Hyperpolarised

Hyperpolarised refers to a non-equilibrium polarisation of nuclear spins in a sample, achieving populations of spin states far greater than those expected from thermal equilibrium at a given temperature and magnetic field. This enhanced spin polarisation translates into dramatically increased nuclear magnetic resonance (NMR) signals, enabling more sensitive measurements and imaging. The hyperpolarised state is typically transient, relaxing back toward equilibrium at a rate determined by the spin-lattice relaxation time (T1).

Hyperpolarisation is produced by several methods. Dynamic nuclear polarisation transfers polarisation from electron spins to nuclei using microwave irradiation at very low temperatures and high magnetic fields. Spin-exchange optical pumping uses circularly polarised light to polarise alkali metals, which then transfer polarisation to noble gas nuclei such as 129Xe or 3He. Para-hydrogen induced polarisation harnesses the spin order of enriched para-hydrogen, often via catalytic transfer (PHIP) or SABRE approaches to polarise a range of molecules.

Applications include enhanced MRI and magnetic resonance spectroscopy, particularly in metabolic imaging with hyperpolarised 13C compounds like [1-13C]pyruvate that reveal real-time metabolic fluxes. Hyperpolarised gases provide high-contrast lung imaging, while other hyperpolarised substrates are investigated for tumour biology, perfusion, and organ function. The technology supports preclinical research and, in some cases, clinical trials, but requires on-site production and rapid delivery to the patient due to the fleeting nature of the polarisation.

Limitations include short lifetimes of the hyperpolarised state, logistical and cost considerations, and the need for specialised equipment and protocols. Development has progressed since the mid-20th century, beginning with the discovery of polarisation transfer effects (Overhauser effect) and continuing through advances in DNP, SEOP, and PHIP/SABRE, yielding a growing set of tools for sensitive molecular imaging.