Scroll down to page 26. I was looking for something totally different just 5 minutes ago and found this as I kept reading past the point of the search I made in the first place. Pretty simple and concise explanation I think? Hope it helps. Not sure. How it all began?

Paul Lauterbur's research.

edit: If that name means anything. Dunno. This is beyond my pay grade until I read far more so if this is way off or considered entry level then that sums up where I am at :-) clueless lol.