The hyperpolarization of nuclear spins using para-hydrogen (pH2) is a fascinating technique that allows increasing spin polarization and, as a result, the magnetic resonance signal by several orders of magnitude. Entirely new applications become available. Signal Amplification By Reversible Exchange (SABRE) is a relatively new method that is based on the reversible exchange of a substrate, catalyst and para-hydrogen. SABRE is particularly interesting for in vivo medical or industrial applications e.g., fast and low-cost trace analysis or continuous signal enhancement. Ever since its discovery, many attempts were undertaken to model and understand SABRE, with various degrees of simplifications.
In this work, we reduce the simplifications further, taking into account nonlinear chemical and physical (CAP) dynamics. These simulations were realized using an open source software (MOIN spin library). The CAP master equation presented here takes several effects into account, that were hitherto neglected. Explicitly included are now (i) chemical processes (substrate exchange, hydrogen exchange, hydrogen entering and leaving the system), (ii) coherent spin evolution and (iii) relaxation. The model is quite flexible; it is not restricted to a specific magnetic field range, number of spins or other experimental conditions.
Using CAP, we confirmed the experimental observation that an increase of pH2 pressure and flow tremendously increases SABRE polarization. Furthermore, it turned out that the NMR-invisible, transient complex plays a crucial role in the process of hyperpolarization; the NMR properties of the transient complex (J-couplings) drastically change the efficiency of SABRE. Today, little is known about the transient complex, and several parameters had to be guessed (kinetic rates, coupling constants, relaxation rates). This may change in the future if more experimental data become available.
To conclude: the CAP model promises to be very useful for understanding and optimizing SABRE, just as well as to elucidate yet unknown parameters of the SABRE system.