In MRI and NMR spectroscopy, an observable nuclear spin polarization (magnetization) is created by an RF pulse or a train of pulses applied to a sample in a homogeneous magnetic field at the resonance (Larmor) frequency of the nuclei. At thermal equilibrium, nuclear spins precess randomly about the direction of the applied field but become abruptly phase coherent when any of the resultant polarization is created orthogonal to the field. This transverse magnetization can induce a signal in an RF coil that can be detected and amplified by an RF receiver. The RF pulses cause the population of spin-states to be perturbed from their thermal equilibrium value. The return of the longitudinal component of the magnetization to its equilibrium value is termed "spin-lattice" relaxation while the loss of phase-coherence of the spins is termed "spin-spin" relaxation, which is manifest as an observed free induction decay (FID).