Relaxation in NMR is a phenomenon describing the time dependence involved in signal intensity after an induced RF (radiofrequency) pulse is applied [153]. After application of a 90° RF pulse, the bulk magnetization will move to the transverse (xy) plane and will gradually return to its original equilibrium position along the longitudinal (z) axis [154]. This process is described in Figure 4, and is termed T1 relaxation. The details are beyond the scope of the manuscript and interested readers are directed to [155] and references therein. Relaxation times for NMR are even more complicated and exist in two categories: T1 and T2. T1 refers to the rate of longitudinal (or spin-lattice) Z-axis relaxation as the system returns to equilibrium. A second component also contributes, i.e., T2 relaxation and refers to the rate of transverse (or spin-spin) relaxation [154] which occurs in the XY plane. T2 is independent of the longitudinal relaxation (T1) and represents the loss of coherence in the precessing spins. Therefore NMR relaxation spectroscopy can be based on T1 and/or T2 [156], and is collectively referred to as “relaxation edited NMR” [157].