The nanobots we have need a light as power source and only work in a strong magnetic field. https://www.youtube.com/watch?v=MQ9aXoah5DA
Just try to avoid having a endoscopic lamp inserted into you while you sit inside a MRT and you should be good, no fear needed.
This is, more or less, a Idea in the long runnig german pulp scifi Perry Rhodan https://www-perrypedia-de.translate.goog/wiki/Viren?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=de