Parahydrogen‐Induced Hyperpolarization of Gases

Abstract

Imaging of gases is a major challenge for any modality including MRI. NMR and MRI signals are directly proportional to the nuclear spin density and the degree of alignment of nuclear spins with applied static magnetic field, which is called nuclear spin polarization. The level of nuclear spin polarization is typically very low, i.e., one hundred thousandth of the potential maximum at 1.5 T and a physiologically relevant temperature. As a result, MRI typically focusses on imaging highly concentrated tissue water. Hyperpolarization methods transiently increase nuclear spin polarizations up to unity, yielding corresponding gains in MRI signal level of several orders of magnitude that enable the 3D imaging of dilute biomolecules including gases. Parahydrogen‐induced polarization is a fast, highly scalable, and low‐cost hyperpolarization technique. The focus of this Minireview is to highlight selected advances in the field of parahydrogen‐induced polarization for the production of hyperpolarized compounds, which can be potentially employed as inhalable contrast agents.

Chemical Reaction Monitoring Using Zero-Field Nuclear Magnetic Resonance Enables Study of Heterogeneous Samples in Metal Containers

“…We used chemical reaction with para-enriched hydrogen gas for this demonstration; a number of alternative approaches have been shown in the context of ZULF NMR [35,61]. A closely related analogue to hydrogenative PHIP (including its heterogeneous version [62]) is signal amplification by reversible exchange (SABRE) [63][64][65], in which the hyperpolarized spin order of para-enriched hydrogen is transferred to other molecules via reversible exchange on a suitable metal-containing catalyst. Another option is to pre-polarize the molecules in an external magnetic field prior to detection.…”

Chemical Reaction Monitoring using Zero‐Field Nuclear Magnetic Resonance Enables Study of Heterogeneous Samples in Metal Containers

“…We used chemical reaction with para‐enriched hydrogen gas for this demonstration; a number of alternative approaches have been shown in the context of ZULF NMR . A closely related analogue to hydrogenative PHIP (including its heterogeneous version) is signal amplification by reversible exchange (SABRE), in which the hyperpolarized spin order of para‐enriched hydrogen is transferred to other molecules via reversible exchange on a suitable metal‐containing catalyst. Another option is to pre‐polarize the molecules in an external magnetic field prior to detection.…”

Heterogeneous Parahydrogen‐Induced Polarization of Diethyl Ether for Magnetic Resonance Imaging Applications

“…In this approach, parahydrogen ( p ‐H 2 ) is added to a double or triple chemical bond of an unsaturated substrate in a pairwise manner with retention of nuclear spins correlation [19] . Pairwise p ‐H 2 addition can be mediated by either homogeneous [20, 21] or heterogeneous [22–26] catalysts, and both these approaches can be utilized for hyperpolarization of gases [27–31] . While homogeneous catalysts provide higher polarization levels for reaction products in liquid phase (up to 50–60 %), [32, 33] less than 1 % polarization is typically achieved for HP gases produced using this approach [27] .…”

“…[19] Pairwise p-H 2 addition can be mediated by either homogeneous [20,21] or heterogeneous [22][23][24][25][26] catalysts, and both these approaches can be utilized for hyperpolarization of gases. [27][28][29][30][31] While homogeneous catalysts provide higher polarization levels for reaction productsi nl iquid phase (up to 50-60 %), [32,33] less than 1% polarization is typically achieved for HP gases produced using this approach. [27] Heterogeneous catalysts usuallyp rovide similar P 1H of % 1%;h owever,r ecent advancesi nc atalysis may potentially enable substantially greater P 1H values.…”