An MRI will be never the sensitive, the technology does not have that ability. i.e. the noise is far larger than the signal, so no improvement in technology will overcome that. (It has to read a signal from a single neuron amongst trillions of them.)

It would, at a minimum, require brain surgery and probes. But that's unlikely to be possible without damaging the person.

You have nothing to worry about - ever.

Maybe if they froze, and then took apart a person, atom by atom, and uploaded everything to a simulator.