Temperature-Dependent Nuclear Spin Relaxation Due to Paramagnetic Dopants Below 30 K: Relevance to DNP-Enhanced Magnetic Resonance Imaging

Abstract

Dynamic nuclear polarization (DNP) can increase nuclear magnetic resonance (NMR) signal strengths by factors of 100 or more at low temperatures. In magnetic resonance imaging (MRI), signal enhancements from DNP potentially lead to enhancements in image resolution. However, the paramagnetic dopants required for DNP also reduce nuclear spin relaxation times, producing signal losses that may cancel the signal enhancements from DNP. Here we investigate the dependence of 1H NMR relaxation times, including T1ρ and T2 under conditions of Lee-Goldburg 1H-H decoupling and pulsed spin locking, on temperature and dopant concentration in frozen solutions that contain the trinitroxide compound DOTOPA. We find that relaxation times become longer at temperatures below 10 K, where DOTOPA electron spins become strongly polarized at equilibrium in a 9.39 T magnetic field. We show that the dependences of relaxation times on temperature and DOTOPA concentration can be reproduced qualitatively (although not quantitatively) by detailed simulations of magnetic field fluctuations due to flip-flop transitions in a system of dipole-coupled electron spin magnetic moments. These results have implications for ongoing attempts to reach submicron resolution in inductively-detected MRI at very low temperatures.

Enhanced spatial resolution in magnetic resonance imaging by dynamic nuclear polarization at 5 K

“…This analysis indicates several directions for further improvements: 1) Lower sample temperatures may lead to slower signal decays during τ LG and t PSL ( 34 ), resulting in larger signals; 2) a cryogenic preamplifier may result in substantially lower noise levels; 3) paramagnetic dopants that are more efficient at very low temperatures may allow smaller values of τ DNP and provide larger values of ε DNP . More efficient dopants may also allow higher protonation levels.…”

“…The sequence began with a train of 64 RF pulses to destroy 1 H spin polarization and suppress background signals (200-μs pulse lengths, 156-kHz RF amplitude, 1.0-ms delay between pulses). Microwaves were then applied for a period τ DNP , optimized as discussed below, followed by a 100-ms delay for reequilibration of electron spin polarization to maximize transverse 1 H spin relaxation times ( 34 ). To create a field of view in the x direction ( FOV x ) that was experimentally feasible, as discussed below, slice selection in the x direction was performed by applying a train of eight π pulses in the presence of a strong x gradient pulse (1.12 kHz/μm), with 50-μs delays for dephasing of transverse 1 H polarization between pulses and with RF amplitudes alternating between 167 and 83 kHz.…”

“…Fluctuating hyperfine fields from these compounds produce large reductions in transverse 1 H spin relaxation times ( T 2H ), again leading to signal losses during phase-encoding and NMR detection periods that cancel out the signal gains from DNP. These signal losses can be minimized by operating at T < 10 K, where the flip-flop transitions among unpaired electron spins that produce short T 2H values are largely suppressed because the electron spins become strongly polarized ( 34 ). A third complication, discussed further below, is that build-up times for nuclear spin polarization (τ DNP ) are long at low temperatures, increasing the time between scans during MRI data acquisition.…”

Advanced high resolution three-dimensional imaging to visualize the cerebral neurovascular network in stroke

“…At low temperatures of tens of degrees Kelvin, the strength of the nuclear magnetic resonance signal can be enhanced by a large spin polarization at thermal equilibrium, thus reducing thermal noise. Meanwhile, dynamic nuclear polarization (DNP) can transfer polarization from electron spin to nuclear spin, and finally, obtain a resolution better than 1 μm [190,191]. The whole brain MRI scan was reduced to 11 minutes, showing both nerve and vascular lesions at the same time [191].…”

“…Meanwhile, dynamic nuclear polarization (DNP) can transfer polarization from electron spin to nuclear spin, and finally, obtain a resolution better than 1 μm [190,191]. The whole brain MRI scan was reduced to 11 minutes, showing both nerve and vascular lesions at the same time [191]. Robert Tycko et al obtained the 1 H MRI image with an isotropic resolution of 2.8 µm in glycerin/water at 28 K [192].…”