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TheQuantumOriginsOfFreeInductionDecaySignalAndSpinNoise

The Quantum Origins Of Free Induction Decay Signal And Spin Noise

D.I.Hoult and N.S.Ginsberg, Journal of Magnetic Resonance, 148, 182-199, 2001.

Many people working in the field of MR believe that MR signal is generated via a process of stimulated emission, because this is what it says in many text books. This article explains that the observed MR signal is not an emission but a Faraday induction and how this confusion has arisen. The difference is best illustrated by considering the near and far field terms of Maxwells equations. An emission process would correspond to the far field term and has a different distance dependency to the near field term. This observation has implications for the scaling of the MR process in high field scanners. In particular the far-field term, while negligible at small field strength, may grow to dominate the near field signal due to a polynomial dependency on field strength, introducing an unwanted source of background.

NAT 8/3/2005

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