Return to equilibrium of net magnetization is called Relaxation. During relaxation, electromagnetic energy is retransmitted: this RF emission is called the NMR signal. Relaxation combines 2 different mechanisms :
Longitudinal relaxation is due to energy exchange between the spins and surrounding lattice (spin-lattice relaxation), re-establishing thermal equilibrium. As spins go from a high energy state back to a low energy state, RF energy is released back into the surrounding lattice.
The recovery of longitudinal magnetization follows an exponential curve. The recovery rate is characterized by the tissue-specific time constant T1. After time T1, longitudinal magnetization has returned to 63 % of its final value. With a 1.5 T field strength, T1 values are about 200 to 3000 ms. T1 values are longer at higher field strengths.
Transverse relaxation results from spins getting out of phase. As spins move together, their magnetic fields interact (spin-spin interaction), slightly modifying their precession rate. These interactions are temporary and random. Thus, spin-spin relaxation causes a cumulative loss in phase resulting in transverse magnetization decay. Transverse magnetization decay is described by an exponential curve, characterized by the time constant T2. After time T2, transverse magnetization has lost 63 % of its original value.
T2 is tissue-specific and is always shorter than T1. Transverse relaxation is faster than longitudinal relaxation. T2 values are unrelated to field strength.