³He and ¹²⁹Xe are the nuclei normally selected for hyperpolarized noble gases MRI for lungs, but in principle, any nucleus with non-zero spin (like ¹H, ²H, ³He, ¹³C, ¹⁴N, ¹⁵N, ¹⁹F, ³¹P, ¹²⁹Xe, ¹³¹Xe…) can be polarized above thermal equilibrium. However, the longitudinal relaxation rate of these nuclei can be rapid, causing significant decay of the spin polarization. The monatomic structure of noble gases, such as ³He, ¹⁹Ne, ²¹Ne, ³⁷Ar, ⁸³Kr, ¹²⁹Xe, and ¹³¹Xe, enable long polarization half-lives compared to other nuclei. Among the noble gas isotopes, ³He and ¹²⁹Xe are the only nuclei with nuclear spin 1/2, resulting in the T ₁ (longitudinal relaxation time) of many hours or even days at the standard temperature and pressure. Hyperpolarized ⁸³Kr (I = 9/2) has also been reported as a third noble gas probe for lung imaging, but quadrupolar interaction and the presence of a breathable concentration of molecular oxygen heavily affect the T ₁ (40). Therefore, hyperpolarized ³He and ¹²⁹Xe have generally been used for the medical lung imaging so far.

For comparison, the basic physical properties of ³He and ¹²⁹Xe are listed in Table 1. For direct imaging of gas spaces, like lung MRI, ³He has an advantage over ¹²⁹Xe, because the gyromagnetic ratio of ³He is 2.8 times larger than that of ¹²⁹Xe. However, the diffusion coefficient of ¹²⁹Xe is 30 times lower than that of ³He, and the lower diffusion constant would result in less signal attenuation during high-resolution imaging. ³He and ¹²⁹Xe cover the spectrum from insoluble to highly soluble. ³He is much less soluble than ¹²⁹Xe due to its smaller size. Since xenon is lipophilic, it has a higher solubility in blood-, oil-, and lipid-containing tissues. ¹²⁹Xe is also extremely sensitive to its environment, which results in a chemical shift greater than 200 ppm in the body and up to 7500 ppm in different chemical compounds (41). Due to the relatively high solubility and the large chemical shift of xenon in biological tissues, such as in the lungs and the brain (42, 43), dissolved phase hyperpolarized xenon MRI and molecular imaging have shown a great potential in the study of gas exchange in the lungs, brain perfusion, and potential detection of diseases with high sensitivity (44–54). Such solubility and chemical shift properties make ¹²⁹Xe a great probe for a variety of biomedical and material science applications.

For SEOP, the first step is the transfer of circularly polarized laser light into electronic spin polarization, i.e., optical pumping. Figure 1 illustrates the principle of optically pumping Rb. In principal, any alkali metal vapor can be optically pumped. Rb is commonly used since the corresponding pumping laser diode arrays (LDA) are routinely manufactured in high power configurations. When the Rb pumping cell is placed in an oven at a temperature up to 160°C, the Rb is vaporized. Since the Rb vapor experiences an externally applied magnetic field (25 G) produced by a pair of Helmholtz coils, the ground state 5²S_1/2 is split into two Zeeman sublevels, and , i.e., electronic spin “down” or “up.” These two Zeeman states have a roughly equal population at room temperature. When the Rb vapor absorbs the circularly polarized laser light centered at 795 nm, a D₁ transition of the Rb is observed, i.e., . The circularly polarized light is right handed (σ+), and therefore only the ground state with a sublevel is pumped into the excited state, . Once the valence electron has been optically pumped to the excited state , collisional mixing equalizes the excited state populations. N₂ gas molecules have many degrees of freedom in terms of rotational and vibrational levels. Thus, nitrogen gas will quench the two sublevels of the excited state back to the ground state. As the sublevel continues to absorb the circularly polarized light an excess of Rb atoms are optically pumped into the Zeeman sublevel while the other sublevel is depleted. Therefore, an electronic spin polarization of nearly 100% can be achieved for Rb.

The second step of SEOP is spin exchange between the polarized electronic spin of Rb and the xenon nucleus, as shown in Fig. 2. In the high-temperature and high-pressure optical pumping cell, the collision between polarized Rb atoms and xenon atoms results in the transfer of angular momentum from the electronic spin to the Xe nuclear spin. During this collision, the electron wave function of the Rb overlaps the nuclear wave function of xenon which causes spin exchange between the electronic spin and nuclear spin. Binary collisions dominate the spin exchange at high pressure, while three-body collision (by forming a Rb/Xe van der Waals molecule) dominate at low pressure (a few tens of torr) (5).

Generally, the polarization build-up time for ³He is much longer than that of ¹²⁹Xe, and accordingly, the spin-exchange optical pumping for ³He is normally done in a batch mode, while hyperpolarized ¹²⁹Xe can be produced in a continuous flow mode. A hybrid optical pumping technique has been developed for ³He to shorten the time needed to polarize the gas (55). In this chapter, we will focus on the building of a xenon polarizer in a continuous flow mode.