Parahydrogen (pH 2 ) is a convenient and cost‐efficient source of spin order to enhance the magnetic resonance signal. Previous work showed that transient interaction of pH 2 with a metal organic complex in a signal amplification by reversible exchange (SABRE) experiment enabled more than 10 % polarization for some 15 N molecules. Here, we analyzed a variant of SABRE, consisting of a magnetic field alternating between a low field of ∼1 μT, where polarization transfer is expected to take place, and a higher field >50 μT (alt‐SABRE). These magnetic fields affected the amplitude and frequency of polarization transfer. Deviation of a lower magnetic field from a “perfect” condition of level anti‐crossing increases the frequency of polarization transfer that can be exploited for polarization of short‐lived transient SABRE complexes. Moreover, the coherences responsible for polarization transfer at a lower field persisted during magnetic field variation and continued their spin evolution at higher field with a frequency of 2.5 kHz at 54 μT. The latter should be taken into consideration for an efficient alt‐SABRE. Theoretical and experimental findings were exemplified with Iridium N‐heterocyclic carbene SABRE complex and 15 N‐acetonitrole, where a 30 % higher 15 N polarization with alt‐SABRE compared to common SABRE was reached.

Signal Amplification By Reversible Exchange (SABRE) is a new and rapidly developing hyperpolarization technique. The recent discovery of Spin-Lock Induced Crossing SABRE (SLIC-SABRE) shows that high field hyperpolarization transfer techniques developed so far were optimized for singlet spin order that does not coincide with the experimentally produced spin state. Here, we investigate the SLIC-SABRE approach and the most advanced quantitative theoretical SABRE model. It is the goal to achieve the highest possible polarization with SLIC-SABRE at high field using the standard SABRE system, IrIMes catalyst with pyridine. We demonstrate the accuracy of SABRE model describing the effects of various physical parameters such as the amplitude and frequency of the radio-frequency field, and the effects of chemical parameters such as the exchange rate constants. The combined use of experiments and theory allows to determine the effective lifetime of SABRE-complex. Furthermore, the entropy and enthalpy of the SABRE-complex dissociation reaction based on the temperature dependence of SLIC-SABRE signal can be accessed. We show, for the first time, that this SLIC-SABRE model can be useful for the evaluation of the chemical exchange parameters that are very important for the production of highly polarized contrast agents via SABRE.

Among the hyperpolarization techniques geared toward in vivo magnetic resonance imaging, parahydrogen-induced polarization (PHIP) shows promise due to its low cost and fast speed of contrast agent preparation. The synthesis of 13 C-labeled, unsaturated precursors to perform PHIP by side arm hydrogenation has recently opened new possibilities for metabolic imaging owing to the biological compatibility of the reaction products, although the polarization transfer between the parahydrogen-derived protons and the 13 C heteronucleus must yet to be better understood, characterized, and eventually optimized. In this realm, a new experimental strategy incorporating pulse-programmable magnetic field cycling has been developed. The approach is evaluated by measuring the 13 C polarization of ethyl acetate-1-13 C, i.e. the product of pairwise addition of parahydrogen to vinyl acetate-1-13 C, resulting from zero-crossing magnetic field sweeps of various durations, amplitudes, and step sizes. The results demonstrate (i) the profound effect these parameters have on the 1 H to 13 C polarization transfer efficiency and (ii) the high reproducibility of the technique.