Micron-scale magnetic resonance imaging with dynamic nuclear polarization at 5 K

Spatial resolution in inductively-detected magnetic resonance imaging (MRI) is limited by the relatively low sensitivity of conventional nuclear magnetic resonance (NMR). At room temperature, isotropic resolution in three-dimensional (3D) ¹H MRI better than about 3.0 μm has proven difficult to achieve, even with microcoils, optimized radio-frequency pulse sequences, paramagnetic doping, and long measurement times. In principle, NMR sensitivity and consequently MRI resolution can be improved greatly by operating at low temperatures, especially temperatures well below 100 K where large nuclear spin hyperpolarizations can be achieved by cross-effect dynamic nuclear polarization (DNP). At low temperatures, strong static dipole-dipole couplings among ¹H nuclear spins broaden the NMR signals, potentially abrogating any sensitivity advantages. However, deleterious effects of ¹H-¹H dipole-dipole couplings can be largely overcome by pulsed spin-lock detection and by homonuclear decoupling through Lee-Goldburg irradiation. Results of recent experiments that demonstrate these principles will be described. In particular, in experiments at 28 K without DNP and 5 K with DNP, we have acquired 3D images of test samples with isotropic resolution of 2.8 μm and 1.7 μm, respectively. Likely avenues for further progress will also be discussed. With improvements in DNP dopants (a.k.a. "polarizing agents") and receiver electronics, isotropic resolution equal to 1.0 μm or less in 3D ¹H MRI seems attainable for total sample volumes on the order of 10⁵ μm³. Useful applications in imaging of single biological cells or cell clusters may then become possible.