Molecular hydrogen and catalytic combustion in the production of hyperpolarized ^83Kr and ¹²⁹Xe MRI contrast agents

Hyperpolarized (hp) ^83Kr 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 ^83Kr that enable unique MRI contrast also complicate the production of hp ^83Kr. This work presents a previously unexplored approach in the generation of hp ^83Kr that can likewise be used for the production of hp ¹²⁹Xe. 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 ^83Kr and P = 63% for ¹²⁹Xe. The results were reproduced over many SEOP cycles despite the laser-induced on-resonance formation of rubidium hydride (RbH). Following SEOP, the H₂ was reactively removed via catalytic combustion without measurable losses in hyperpolarized spin state of either ^83Kr or ¹²⁹Xe. Highly spin-polarized ^83Kr can now be purified for the first time, to our knowledge, to provide high signal intensity for the advancement of in vivo hp ^83Kr MRI. More generally, a chemical reaction appears as a viable alternative to the cryogenic separation process, the primary purification method of hp ¹²⁹Xe for the past 2 1/2 decades. The inherent simplicity of the combustion process will facilitate hp ¹²⁹Xe production and should allow for on-demand continuous flow of purified and highly spin-polarized ¹²⁹Xe.