Hyperpolarized molecules in solution.

Hyperpolarization is a technique to enhance the nuclear polarization and thereby increase the available signal in magnetic resonance (MR). This chapter provides an introduction to the concept of hyperpolarization as well as an overview of dynamic nuclear polarization (DNP) and para-hydrogen induced polarization (PHIP), two methods used to generate hyperpolarized molecules in aqueous solution.

Applications of hyperpolarized agents in solutions.

This chapter provides an overview of pulse sequences adapted to hyperpolarized MR imaging. Applications of hyperpolarized agents in aqueous solution are reviewed. Vascular (e.g., angiography, perfusion, and catheter tracking) as well as metabolic (e.g., oncology, cardiology, neurology, and pH mapping) applications are covered. Due to the rapid development of new applications for hyperpolarized agents, a review format has been used for this chapter instead of a strict protocol/procedure structure.

Trityl biradicals and 13C dynamic nuclear polarization.

The objective of this study is to investigate if trityl biradicals could lead to more efficient dynamic nuclear polarization (DNP) for low gamma nuclear spins at low temperature (approximately 1 K) than a trityl monoradical. Three novel trityl biradicals of different size are synthesized, characterized and employed for hyperpolarization of [1-(13)C]pyruvic acid at 3.35 T and 4.64 T. Intramolecular electron-electron distances are obtained via dipolar couplings from electron paramagnetic resonance (EPR) spectroscopy at X-band and W-band that match well with calculated molecular structures. Steady-state DNP levels and build-up times are measured as function of radical concentration, magnetic field strength and microwave frequency for each biradical. Similar maximal DNP is obtained with all studied biradicals whereas a twice as high polarization is achievable with the monoradical. Both the biradicals and the monoradical show approximately a doubling of the polarization when increasing the field strength from 3.35 T to 4.64 T. Biradical concentrations at maximum polarization are several times lower than the optimum monoradical concentration, but the penalty is a much longer build-up time. Adding a small amount of Gd(3+) to the samples (molar fraction of typically 100 ppm) has the same effect on DNP with the biradicals as with the monoradical. The electron longitudinal relaxation time T(1e) is found to be independent of the radical type and the field strength in this study. The same dependence of T(1e) on the trityl concentration is observed for all radicals. A considerable shortening of the (13)C longitudinal relaxation time is observed for biradicals which agrees with the shortened build-up time compared to the monoradical at the same trityl concentration. This is probably the reason for lower DNP levels with trityl biradicals.

Dynamic Nuclear Polarization of [1-13C]pyruvic acid at 4.6 tesla.

Dynamic Nuclear Polarization (DNP) of the (13)C nucleus has been investigated for [1-(13)C]pyruvic acid, doped with the trityl radical OX063Me, at 4.64 T and 1.15K. The dependence of the polarization on microwave frequency, radical concentration and electron saturation was studied. For optimized conditions, a (13)C polarization equal to 64+/-5% was obtained, an increase by more than a factor of two compared with earlier results at 3.35 T of the same system. It was furthermore observed that the addition of gadolinium, which resulted in a twofold polarization increase at 3.35 T, only resulted in a minor improvement at 4.64 T. The dependence of the electron saturation on microwave frequency and microwave power was quantified by first moment measurements which were obtained by nucleus-electron double resonance (NEDOR) experiments. Complete electron saturation was observed for a microwave frequency close to the centre frequency of the ESR line, and by using maximum power of the microwave source. The DNP build-up time at 4.64 T (approximately 3000 s) was prolonged by approximately a factor three over the build-up time at 3.35 T (approximately 1200 s). However, after approximately 20 min of microwave irradiation the polarization at 4.64 T exceeded the polarization at 3.35 T.

Hyperpolarization of 13C through order transfer from parahydrogen: a new contrast agent for MRI.

The order within proton pairs in organic molecules, resulting from hydrogenation with parahydrogen, 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, in relatively low magnetic field, we describe the procedure of order transfer to the 13C spins through a sudden drop from the initial field to zero field followed by an adiabatic remagnetization. The expected final polarizations in the absence of relaxation are given for several compounds. Finally, we show an example of MR images observed in vivo on animals as an illustration of the contrast capacity of this new method.