NMR signal reception: Virtual photons and coherent spontaneous emission

Abstract

In portions of the magnetic resonance community, there is a misunderstanding of the process of nuclear magnetic resonance (NMR) signal generation and reception, and even in accepted texts, it is frequently described in terms of absorption and emission of radio waves, or radiation, by a two-level quantum system. While this explanation can be refuted, for those who do understand that the NMR free induction decay signal is easily explained by Faraday's law of induction, reconciling the presence of an induced electromotive force with an apparent absence of transitions between nuclear energy levels causes conceptual problems. This difficulty is examined, and an explanation of the signal given whereby Faraday's law is explained simply in terms of an exchange of virtual photons. The article thus attempts to reconcile the standard engineering approach to signal reception with a quantum mechanical description of the NMR phenomenon. Radiation damping and its relations to detection of the induced signal and to signal-to-noise ratio are then scrutinized, and the misleading nature of the appellation noted. In the process, it is shown that while damping is inherently necessary for signal detection and the transfer of energy that it entails, the degree of such damping depends on the efficiency of the detector—in other words, of the preamplifier in the NMR receiver—and can therefore be minimized. Mathematics in the article is kept to a minimum; proofs of the Principle of Reciprocity description of Faraday's law for reception of both signal and noise from a conducting sample are given in an appendix. ©1997 John Wiley & Sons, Inc. Concepts Magn Reson 9: 277–297, 1997