T2
From ISMRMWiki
T2 decay occurs due to interactions with the surrounding molecular environment . T2 is the time constant of the transverse magnetization signal relaxation has decayed due to loss of phase coherence. T2 decay is very important for the Bloch equations. On really small scales, there will be local magnetic field due to surrounding atoms that is different from the macroscopic magnetic field. This local environment is the cause of T2 decay (i.e. so called spin-spin relaxation).
