REAL mind reading machines and their techniques

Lately, a large amount of content coming our way all share a related topic: mind reading.  There are titles like  "Brain Scanners Can See Your Decisions Before You Make Them" and "Brain scanning may be used in security checks".  The brain scanners that are currently used by scientists are not the kind of mind reading machines we see in science fiction movies, but they are still quite amazing.  With all the research and information scientists now have on the human brain, they are able to translate data from brain scans to be able to know exactly what you saw or what you were thinking.  In the future, could robots use this research and develop new technology to help them in their destruction of mankind?

"Scientists are one step closer to knowing what you’ve seen by reading your mind"

So how exactly can a scientist read your mind?  It's all done through careful analysis of the brain's neural activities and patterns.  To do this, a functional magnetic resonance imager, or fMRI, is used.   So, how does a fMRI work?