people are claiming, but it's a huge gain nonetheless.