Abstract
Spatial resolution in MRI is ultimately limited by the signal detection sensitivity of NMR, since resolution equal to ρiso in all three dimensions requires the detection of NMR signals from a volume ρiso3. With inductively detected NMR at room temperature, it has therefore proven difficult to achieve isotropic resolution better than ρiso = 3.0 μm, even with radio-frequency microcoils, optimized samples, high magnetic fields, optimized pulse sequence methods, and data acquisition times around 60 h. Here we show that spatial resolution can be improved and data acquisition times can be reduced substantially by performing MRI measurements at 5 K and using dynamic nuclear polarization (DNP) to enhance sensitivity. We describe the experimental apparatus and methods, and we report images of test samples with ρiso = 2.6 μm and ρiso = 1.7 μm, with signal-to-noise ratios greater than 15, acquired in 31.5 and 81.6 h, respectively. Image resolutions are verified by quantitative comparisons with simulations. These results establish a promising direction for high-resolution MRI of small samples. With further improvements in the experimental apparatus and in paramagnetic dopants for DNP, DNP-enhanced low-temperature MRI with ρiso < 1.0 μm is likely to become feasible, potentially enabling informative studies of structures within typical eukaryotic cells, cell clusters, and tissue samples.