Hyperpolarization (physics)

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Hyperpolarization is the nuclear spin polarization of a material far beyond thermal equilibrium conditions.[1] It can be applied to gases such as 129Xe and 3He, and small molecules where the polarization levels can be enhanced by a factor of 104-105 above thermal equilibrium levels. Hyperpolarized noble gases are typically used in magnetic resonance imaging (MRI) of the lungs.[2] Hyperpolarized small molecules are typically used for in vivo metabolic imaging. For example, a hyperpolarized metabolite can be injected into animals or patients and the metabolic conversion can be tracked in real-time.

Spin-exchange optical pumping[edit]

3He and 129Xe are generally hyperpolarized using a process called spin-exchange optical pumping (SEOP). During this process, circularly polarized infrared laser light, tuned to the appropriate wavelength, is used to excite electrons in an alkali metal, such as caesium or rubidium inside a sealed glass vessel. The angular momentum is transferred from the alkali metal electrons to the noble gas nuclei through collisions. Nitrogen is used as a quenching gas, which prevents the fluorescence of the polarized alkali metal, which would lead to de-polarization of the noble gas. Great improvements in 129Xe hyperpolarization technology have achieved > 50% level at flow rates of 1–2 L/min, which enables human clinical applications.[3]

Metastability exchange optical pumping[edit]

3He can also be hyperpolarized using metastability exchange optical pumping (MEOP).[4] This process is able to polarize 3He nuclei in the ground state with optically pumped 3He nuclei in the metastable state. MEOP only involves 3He nuclei at room temperature and at low pressure (≈a few mbars). The process of MEOP is very efficient (high polarization rate), however, compression of the gas up to atmospheric pressure is needed.

Dynamic nuclear polarization[edit]

Compounds containing NMR-sensitive nuclei, such as 13C or 15N, can be hyperpolarized using Dynamic Nuclear Polarisation (DNP). DNP is typically performed at low temperature (≈1 K) and high magnetic field (≈3 T). The compound is subsequently thawed and dissolved to yield a room temperature solution containing hyperpolarized nuclei.[5] This liquid can be used in in vivo metabolic imaging[6] for oncology[7] and other applications. The 13C polarization levels in solid compounds can reach up to ≈64% and the losses during dissolution and transfer of the sample for NMR measurements can be minimized to a few percent.[8] Compounds containing NMR-active nuclei can also be hyperpolarized using chemical reactions with para-hydrogen, see Para-Hydrogen Induced Polarization (PHIP).

Parahydrogen induced polarization[edit]

Molecular hydrogen, H2, contains two different spin isomers, para-hydrogen and ortho-hydrogen, with a ratio of 25:75 at room temperature. Creating para-hydrogen induced polarization (PHIP)[9] means that this ratio is increased, in other words that para-hydrogen is enriched. This can be accomplished by cooling hydrogen gas and then inducing ortho-to-para conversion via an iron-oxide or charcoal catalyst. When performing this procedure at ~70 K (i.e. with liquid nitrogen), para-hydrogen is enriched from 25% to ca. 50%. When cooling to below 20 K and then inducing the ortho-to-para conversion, close to 100% parahydrogen can be obtained.[citation needed]

For practical applications, the PHIP is most commonly transferred to organic molecules by reacting the hyperpolarized hydrogen with precursor molecules in the presence of a transition metal catalyst. Proton NMR signals with ca. 10,000-fold increased intensity[citation needed] can be obtained compared to NMR signals of the same organic molecule without PHIP and thus only "thermal" polarization at room temperature.

Signal amplification by reversible exchange (SABRE)[edit]

Signal amplification by reversible exchange (SABRE) is a technique to hyperpolarize samples without chemically modifying them. Compared to orthohydrogen or organic molecules, a much greater fraction of the hydrogen nuclei in parahydrogen align with an applied magnetic field. In SABRE, a metal center reversibly binds to both the test molecule and a parahydrogen molecule facilitating the target molecule to pick up the polarization of the parahydrogen.[10] This technique can be improved and utilized for a wide range of organic molecules by using an intermediate "relay" molecule like ammonia. The ammonia efficiently binds to the metal center and picks up the polarization from the parahydrogen. The ammonia then transfers it other molecules that don't bind as well to the metal catalyst.[11] This enhanced NMR signal allows the rapid analysis of very small amounts of material.

