I don't know much about MRI technology, but would you get an improvement in resolution if the whole machine was scaled down (i.e. because it only needs to fit someone's head in it, rather than their whole body)?
There's already head-only MRI machines. Like all MRI machines, they are phenomenally expensive to operate, eat shittons of energy, and only make sense if they're in constant use. But yes, reducing bore size improves resolution.