vessels (also called cells), and therefore different pressures, are used depending on the application, one amagat of total pressure of noble gas and nitrogen is sufficient for SEOP and 0.1 amagat of nitrogen density is needed to quench fluorescence. Great improvements in 129Xe hyperpolarization technology have achieved > 50% level at flow rates of 1–2 L/min, which enables human clinical applications. History The discovery of SEOP took decades for all the pieces to fall into place to create a complete technique. First, in 1897, Zeeman's studies of sodium vapor led to the first result of optical pumping. The next piece was found in 1950 when Kastler determined a method to electronically spin-polarize rubidium alkali metal vapor using an applied magnetic field and illuminating the vapor with resonant circularly polarized light. Ten years later, Marie-Anne Bouchiat, T. M. Carver, and C. M. Varnum performed spin exchange, in which the electronic spin polarization was transferred to nuclear spins of a noble gas (3He and 129Xe) through gas-phased collisions. Since then, this method has been greatly improved and expanded to use with other noble gases and alkali metals. Theory To explain the processes of excitation, optical pumping, and spin exchange easier, the most common alkali metal used for this process, rubidium, will be used as an example. Rubidium has an odd number of electrons, with only one in the outermost shell that can be excited under the right conditions. There are two transitions that can occur, one referred to as the D1 line where the transition occurs from the 52S1/2 state to the 52P3/2 state and another referred to the D2 line where the transition occurs from the 52S1/2 to the 52P1/2 state. The D1 and D2 transitions can occur if the rubidium atoms are illuminated with light at a wavelength of 794.7 nm and 780 nm, respectively (Figure 1). While it is possible to cause either excitation, laser technology is well-developed for causing the D1 transition to occur. Those lasers are said to be tuned to the D1 wavelength (794.7 nm) of rubidium. In order to increase the polarization level above thermal equilibrium, the populations of the spin states must be altered. In the absence of magnetic field, the two spin states of a spin I = ½ nuclei are in the same energy level, but in the presence of a magnetic field, the energy levels split into ms = ±1/2 energy