Hyperpolarisation criteria in magnetic resonance.

Nuclear Magnetic Resonance (NMR) techniques display an inherently low sensitivity due to a small equilibrium magnetisation. Nowadays this issue is easily overcome through the use of hyperpolarisation methods. This however raises the question as to what precisely do we mean by "hyperpolarisation". Recently a formal definition of hyperpolarisation has been given based on the von Neumann entropy of a system. Ideally this definition should conform with the general usage in the magnetic resonance community, where hyperpolarisation is often used synonymously with "larger" NMR signals. Within this article I show that an entropy-based hyperpolarisation criterion does not always conform with the general usage. Based on this observation I introduce an alternative hyperpolarisation criterion utilising the concept of latent polarisation, where latent polarisation is a measure of the highest possible amount of polarisation that may be extracted from a system. I show that a hyperpolarisation criterion based on latent polarisation correlates more strongly with the general usage within the magnetic resonance community. Ultimately however our results show that there are several possible notions of hyperpolarisation, and the choice depends upon the questions of interest.