Hyperpolarization (physics)

In physics , hyperpolarization is understood as an orderly alignment of nuclear spins in a material sample far beyond thermal equilibrium .

definition

A spin of the size has possible orientations relative to a given quantization axis (see directional quantization ). If the particles are in an external magnetic field , energy differences arise between these orientations due to the magnetic moment associated with the spin , which are typical for a given atom and its molecular environment. These differences in energy basically cause polarization , i.e. differences between the occupation numbers of the individual orientations. Even in strong fields, the energy differences are very small compared to the thermal energy of the particles. The resulting differences in occupation can be calculated using the Boltzmann statistics ; they are always below 1 per thousand. ${\ displaystyle I}$ ${\ displaystyle 2I + 1}$

For nuclear magnetic resonance spectroscopy , however, the greatest possible difference in population is important, since otherwise the absorption is completely canceled out by the induced emission, i.e. the sample becomes more or less transparent. The sensitivity of nuclear magnetic resonance experiments is therefore comparatively low.

A sample in which one or more spin states outweighs one or the other considerably more than their energy difference predicts via the Boltzmann statistics is called hyperpolarized.

realization

Hyperpolarization effects can be generated experimentally in noble gases . This is often Helium - isotope ³ He, or the xenon isotope ¹²⁹ used Xe.

The hyperpolarization is achieved via the detour of the polarization of electron spins of alkali metals . The magnetic moments of the shell electrons of a gaseous alkali metal (often rubidium ) are aligned by optical pumping and the alkali metal gas is mixed with the noble gas. Collisions between the noble gas atoms and the alkali metal atoms cause the aligned electron spins of the alkali metals to align the nuclear spins of the noble gases ( hyperfine interactions ). As a result, degrees of polarization of up to 70% could be achieved experimentally, which is several orders of magnitude higher than the equilibrium polarization specified by the Boltzmann statistics.

Another possibility is the so-called dynamic nuclear polarization ( dynamic nuclear polarization is). This method can in solids polarization of electron spins transferred to the nuclear spin. For this purpose (mostly organic ) radicals are introduced into the sample, and the sample is then exposed to strong microwave radiation in the range of the resonance frequency of the radical electrons at very low temperatures and medium magnetic fields (for example 1.1 Kelvin and 3 Tesla). This corresponds to the structure of an electron spin resonance experiment . Due to interactions between electrons and nuclei (Overhauser effect , solid effect , thermal mixing and others), a high polarization of the nuclei can be achieved after sufficiently long irradiation. There are promising attempts to transfer this polarization of the solid into the liquid phase by rapid thawing and dissolving.

Applications

One application of hyperpolarization is in the field of nuclear magnetic resonance (NMR) or magnetic resonance tomography (MRT). Due to the large number of aligned nuclear spins, the hyperpolarized gas can be recognized very well with magnetic resonance methods, which is why it is suitable as a contrast medium for certain applications. So far it has been successfully demonstrated that good MRI images of the human lungs can be made after inhalation of hyperpolarized xenon.

The method mentioned in the previous section with a solution of molecules with hyperpolarized atomic nuclei can e.g. B. use in oncological diagnostics. The metabolism of the hyperpolarized molecules is tracked with dynamic imaging. As an example, a degree of polarization in the solid state of (64 ± 5)% could be achieved for ¹³ C spins in a molecule suitable for oncology ( pyruvic acid ). Losses during the dissolution of the sample and transfer for the subsequent NMR or MRT measurement can be reduced to a few percent.

Hyperpolarization can also be used successfully in structural biology research.

literature

Thad G. Walker, William Happer: Spin-exchange optical pumping of noble-gas nuclei . In: Reviews of Modern Physics . tape 69 , no. 2 , 1997, p. 629-642 , doi : 10.1103 / RevModPhys.69.629 .

Web links

Article of January 22, 2014 in the NZZ on hyperpolarization

Individual evidence

↑ Jan H. Ardenkjær-Larsen, Björn Fridlund, Andreas Gram, Georg Hansson, Lennart Hansson, Mathilde H. Lerche, Rolf Servin, Mikkel Thaning, Klaes Golman: Increase in signal-to-noise ratio of> 10,000 times in liquid-state NMR . In: Proceedings of the National Academy of Sciences . tape 100 , no. 18 , 2003, p. 10158-10163 , doi : 10.1073 / pnas.1733835100 .
↑ SE Day et al .: Detecting tumor response to treatment using hyperpolarized ¹³ C magnetic resonance imaging and spectroscopy. In: Nat. Med. 13, No. 11, 2007, pp. 1382-1387, PMID 17965722 .
↑ K. Golman et al .: Molecular imaging with endogenous substances. In: Proc. Natl. Acad. Sci. USA 100, No. 18, 2003, pp. 10435-10439, PMID 12930896 .
↑ H. Jóhannesson et al .: Dynamic Nuclear Polarization of [1- ¹³ C] pyruvic acid at 4.6 tesla. In: J. Magn. Reson. 197, No. 2, 2009, pp. 167-175, PMID 19162518 .
↑ Thibault Viennet, Aldino Viegas, Arne Kuepper, Sabine Arens, Vladimir Gelev, Ognyan Petrov, Tom N. Grossmann, Henrike Heise, Manuel Etzkorn: Selective Protein Hyperpolarization in Cell Lysates Using Targeted Dynamic Nuclear Polarization. In: Angew. Chem. Int. Ed. June 2016, doi: 10.1002 / anie.201603205 .

[1] Jan H. Ardenkjær-Larsen, Björn Fridlund, Andreas Gram, Georg Hansson, Lennart Hansson, Mathilde H. Lerche, Rolf Servin, Mikkel Thaning, Klaes Golman: Increase in signal-to-noise ratio of> 10,000 times in liquid-state NMR . In: Proceedings of the National Academy of Sciences . tape 100 , no. 18 , 2003, p. 10158-10163 , doi : 10.1073 / pnas.1733835100 .

[2] SE Day et al .: Detecting tumor response to treatment using hyperpolarized ¹³ C magnetic resonance imaging and spectroscopy. In: Nat. Med. 13, No. 11, 2007, pp. 1382-1387, PMID 17965722 .

[3] K. Golman et al .: Molecular imaging with endogenous substances. In: Proc. Natl. Acad. Sci. USA 100, No. 18, 2003, pp. 10435-10439, PMID 12930896 .

[4] H. Jóhannesson et al .: Dynamic Nuclear Polarization of [1- ¹³ C] pyruvic acid at 4.6 tesla. In: J. Magn. Reson. 197, No. 2, 2009, pp. 167-175, PMID 19162518 .

[5] Thibault Viennet, Aldino Viegas, Arne Kuepper, Sabine Arens, Vladimir Gelev, Ognyan Petrov, Tom N. Grossmann, Henrike Heise, Manuel Etzkorn: Selective Protein Hyperpolarization in Cell Lysates Using Targeted Dynamic Nuclear Polarization. In: Angew. Chem. Int. Ed. June 2016, doi: 10.1002 / anie.201603205 .