Recycling and imaging of nuclear singlet hyperpolarization.

The strong enhancement of NMR signals achieved by hyperpolarization decays, at best, with a time constant of a few minutes. Here, we show that a combination of long-lived singlet states, molecular design, magnetic field cycling, and specific radiofrequency pulse sequences allows repeated observation of the same batch of polarized nuclei over a period of 30 min and more. We report a recycling protocol in which the enhanced nuclear polarization achieved by dissolution-DNP is observed with full intensity and then returned to singlet order. MRI experiments may be run on a portion of the available spin polarization, while the remaining is preserved and made available for a later use. An analogy is drawn with a "spin bank" or "resealable container" in which highly polarized spin order may be deposited and retrieved.