Stacked planar micro coils for single-sided NMR applications

Abstract

This paper describes planar micro structured coils fabricated in a novel multilayer assembly for single-sided NMR experiments. By arranging the coil’s turns in both lateral and vertical directions, all relevant coil parameters can be tailored to a specific application. To this end, we implemented an optimization algorithm based on simulations applying finite element methods (FEMs), which maximizes the coil’s sensitivity and thus signal-to-noise ratio (SNR) while incorporating boundary conditions such as the coil’s electrical properties and a localized sensitivity needed for single-sided applications. Utilizing thin-film technology and microstructuring techniques, the planar character is kept by a sub-millimeter overall thickness. The coils are adapted to the Profile NMR-MOUSE® magnet with a homogeneous slice of about 200μm in height and a uniform depth gradient of about 20T/m. The final design of a coil with 20 turns, separated in four layers with five turns each, and an outer dimension of 4×4mm2 is able to measure a sample volume almost five times smaller than that of a state-of-the-art 14×16mm2 Profile NMR-MOUSE® coil with the same SNR. This allows for volume-limited measurements with high SNR and enables different future developments. The minimal dead time of 4μs facilitates further improvements of the SNR by echo adding techniques and the characterization of samples with short T2 relaxation times. Measurements on solid polymers like polyethylene (PE) and polypropylene (PP) with T2 components as short as 200μs approve the overall beneficial coil properties. Furthermore the ability to perform depth profiling with microscopic resolution is demonstrated.