Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in a number of practical applications, with medical MRI being the most widely-known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the dramatic signal enhancement provided by the rapidly-developing field of spin hyperpolarization. Such techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity compared to other analytical techniques. This provides new impetus for existing applications, and, even more importantly, this opens the gates to numerous novel and exciting possibilities in the broad fields of fundamental and applied magnetic resonance. There are many different techniques that fall under the umbrella term “hyperpolarization”. Existing reviews cover the various subfields, but they are mostly addressed separately, and are seldom perceived as integral parts of the same field. In this review we attempt to unify the many methods that are used to hyperpolarize nuclear spins into one picture. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization; to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target nuclear spins. After outlining the inner workings of hyperpolarization, we give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues and possible future directions. While a substantial progress has been achieved in the field of spin hyperpolarization in recent years, the continuing growth of activity indicates that this is just the beginning. It is unlikely that fundamentally new sources of hyperpolarization will be uncovered in the near future, but we expect the field to flourish as new ways to improve and utilize current hyperpolarization techniques are identified and implemented. There is great scope for cross-fertilization between known methods, and developments in one area (e.g., prolonging polarization lifetimes, or creating more efficient excitation-detection schemes) can have a very broad impact across the entire field of hyperpolarization. We hope this review will facilitate this process, since advances in hyperpolarization will help to overcome existing challenges in magnetic resonance and enable novel applications.