Nuclear spin hyperpolarization is an important resource for increasing the sensitivity of NMR spectroscopy and MRI. Signal enhancements can be as large as 3-4 orders of magnitude. In hyperpolarization experiments, it is often desirable to transfer the initial polarization to other nuclei of choice, either protons or insensitive nuclei such as (13)C and (15)N. This situation arises primarily in Chemically Induced Dynamic Nuclear Polarization (CIDNP), Para-Hydrogen Induced Polarization (PHIP), and the related Signal Amplification By Reversible Exchange (SABRE). Here we review the recent literature on polarization transfer mechanisms, in particular focusing on the role of Level Anti-Crossings (LACs) therein. So-called "spontaneous" polarization transfer may occur both at low and high magnetic fields. In addition, transfer of spin polarization can be accomplished by using especially designed pulse sequences. It is now clear that at low field spontaneous polarization transfer is primarily due to coherent spin-state mixing under strong coupling conditions. However, thus far the important role of LACs in this process has not received much attention. At high magnetic field, polarization may be transferred by cross-relaxation effects. Another promising high-field technique is to generate the strong coupling condition by spin locking using strong radio-frequency fields. Here, an analysis of polarization transfer in terms of LACs in the rotating frame is very useful to predict which spin orders are transferred depending on the strength and frequency of the B1 field. Finally, we will examine the role of strong coupling and LACs in magnetic-field dependent nuclear spin relaxation and the related topic of long-lived spin-states.

中文翻译：

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能级反交叉在核自旋超极化中的作用

核自旋超极化是提高 NMR 光谱和 MRI 灵敏度的重要资源。信号增强可以达到 3-4 个数量级。在超极化实验中，通常需要将初始极化转移到其他选择的原子核，质子或不敏感的原子核，如 (13)C 和 (15)N。这种情况主要出现在化学诱导动态核极化 (CIDNP)、对氢诱导极化 (PHIP) 和相关的可逆交换信号放大 (SABRE) 中。在这里，我们回顾了最近关于极化转移机制的文献，特别关注其中的 Level Anti-Crossings (LAC) 的作用。所谓的“自发”极化转移可能发生在低磁场和高磁场中。此外，自旋极化的转移可以通过使用特别设计的脉冲序列来实现。现在很清楚，在低场自发极化转移主要是由于强耦合条件下的相干自旋态混合。然而，到目前为止，LAC 在这一过程中的重要作用并未受到太多关注。在高磁场下，极化可能通过交叉弛豫效应转移。另一种有前途的高场技术是通过使用强射频场的自旋锁定来产生强耦合条件。在这里，根据旋转坐标系中的 LAC 对极化转移的分析对于预测根据 B1 场的强度和频率转移哪些自旋顺序非常有用。最后，