Super-resolution Surface Microscopy of Conductors using Magnetic Resonance

The spatial resolution of traditional Magnetic Resonance Imaging (MRI) techniques is typically dictated by the strength of the applied magnetic field gradients, resulting in hard resolution limits of the order of 20-50 μm in favorable circumstances. We demonstrate here a technique which is suitable for the interrogation of regions at specified distances below the surface of conducting objects with a resolution well below these limiting values. This approach does not rely on magnetic field gradients, but rather on the spatial variation of the radiofrequency field within a conductor. Samples of aluminium and lithium metal with different sizes and morphologies are examined with this technique using ²⁷Al and ⁷Li NMR. In this implementation, the slice selectivity depends on the conductivity of the material, as well as on the frequency of operation, although in the most general case, the technique could also be used to provide spatial selectivity with arbitrary B ₁ field distributions in non-conductors.