fMRI is "functional MRI", which is a way of scanning not only the physical structure of an organ, but its function. Scientists are actually able to see which parts of the brain respond to various stimuli. So, for instance, if you show a normal person a sad scene from a movie, you can see where the person's brain processes the emotion.
A psychopath either would not feel the sadness, and their brain would have no visible activity, or they would get pleasure out of witnessing the sadness, and a different part of the brain would be active.
Do they have to inject anything to get an f MRI? I had a couple of MRIs for a brain bleed and they put in some kind of enhancement.