The order within proton pairs in organic molecules, resulting from hydrogenation with para-hydrogen, can be transferred in great part to nearby carbon-13 spins through adequate field manipulations. The molecules with hyperpolarized 13C thus obtained can be used as new contrast agents of high efficiency in MRI. After a brief presentation of the hydrogenation process and apparatus, and a summary of the order transfer to the 13C spins through field-cycling, we describe the use of radio-frequency irradiation, by CW and by pulses, to produce such an order transfer. We consider in turn the various steps involved, their rationale and their optimization. The final polarizations expected from both methods in the absence of relaxation are compared. Finally, some MRI pictures observed in vivo on animals are shown as an illustration of the contrast capacity of these methods. To cite this article: M. Goldman et al., C. R. Chimie 9 (2006).