Hyperpolarized

Hyperpolarized describes a state in which the population differences between nuclear spin states are greatly enhanced relative to thermal equilibrium, producing a much stronger magnetic resonance signal than would be expected at a given temperature. In NMR and MRI, hyperpolarization can raise signal levels by several orders of magnitude, enabling detection of low-concentration compounds and observation of fast biochemical processes that are otherwise difficult to study with conventional MRI.

Several techniques generate hyperpolarization. Dynamic nuclear polarization (DNP) cools a sample to cryogenic temperatures and uses microwave irradiation to transfer polarization from electron spins to nuclei, after which the hyperpolarized substance is dissolved and rapidly delivered to an MRI scanner. 13C and 15N nuclei are common targets; a notable application is hyperpolarized [1-13C]pyruvate for metabolic imaging. Spin-exchange optical pumping (SEOP) polarizes noble gases such as 129Xe and 3He for MRI. Parahydrogen-induced polarization (PHIP) and SABRE employ parahydrogen to transfer polarization to target molecules, enabling faster polarization under milder conditions in some cases.

Applications of hyperpolarized MRI include real-time metabolic imaging, cancer research, and cardiology, where tracking substrate conversion (for example, pyruvate to lactate or bicarbonate) provides insight into energetic state and tissue viability. Limitations include the transient nature of the polarization, with NMR signals decaying according to T1 relaxation times; the need for specialized equipment and rapid delivery systems; and the non-universality of polarization methods across different molecules. Consequently, hyperpolarization complements standard MRI rather than replacing it, offering high-sensitivity measurements for selected probes.