In the Scientific American there is an old demo of NMR, with a coil, a magnetron magnet, a tube oscillator/amplifier and an oscilloscope. It uses US 60Hz as the sweep. Ordinary water is too weak for it to work, so they dissolve a paramagnetic salt, either copper or iron, I forget. This decreases the relaxation time of the proton signal. The same energy being absorbed or emitted in a shorter time is a stronger signal. This also makes the line width of the signal wider and this demo really is only the demo of a signal, it's otherwise useless.
This tells the specifics of Gadolinium but it's really the same. Paramagnetic salts (unpaired electrons) cause shorter brighter signals, and T1 weighted images are short delay echo sequences so the areas the agent is turn out bright. With long delay echo sequences like T2 weighted images, the energy from that area is gone so you would expect darker than normal areas.
https://radiopaedia.org/articles/gadoli ... ts?lang=gb
