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Profiles of paint layer samples obtained in the fringe field of a high field magnet by means of very short broadband frequency modulated pulses.

In this article we describe the acquisition of depth profiles, in particular of paint layers, in the static gradient of a high field magnet, providing a superior sensitivity. The main objective are reference profiles that help to understand scans made with non-invasive unilateral NMR, which often suffers from poor signal-to-noise ratio when working with real samples. Various technical aspects like the coil geometry and the limit of resolution are investigated. A major advancement is the use of frequency modulated pulses that are very broadband and at the same time very short (25 μs). The latter is necessary to allow the acquisition of a CPMG echo train of old, rigid paint material. Despite being far from adiabatic, they provide uniform excitation and refocusing over 1 MHz, which corresponds to about 400 μm with the used gradient. We show that the uniformity is even sufficient to obtain biexponential relaxation profiles. With these tools a paint sample from a restoration campaign is analyzed with different contrast criteria: the original and two layers from former restoration attempts can be visualized, and furthermore the relaxation profiles allow to study the migration of plasticizing molecules.