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Abstract

One of the greatest limitations of NMR is its lack of sensitivity, achieving a thermal polarisation of only ppm at clinical field strengths. This limits the use of NMR for high sensitivity applications, including spectroscopy at zero to ultra-low field (ULF) and characterisations of mechanisms, such as spin-spin relaxation. Hyperpolarisation techniques, such as SABRE-SHEATH make these applications feasible by increasing nuclear polarisation to the order of percent. Aims: Using SABRE-SHEATH, we aim to empirically characterise the relaxation processes of [1-¹³C]pyruvate and [2-¹³C]pyruvate in comparison to what would be predicted from the evolution of their product operators, derived for arbitrary magnetic field. Methods: We hyperpolarised a 50 mM solution of 13C labelled pyruvate salt with the IrIMes catalyst and co- ligand DMSO in methanol, using SABRE-SHEATH. We controlled the guiding BHyp and detection BDet fields in a home-built ULF MRI setup, inside a moderately shielded room. Hyperpolarisation was performed with the BHyp field along the detection axis. For detection, the BHyp field was switched off and an orthogonal BDet field in the range of nTs-μTs was switched on non-adiabatically. Detection was performed, using an ultra-sensitive SQUID system. The NMR spectra were then modelled, using a density matrix approach. Results: The results from the strong-coupling regime (zero-field) and from the weak-coupling regime (μTs) were then compared with the model. Both the empirical data and model agreed well with the FID evolution of [1-¹³C]pyruvate, although there were some unexpected signals detected from [2- ¹³C]pyruvate. However, the empirical data also revealed unexpected behaviour of the spin-spin relaxation, manifested by the variable linewidths of the ¹³C multiplets. Conclusions: The implication of this is that hyperpolarisation enables single shot long acquisitions, through increased SNR that reveal features otherwise impossible to detect, enabling a better understanding of ULF spin evolution and future optimisation of hyperpolarisation.