Hyperpolarized silicon particles have been shown to exhibit long spin-lattice relaxation times at room temperature, making them interesting as novel MRI probes. Demonstrations of hyperpolarized silicon particle imaging have focused on large micron size particles (average particle size (APS) = 2.2 μm) as they have, to date, demonstrated much larger polarizations than nanoparticles. We show that also much smaller silicon-29 particles (APS = 55 ± 12 nm) can be hyperpolarized with superior properties. A maximum polarization of 12.6% in the solid state is reported with a spin-lattice relaxation time of 42 min at room temperature thereby opening a new window for MRI applications.

Dissolution DNP has become an important method to generate highly polarized substrates such as pyruvic acid for in vivo imaging and localized spectroscopy. In a quest to further increase the polarization levels, which is important for in vivo MRI employing C detection, we describe the design and implementation of a new DNP polarizer that is suitable for dissolution operation at 7 T static magnetic field and a temperature of 1.4 K. We describe all important sample preparation steps and experimental details necessary to optimize trityl based samples for use in our polarizer at this higher field. In [1-C]-pyruvic acid polarization levels of about 56% are achieved, compared to typical polarization levels of about 35-45% at a standard field of 3.4 T. At the same time, the polarization build-up time increases significantly from about 670 s at 3.4 T to around 1300-1900 s at 7 T, depending on the trityl derivate used. We also investigate the effect of adding trace amounts of Gd to the samples. While one trityl compound does not exhibit any benefit, the other profits significantly, boosting achievable polarization by 6%.

Due to the inherently long relaxation time of C spins in diamond, the nuclear polarization enhancement obtained with dynamic nuclear polarization can be preserved for a time on the order of about one hour, opening up an opportunity to use diamonds as a new class of long-lived contrast agents. The present communication explores the feasibility of usingC spins in directly hyperpolarized diamonds for MR imaging including considerations for potential in vivo applications.

Micro-and nanoparticles of elemental, crystalline silicon represent an attractive target for a wide range of applications spanning from quantum computing to contrast agents for biomedical imaging applications. To overcome the low sensitivity of the 29 Si nuclei in magnetic resonance, dynamic nuclear polarization (DNP), which exploits the endogenous surface defects as a source of polarization, can be used to temporarily boost nuclear polarization of the 29 Si spin bath. In the present work, we have assessed a number of commercially available silicon micro-and nanoparticles concerning properties and characteristics under DNP conditions. It has been found that optimal physical and chemical conditions, including surface-defect concentration adjusted to the particle size, are necessary to achieve a high level of polarization enhancement.

The spin dynamics of dissolution DNP samples consisting of 4.5 M [ 13 C]urea in a mixture of (1/1)Vol glycerol/water using 4-Oxo-TEMPO as a radical was investigated. We analyzed the DNP dynamics as function of radical concentration at 7 T and 3.4 T static magnetic field as well as function of deuteration of the solvent matrix at the high field. The spin dynamics could be reproduced in all cases, at least qualitatively, by a thermodynamic model based on spin temperatures of the nuclear Zeeman baths and an electron non-Zeeman (dipolar) bath. We find, however, that at high field (7 T) and low radical concentrations (25 mM) the nuclear spins do not reach the same spin temperature indicating a weak coupling of the two baths. At higher radical concentrations, as well as for all radical concentrations at low field (3.4 T), the two nuclear Zeeman baths reach the same spin temperature within experimental errors. Additionally, the spin system was prepared with different initial conditions. For these cases, the thermodynamic model was able to predict the time evolution of the system well.While the DNP profiles do not give clear indications to a specific polarization transfer mechanism, at high field (7 T) increased coupling is seen. The EPR line shapes cannot clarify this in absence of ELDOR type experiments, nevertheless DNP profiles and dynamics under frequency-modulated microwave irradiation illustrate the expected increase in coupling between electrons with increasing radical concentration.