"What's stopping you?"

Typically the State medical device licensing boards will stop you. Try this: get 1000 families to chip in for an MRI and techs to run it for the benefit of the 1000 and their families. When you apply for State permit to turn it on, that board will look at competition in the area and not allow it. The rules don't allow too much competition to drive down prices, so only the mega-corps typically can play the game.