See also[edit]

External links[edit]


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  2. ^ Altes, Talissa; Salerno, Michael (2004). "Hyperpolarized Gas Imaging of the Lung". J Thorac Imaging. 19: 250–258. doi:10.1097/01.rti.0000142837.52729.38.
  3. ^ F. William Hersman; et al. (2008). "Large Production System for Hyperpolarized 129Xe for Human Lung Imaging Studies". Acad. Radiol. 15: 683–692. doi:10.1016/j.acra.2007.09.020. PMC 2475596. PMID 18486005.
  4. ^ Katarzyna Suchanek; et al. (2005). "Hyperpolarized Gas Imaging of the Lung". Optica Applicata. 35: 263–276.
  5. ^ Jan H. Ardenkjær-Larsen; Björn Fridlund; Andreas Gram; Georg Hansson; Lennart Hansson; Mathilde H. Lerche; Rolf Servin; Mikkel Thaning; Klaes Golman (2003). "Increase in signal-to-noise ratio of > 10,000 times in liquid-state NMR". Proc. Natl. Acad. Sci. U.S.A. 100 (18): 10158–10163. Bibcode:2003PNAS..10010158A. doi:10.1073/pnas.1733835100. PMC 193532. PMID 12930897.
  6. ^ Klaes Golman; Jan H. Ardenkjær-Larsen; J. Stefan Petersson; Sven Månsson; Ib Leunbach (2003). "Molecular imaging with endogenous substances". Proc. Natl. Acad. Sci. U.S.A. 100 (18): 10435–10439. Bibcode:2003PNAS..10010435G. doi:10.1073/pnas.1733836100. PMC 193579. PMID 12930896.
  7. ^ Day SE, Kettunen MI, Gallagher FA, Hu DE, Lerche M, Wolber J, Golman K, Ardenkjaer-Larsen JH, Brindle KM (2007). "Detecting tumor response to treatment using hyperpolarized 13C magnetic resonance imaging and spectroscopy". Nat. Med. 13 (11): 1382–1387. doi:10.1038/nm1650. PMID 17965722.
  8. ^ Haukur Jóhannesson; Sven Macholl; Jan H. Ardenkjær-Larsen (2009). "Dynamic Nuclear Polarization of [1-13C]pyruvic acid at 4.6 tesla". J. Magn. Reson. 197 (2): 167–175. Bibcode:2009JMagR.197..167J. doi:10.1016/j.jmr.2008.12.016. PMID 19162518.
  9. ^ Natterer, Johannes; Bargon, Joachim (1997). "Parahydrogen induced polarization". Progress in Nuclear Magnetic Resonance Spectroscopy. 31: 293–315. doi:10.1016/s0079-6565(97)00007-1.
  10. ^ Eshuis, Nan; Aspers, Ruud L.E.G.; van Weerdenburg, Bram J.A.; Feiters, Martin C.; Rutjes, Floris P.J.T.; Wijmenga, Sybren S.; Tessari, Marco (2016). "Determination of long-range scalar 1 H– 1 H coupling constants responsible for polarization transfer in SABRE". Journal of Magnetic Resonance. Elsevier BV. 265: 59–66. Bibcode:2016JMagR.265...59E. doi:10.1016/j.jmr.2016.01.012. ISSN 1090-7807.
  11. ^ Iali, Wissam; Rayner, Peter J.; Duckett, Simon B. (2018). "Using para hydrogen to hyperpolarize amines, amides, carboxylic acids, alcohols, phosphates, and carbonates". Science Advances. American Association for the Advancement of Science (AAAS). 4 (1): eaao6250. Bibcode:2018SciA....4O6250I. doi:10.1126/sciadv.aao6250. ISSN 2375-2548.