Radical-induced hetero-nuclear mixing and low-field 13^{13}C relaxation in solid pyruvic acid

Radicals serve as a source of polarization in dynamic nuclear polarization, but may also act as polarization sink, in particular at low field. Additionally, if the couplings between the electron spins and different nuclear reservoirs are stronger than any of the reservoirs’ couplings to the lattice, radicals can mediate hetero-nuclear polarization transfer. Here, we report radical-enhanced $^{13}$C relaxation in pyruvic acid doped with trityl. Up to 40 K, we find a linear carbon $T_{1}$ field dependence between 5 mT and 2 T. We model the dependence quantitatively, and find that the presence of trityl accelerates direct hetero-nuclear polarization transfer at low fields, while at higher fields $^{13}$C relaxation is diffusion limited. Measurements of hetero-nuclear polarization transfer up to 600 mT confirm the predicted radical-mediated proton–carbon mixing

Radical-Induced Low-Field 1H Relaxation in Solid Pyruvic Acid Doped with Trityl-OX063

In dynamic nuclear polarization (DNP), radicals such as trityl provide a source for high nuclear spin polarization. Conversely, during the low-field transfer of hyperpolarized solids, the radicals’ dipolar or Non-Zeeman reservoir may act as a powerful nuclear polarization sink. Here, we report the low-temperature proton spin relaxation in pyruvic acid doped with trityl, for fields from 5 mT to 2 T. We estimate the heat capacity of the radical Non-Zeeman reservoir experimentally and show that a recent formalism by Wenckebach yields a parameter-free, yet quantitative model for the entire field range

Radical-induced Hetero-Nuclear Mixing and Low-field 13^{13}C Relaxation in Solid Pyruvic Acid

Radicals serve as source in dynamic nuclear polarization, but may also act as polarization sink. If the coupling between the electron spins and different nuclear reservoirs is stronger than any of the reservoirs' couplings to the lattice, radicals can mediate hetero-nuclear mixing. Here, we report radical-enhanced $^{13}$C relaxation in pyruvic acid doped with trityl. We find a linear dependence of the carbon $T_1$ on field between 5 mT and 2 T. We extend a model, employed previously for protons, to carbon, and predict efficient proton-carbon mixing via the radical Non-Zeeman reservoir, for fields from 20 mT to beyond 1 T. Discrepancies between the observed carbon relaxation and the model are attributed to enhanced direct hetero-nuclear mixing due to trityl-induced linebroadening, and a field-dependent carbon diffusion from the radical vicinity to the bulk. Measurements of hetero-nuclear polarization transfer up to 600 mT confirm the predicted mixing as well as both effects inferred from the relaxation analysis.Comment: 6 pages, 3 figure

Deuterium metabolic imaging and hyperpolarized 13C-MRI of the normal human brain at clinical field strength reveals differential cerebral metabolism.

Deuterium metabolic imaging (DMI) and hyperpolarized 13C-pyruvate MRI (13C-HPMRI) are two emerging methods for non-invasive and non-ionizing imaging of tissue metabolism. Imaging cerebral metabolism has potential applications in cancer, neurodegeneration, multiple sclerosis, traumatic brain injury, stroke, and inborn errors of metabolism. Here we directly compare these two non-invasive methods at 3 T for the first time in humans and show how they simultaneously probe both oxidative and non-oxidative metabolism. DMI was undertaken 1-2 h after oral administration of [6,6'-2H2]glucose, and 13C-MRI was performed immediately following intravenous injection of hyperpolarized [1-13C]pyruvate in ten and nine normal volunteers within each arm respectively. DMI was used to generate maps of deuterium-labelled water, glucose, lactate, and glutamate/glutamine (Glx) and the spectral separation demonstrated that DMI is feasible at 3 T. 13C-HPMRI generated maps of hyperpolarized carbon-13 labelled pyruvate, lactate, and bicarbonate. The ratio of 13C-lactate/13C-bicarbonate (mean 3.7 ± 1.2) acquired with 13C-HPMRI was higher than the equivalent 2H-lactate/2H-Glx ratio (mean 0.18 ± 0.09) acquired using DMI. These differences can be explained by the route of administering each probe, the timing of imaging after ingestion or injection, as well as the biological differences in cerebral uptake and cellular physiology between the two molecules. The results demonstrate these two metabolic imaging methods provide different yet complementary readouts of oxidative and reductive metabolism within a clinically feasible timescale. Furthermore, as DMI was undertaken at a clinical field strength within a ten-minute scan time, it demonstrates its potential as a routine clinical tool in the future