Brain Reading: fMRI
fMRI or functional magnetic resonance imaging, is one of the newest brain imaging technologies for the first decade of the 21st century.
fMRI works based on detection of the dynamic regulation of blood flow in the brain. Medically, this is termed the haemodynamic response, however it is simply tracking brain activity based on increasing and decreasing demand for oxygen and glucose in the haemoglobin of the blood in the brain.
Functional magnetic resonance imaging thusly does not monitor the electrical signals in the brain at all. It cannot tell what signal the neurons are sending, but what it can do is detect exactly which neurons are active, how active they are, and the duration of the activity. This allows for detection of general concepts, and the use of specific areas of the brain known to be dedicated for certain purposes.
Because fMRI does not track actual electrical or chemical interactions between neurons, it will never be able to tell exactly what you are thinking. However, it is reasonably good at detecting general concepts, provided the function of the area of the brain where activity is high, is understood.
Additionally fMRI suffers from imprecision based on differing oxygen requirements of different parts of the brain, and can be thrown off entirely by an imbalanced blood sugar level.
Functional magnetic resonance imaging would never be a brain-machine-interface method for interaction; it is purely detection based. The dire fidelity that results from simply tracking oxygenation and energy provision levels makes it all but useless for detecting higher brain functions.
However, the technique excels at defining the edges of specialist brain sections, and allowing the tracking and mapping of common data pathways as they flow through the brain. This directly permits the later use of other brain-scanning and two-way signalling technologies utilising the brain areas that fMRI identified.