Visualizing skin effects in conductors with MRI: 7Li MRI experiments and calculations

Abstract

While experiments on metals have been performed since the early days of NMR (and DNP), the use of bulk metal is normally avoided. Instead, often powders have been used in combination with low fields, so that skin depth effects could be neglected. Another complicating factor of acquiring NMR spectra or MRI images of bulk metal is the strong signal dependence on the orientation between the sample and the radio frequency (RF) coil, leading to non-intuitive image distortions and inaccurate quantification. Such factors are particularly important for NMR and MRI of batteries and other electrochemical devices. Here, we show results from a systematic study combining RF field calculations with experimental MRI of 7 Li metal to visualize skindepth effects directly and to analyze the RF field orientation effect on MRI of bulk metal. It is shown that a certain degree of selectivity can be achieved for particular faces of the metal, simply based on the orientation of the sample. By combining RF field calculations with bulk magnetic susceptibility calculations accurate NMR spectra can be obtained from first principles. Such analyses will become valuable in many applications involving battery systems, but also metals, in general.

Visualizing electromagnetic fields in metals by MRI

“…[12][13][14]17,[22][23][24] Interestingly, the 3d CSI sheds new light on previous bulk metal NMR studies. For instance, in an earlier study, 12 a similar shift difference between NMR peaks was observed at and ⊥ orientations (relative to B 0 ) of the major faces of a thinner metal strip, by carrying out two separate experiments.…”

“…Moreover, the analysis presented here may throw light on previous bulk metal MRI studies. For instance, consider the MRI in orientation in an earlier study: 14 with c = 10mm and ∆y = 31µm, Eq. (5) yields S ac /S ab = 161, rendering the contribution from the ab faces all but invisible in those images.…”

“…Chief among them are the rapidly decaying B 1 inside the metal (Fig.1), and orientation of the bulk metal surface relative to incident B 1 . [9][10][11] Recent studies [12][13][14][15][16][17][18] have overcome these difficulties to demonstrate and highlight bulk metal NMR and MRI, albeit, primarily applied to batteries and electrochemical cells (section S2).…”

“…Since all MRI and CSI images to follow were acquired with the given phantom's bc faces normal (⊥, perpendicular) to B 1 , the images have no contribution from these faces to the magnetic resonance (MR) signal, [13][14][15][16] since B 1 penetration into the metal is maximal when it is parallel ( ) to the metal surface, and minimal when metal surface. 9,13,14 For details on the MRI experiments, including the nomenclature, the reader is referred to Methods section S1.2.…”

“…9,13,14 For details on the MRI experiments, including the nomenclature, the reader is referred to Methods section S1.2. It is straightforward to infer that the walls of high intensity regions in either image emanate from ac faces of the P3 strip, [13][14][15] as we did while measuring the thickness of metal strips (Figs.S2 and S3, section S4). In either image, contributions from spatially unresolved dimensions sum up to yield the high intensity walls.…”