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Parahydrogen-induced polarization of 13 C nuclei by side-arm hydrogenation (PHIP-SAH) for [1- 13 C]acetate and [1- 13 C]pyruvate esters with application of PH-INEPT-type pulse sequences for 1 H to 13 C polarization transfer is reported, and its efficiency is compared with that of polarization transfer based on magnetic field cycling (MFC). The pulse-sequence transfer approach may have its merits in some applications because the entire hyperpolarization procedure is implemented directly in an NMR or MRI instrument, whereas MFC requires a controlled field variation at low magnetic fields. Optimization of the PH-INEPT-type transfer sequences resulted in 13 C polarization values of 0.66 ± 0.04% and 0.19 ± 0.02% for allyl [1- 13 C]pyruvate and ethyl [1- 13 C]acetate, respectively, which is lower than the corresponding polarization levels obtained with MFC for 1 H to 13 C polarization transfer (3.95 ± 0.05% and 0.65 ± 0.05% for allyl [1- 13 C]pyruvate and ethyl [1- 13 C]acetate, respectively). Nevertheless, a significant 13 C NMR signal enhancement with respect to thermal polarization allowed us to perform 13 C MR imaging of both biologically relevant hyperpolarized molecules which can be used to produce useful contrast agents for the in vivo imaging applications.

Among the hyperpolarization techniques geared toward in vivo magnetic resonance imaging, parahydrogen-induced polarization (PHIP) shows promise due to its low cost and fast speed of contrast agent preparation. The synthesis of 13 C-labeled, unsaturated precursors to perform PHIP by side arm hydrogenation has recently opened new possibilities for metabolic imaging owing to the biological compatibility of the reaction products, although the polarization transfer between the parahydrogen-derived protons and the 13 C heteronucleus must yet be better understood, characterized, and eventually optimized. In this realm, a new experimental strategy incorporating pulse-programmable magnetic field sweeping and in situ detection has been developed. The approach is evaluated by measuring the 13 C polarization of ethyl acetate-1- 13 C, i.e ., the product of pairwise addition of parahydrogen to vinyl acetate-1- 13 C, resulting from zero-crossing magnetic field ramps of various durations, amplitudes, and step sizes. The results demonstrate (i) the profound effect these parameters have on the 1 H to 13 C polarization transfer efficiency and (ii) the high reproducibility of the technique.