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In the context of NMR spectroscopy and MRI, the principle of reciprocityprovides a convenient method for determining the reception sensitivity from thetransmitted rf field pattern. The reciprocity principle for NMR was originallydescribed by Hoult et al. [J. Magn. Reson. (1976),24, 71], and can be seen asbeing based on the broader Lorentz reciprocity principle, and similar theoremsfrom antenna theory. One frequent application of the reciprocity principle isthat for a single coil used for both transmission and detection, the transmitand receive fields can be assumed to be equal. This aspect is also where someof the conceptual difficulty of applying the theorem may be encountered. Forexample, the questions of whether one should use the complex conjugate fieldfor detection, or whether one should apply the theorem in the rotating frame orthe laboratory frame are often where considerable confusion may arise, andincorrect results may be derived. We attempt here to provide a helpfuldiscussion of the application of the reciprocity principle in such a way as toclarify some of the confounding questions. In particular, we avoid the use ofthe `negatively rotating frame', which is frequently mentioned in this context,since we consider it to unnecessarily complicate the matter. In addition, wealso discuss the implications of the theorem for magnetic resonance experimentson conducting samples, and metals, in particular.

Aspects of NMR Reciprocity and Applications in Highly Conductive Media