I don't know that MRI would have that effect, like in the machine. I've heard conflicting things about MRI machines and I've never really looked into the design closely myself. Also whether or not a magnetic field has any effect on movement is pretty dependent on what the nanobot is made of, assuming we're not talking about a magnetic field of ionizing strength, and I recognize the amount of energy required to ionize a given substance or covalent bond of a substance is variable depending on the wavelength and other matters like harmonics that might occur not just at the bond but also in the molecule as a whole and reactants. When talking about nanobots we generally are discussing non-metallic or mostly non-metallic molecules aren't we? There's also something that I guess you'd call virtual nanobots that are assembled out of molecules in a substrate and vanish with the forces that created them. Why jump that far ahead though?