Chapter 24: Beyond Spin Exchange Optical Pumping: Hyperpolarization of ¹²⁹Xe via Sublimation Dynamic Nuclear Polarization

Dynamic nuclear polarization (DNP) is a competitive alternative to the established spin exchange optical pumping method to hyperpolarize ¹²⁹Xe. DNP consists of transferring the large electron spin polarization of incorporated free radicals to the surrounding nuclei in a frozen sample by continuous irradiation with microwaves at a frequency close to the electron spin resonance of the radicals. The introduction of a dissolution step to transform the frozen sample into a liquid while preserving the nuclear polarization opened new perspectives for metabolic and molecular imaging. Hyperpolarization via dissolution DNP is nowadays widespread in the biomedical community and it is therefore attractive to develop a method to prepare hyperpolarized ¹²⁹Xe using the same instrumentation. In this chapter, the concept of sublimation DNP to hyperpolarize gases is presented. The physical and technical challenges associated with the preparation of an optimal frozen sample containing ¹²⁹Xe are discussed. It is shown that photo-induced radicals and optimized hardware can boost the polarization level and the throughput. It is currently possible to produce 0.1 L of hyperpolarized ¹²⁹Xe polarized at 25% in about 30 min and there are no fundamental barriers to rapidly scale up this method to the production of several litres per hour at the same polarization level.