Nuclei in NMR samples possess spin magnetic moments, which are oriented isotropically in the absence of magnetic fields, so that no macroscopic magnetization arises. Conversely, in the presence of a magnetic field B0, the spin magnetic moments precess around the magnetic field direction, with an energy which depends on the direction, either parallel or antiparallel, of the magnetic moments with respect to the magnetic field. This difference in energy is responsible for a different population of the spin states, and thus for a net magnetization. The larger the applied magnetic field, the larger the differences in energy and thus the resulting magnetization at equilibrium. If the magnetization is perturbed by a change in the applied magnetic field or by external radiofrequency pulses, it recovers its equilibrium value with a process called spin relaxation. The longitudinal relaxation rate 𝑅 1 is the exponential rate constant for recovering the equilibrium magnetization along the direction of the external magnetic field.