Generalised level anticrossings explain improved ¹⁹F SABRE hyperpolarisation under oscillating magnetic fields

Signal amplification by reversible exchange (SABRE) is a parahydrogen-based hyperpolarisation technique that significantly enhances nuclear magnetic resonance signals without the use of expensive hardware. While conventional SABRE relies on a static polarisation transfer field set near the level anticrossing (LAC) condition, recent work has shown that oscillating fields can substantially boost the hyperpolarisation levels. Here, we develop a new theoretical model that generalises the LAC condition to account for the oscillating polarisation transfer fields, thereby explaining the spin dynamics of SABRE under such conditions. We use the generalised LAC condition to optimise the oscillating fields for maximum polarisation transfer to ¹⁹F and show that the scalar relaxation of the second kind can be simultaneously suppressed. Large-scale spin dynamics simulations and experiments show that this leads to enhanced ¹⁹F hyperpolarisation compared to conventional SABRE, with a 79% improvement observed experimentally. This work demonstrates a generalisable strategy for improving the efficiency of SABRE, advancing its potential for various applications, such as in biomedicine.