Hyperpolarization (physics)

Hyperpolarization is the nuclear spin polarization of a material far beyond thermal equilibrium conditions. 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. Hyperpolarized small molecules are typically used for invivo metabolic imaging. For example, a hyperpolarized metabolite can be injected into animals or patients and the metabolic conversion can be tracked in real-time.

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.

³He can also be hyperpolarized using metastability exchange optical pumping (MEOP). 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.

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. This liquid can be used in in vivo metabolic imaging for oncology 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. Compounds containing NMR-active nuclei can also be hyperpolarized using chemical reactions with para-hydrogen, see Para-Hydrogen Induced Polarization (PHIP).

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