Are you assuming a brain-scanning technology wouldn't be opt-in? I figure the mechanical nature of it would make it difficult to implement in any other way... If someone doesn't want to be scanned, they can take the helmet off.
Maybe in court we can make it like discovery or cross-examination...
"Ok, I'll agree to using the mind-reader, but only if I can use the mind-reader on you as well..."
Daniel Suarez touched on a proto-society developing with fMRI mind-reading in his Daemon series of books. Higher levels of authority required more frequent rounds of time in the mind-reader taking loyalty/polygraph tests.