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Hyperpolarization is the nuclear spin polarization of a material far beyond thermal equilibrium conditions.[1] It is commonly applied to gases such as 129Xe and 3He, 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]

Spin-Exchange Optical Pumping

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

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

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

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.

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 intensitycan be obtained compared to NMR signals of the same organic molecule without PHIP and thus only "thermal" polarization at room temperature.


See also

Dynamic nuclear polarisation
Electron paramagnetic resonance

External links

Europhysics - "Take a breath of polarized noble gas"
University of Virginia - "Hyperpolarized Gas MR Imaging"
Swiss DNP Initiative - "DNP-NMR 13C Hyperpolarization"

References

Leawoods, Jason C.; Yablonskiy, Dmitriy A.; Saam, Brian; Gierada, David S.; Conradi, Mark S. (2001). "Hyperpolarized 3He Gas Production and MR Imaging of the Lung". Concepts in Magnetic Resonance 13: 277–293. doi:10.1002/cmr.1014.
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.
F. William Hersman; et al. (2008). "Large Production System for Hyperpolarized 129Xe for Human Lung Imaging Studies". Acad. Radiol. 15: 683–292. doi:10.1016/j.acra.2007.09.020.
Katarzyna Suchanek; et al. (2005). "Hyperpolarized Gas Imaging of the Lung". Optica Applicata 35: 263–276.
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.
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.
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.
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.
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.

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