Microscopically, T2 relaxation or spin-spin relaxation occurs when spins in the high and low energy state exchange energy but do not loose energy to the surrounding lattice. This results macroscopically in loss of the transverse magnetization. In pure water, The T2 and T1 times are approximately the same, 2-3 seconds. In biological materials, the T2 time is considerably shorter than the T1 time. For CSF, T1=1.9 seconds and T2=0.25 seconds. For brain white matter, T1=0.5 seconds and T2=0.07 seconds (70 msec). T2 relaxation occurs exponentially like T1 relaxation with 63% of the transverse magnetization gone after one T2 period as shown in the graph.