What is Longitudinal and transverse relaxation times?

The return of $M_z$ to $M_0$ after a perturbation is exponential, with the longitudinal relaxation time $T_1$ (spin–lattice relaxation: energy given to the surroundings). The decay of the transverse magnetisation to zero is exponential with the transverse relaxation time $T_2 \le T_1$ (spin–spin relaxation: loss of phase coherence between spins).