Hyperpolarized (hp) 83 Kr is a promising MRI contrast agent for the diagnosis of pulmonary diseases affecting the surface of the respiratory zone. However, the distinct physical properties of 83 Kr that enable unique MRI contrast also complicate the production of hp 83 Kr. This work presents a previously unexplored approach in the generation of hp 83 Kr that can likewise be used for the production of hp 129 Xe. Molecular nitrogen, typically used as buffer gas in spin-exchange optical pumping (SEOP), was replaced by molecular hydrogen without penalty for the achievable hyperpolarization. In this particular study, the highest obtained nuclear spin polarizations were P = 29% for 83 Kr and P = 63% for 129 Xe. The results were reproduced over many SEOP cycles despite the laser-induced on-resonance formation of rubidium hydride (RbH). Following SEOP, the H 2 was reactively removed via catalytic combustion without measurable losses in hyperpolarized spin state of either Xe can be obtained through dynamic nuclear polarization (25) with high spin-polarization levels of up to P = 30% (26) He-N 2 mixture or pure N 2 gas. The buffer gas serves a dual purpose as it prevents destructive radiation trapping, originating from radiative electronic relaxation of rubidium (27, 28), but also causes pressure broadening of the Rb D 1 linewidth. Rubidium line broadening maximizes adsorption of laser light emitted by high-power solid devices, even if those are line narrowed. Following SEOP, hp 129

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