Aspects of NMR Reciprocity and Applications in Highly Conductive Media

TL;DR

This paper clarifies the application of the NMR reciprocity principle, addressing conceptual confusions, and discusses its implications for experiments involving highly conductive media like metals.

Contribution

It provides a clear discussion on applying the NMR reciprocity principle, avoiding common misconceptions, especially in the context of conducting samples and metals.

Findings

Clarifies the correct application of the reciprocity principle in NMR.

Discusses implications for experiments with conductive media.

Avoids unnecessary complexity by excluding the rotating frame.

Abstract

In the context of NMR spectroscopy and MRI, the principle of reciprocity provides a convenient method for determining the reception sensitivity from the transmitted rf field pattern. The reciprocity principle for NMR was originally described by Hoult et al. [J. Magn. Reson. (1976),24, 71], and can be seen as being based on the broader Lorentz reciprocity principle, and similar theorems from antenna theory. One frequent application of the reciprocity principle is that for a single coil used for both transmission and detection, the transmit and receive fields can be assumed to be equal. This aspect is also where some of the conceptual difficulty of applying the theorem may be encountered. For example, the questions of whether one should use the complex conjugate field for detection, or whether one should apply the theorem in the rotating frame or the laboratory frame are often where considerable confusion may arise, and incorrect results may be derived. We attempt here to provide a helpful discussion of the application of the reciprocity principle in such a way as to clarify some of the confounding questions. In particular, we avoid the use of the `negatively rotating frame', which is frequently mentioned in this context, since we consider it to unnecessarily complicate the matter. In addition, we also discuss the implications of the theorem for magnetic resonance experiments on conducting samples, and metals, in particular.