Constant-adiabaticity ultralow magnetic field manipulations of parahydrogen-induced polarization: application to an AA'X spin system

The field of magnetic resonance imaging with hyperpolarized contrast agents is rapidly expanding, and parahydrogen-induced polarization (PHIP) is emerging as an inexpensive and easy-to-implement method for generating the required hyperpolarized biomolecules. Hydrogenative PHIP delivers hyperpolarized proton spin order to a substrate via chemical addition of H2 in the spin-singlet state, but prior to imaging it is typically necessary to transfer the proton polarization to a heteronucleus (usually 13C) in the molecule. Adiabatic ultralow magnetic field manipulations can be used to induce the polarization transfer, but this is necessarily a slow process, which is undesirable since the spins continually relax back to thermal equilibrium. Here we demonstrate constant-adiabaticity field cycling and field sweeping for optimal polarization transfer on a model AA$'$X spin system, [1-13C]fumarate. We introduce a method for calculating constant-adiabaticity magnetic field ramps and demonstrate that they enable much faster spin-order conversion as compared to linear ramps used before. The present method can thus be utilized to manipulate nonthermal order in heteronuclear spin systems.