  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.
