The vision that taxpayers support a system where everyone is granted access to next-to-free / affordable health care has a natural cost in certain areas.
Sure it's great when you're both poor and sick cause you get to be treated like everyone else, you get the day care for your kids, you get the MRI's, the cancer treatment, you're being taken care of at the ER etc.
But at the same time, everyone has to realize that this has a cost in terms of taxes. Taxes for companies, home buyers, CEOs, businesses, cigarettes and alcohol, gas, etc.
It's the old chasm between putting the emphasis on the individual or on the nation/country as a whole. America has traditionally favoured private/capital-oriented solutions so far for most of its services (postal services, health care, public transportation, part of its schools, water, electricity, Internet infrastructure, etc.) and was thus able to keep the burden of taxes lower for individuals.
If it were to move to a system where everyone pitches in for cheap state-driven services, the money would have to come from somewhere and it would be sure to somehow stifle economic growth in certain areas (see the economic situation of France or other countries where the involvement of the state is still strong).
TL;DR: The idea of cheap MRI is nice on paper (and I believe it to be awesome in practice), but there's no magic wand to make that happen, the money has to come from somewhere and it requires that a whole country accepts paying more to the state to get more at the other end, a paradigm shift that will be tough to swallow for the USA given its economic culture.