  Magnetic resonance imaging of $^{13}$C-labeled metabolites enhanced by
parahydrogen-induced polarization (PHIP) can enable real-time monitoring of
processes within the body. We introduce a robust, easily implementable
technique for transferring parahydrogen-derived singlet order into 13C
magnetization using adiabatic radio-frequency sweeps at $\mu$T fields. We
experimentally demonstrate the applicability of this technique to several
molecules, including some molecules relevant for metabolic imaging, where we
show significant improvements in the achievable polarization, in some cases
reaching above 60%. Furthermore, we introduce a site-selective deuteration
scheme, where deuterium is included in the coupling network of a pyruvate ester
to enhance the efficiency of the polarization transfer. These improvements are
enabled by the fact that the transfer protocol avoids relaxation induced by
strongly coupled quadrupolar nuclei.
