The chorus (which you'll now be singing all day, sorry!) :
There's 15N, 39K, 19F, and also 13C,
But of all the atoms East to West, the nucleus I like best is 31P.
Proton spins are everywhere, and MRIs reveal their density.
But all that leaves me unimpressed; the nucleus I like best is 31P.
31P (Phosphorus-31 NMR) is used in nuclear magnetic resonance imaging to study biological membranes in native conditions. It also -- as the song says -- let's you "find the rates of using and creating ATP."