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

Abstract

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

Electron and nuclear spin polarization in Rb-Xe spin-exchange optical hyperpolarization

“…The two distinctly marked data points represent the calculated values of P Xe at the observed EPR frequency of 2.76 kHz, which is due to the work of Bear, using the computed enhancement factor κ of 726 from Schaefer et al [10], and the present calculations using our result, κ = 588 (see below). There is a difference between our P Xe values of roughly 10%-40% and the ones measured in the experiments, P Xe ≈ 60%-70% [1,71,72]. However, as noted already in the context of electron spin polarization, several factors, such as the partial pressure of xenon, xenon and rubidium number densities, laser power, wall relaxation, etc., all play a role in the experimental setups referred to in Fig.…”

“…However, as noted already in the context of electron spin polarization, several factors, such as the partial pressure of xenon, xenon and rubidium number densities, laser power, wall relaxation, etc., all play a role in the experimental setups referred to in Fig. 5 [11,66,70], as well as in more recent studies [1,71,72]. [Eq.…”

Hyperpolarized 129Xe MRI using isobutene as a new quenching gas

“…However, after repeated experiments, renewal of the polarizing cell was found to be necessary every 30 h, which was about three times more often than with N 2 . This observation is similar to that reported by Rogers, Meersmann et al According to their report, removable molecular hydrogen as a new buffer gas could improve the polarization levels of HP Kr and Xe in SEOP, despite necessitating removal of rubidium hydride formed during about 30 h of SEOP.…”

NMR Hyperpolarization Techniques of Gases

“…[61] Since that work, HP xenon has been used to study diffusion in confined spaces or porous media [62] , [63] , [64] ; image such systems as a function of gas flow [65] or 129 Xe chemical shift [66] ; or spectroscopically probe single-crystal surfaces [67] , liquid crystals, [68] or combustion processes. [69] However, the greatest body of materials-related work has concerned the effort to probe void spaces and surfaces in microporous or nanoporous materials with HP 129 Xe, thereby providing information about pore size, pore shape, and gas dynamics in: nanochanneled organic, organometallic, and peptide-based molecular materials [70] (including in macroscopically oriented single crystals [71] ); multi-walled carbon nanotubes [72] ; gas hydrate clathrates [73] ; porous polymeric materials and aerogels [74] ; metal-organic frameworks [75] ; calixarene-based materials and nanoparticles [76] ; organo-clays [77] ; mesoporous silicas [78] ; and zeolites and related materials [79] — efforts that have been aided by computational studies of xenon in confined spaces (e.g., Refs. [80] ).…”