For most of the last forty years, the techniques of Dynamic Nuclear Polarization (DNP) have been confined to particle-physics laboratories building polarized targets, but recently it has been shown that samples similar to a solid target can be transformed into room temperature liquid solutions while retaining a high nuclear polarization. This method of ' 'hyperpolarization' ' is of interest in NMR/MRI/MRS. We describe a 3.35 T DNP/9.4 T MRI installation based on a continuous-flow cryostat, using a standard wide-bore low-field NMR magnet as prepolarizer magnet and a widely available radical as polarizing agent. The interfacing to a rodent scanner requires that the infusion of the polarized solution in the animal be remotely controlled, because of limited access inside the magnet bore. Physiological constraints on the infusion rate can be a serious source of polarization loss, and the discussion of efficiency is therefore limited to that of the prepolarizer itself, i.e., the spin temperatures obtained in the solid state. To put our results in context, we summarize data obtained in targets with different types of radicals, and provide a short review of the DNP mechanisms needed in their discussion.

In recent years methods of creating ‘hyperpolarized’ substances have gained considerable attention in biomedical magnetic resonance and dynamic nuclear polarization (DNP) is one of the most promising, especially for imaging applications. Here we present results of DNP studies on protons and 13C nuclei in frozen solutions of sodium acetate and glycine, dissolved in water–ethanol and water–glycerol, doped with TEMPO free radicals and EHBA-CrV complexes. Up to 14% 13C polarization and close to 50% proton polarization were achieved at ∼1.2 K in a magnetic field of ∼3.5 T under irradiation with ∼97 GHz microwaves, which corresponds to an enhancement of more than 15 000 with respect to thermal equilibrium polarization in a 9.4 T magnet at room temperature. For all investigated samples the main DNP mechanism was found to be thermal mixing. The absolute polarization values achieved are mainly depending on the type of solvent, the water–alcohol ratio, its degree of deuteration and the concentration of paramagnetic centres. This allows application of the so-established sample preparation and DNP procedure to other molecules in future experiments. Two further examples of DNP of molecules in solution underline the general applicability of the method to a wide variety of organic compounds.

Apart from their very classical use to build polarized targets for particle physics, the methods of dynamic nuclear polarization (DNP) have more recently found application for sensitivity enhancement in high-resolution NMR, both in the solid and in the liquid state. It is often thought that the possible signal enhancement in such applications deteriorates when the DNP is performed at higher fields. We show that for a dissolution-DNP method that uses conventional (2,2,6,6-tetramethylpiperidine 1-oxyl) radicals as the paramagnetic agent, this is not the case for fields up to 5 T.