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It is true that in a hypothetical system where we are observing the behavior of hydrogen nuclei (spins) in the presence of an externally applied magnetic field (B0), that the spins will be more likely to assume the same orientation as B0. Thus, the net magnetization present (i.e., the NMV) without any other perturbation will have the same orientation as B0. This, I believe, is what you are all expecting. However, this scenario ONLY pertains in the absence of other effects on the magnetic field B0. Specifically, in a diamagnetic environment (i.e., where the other stuff in the sample, aside from spins, has magnetic susceptibility <0) the resting state is altered so that the preferred (lower energy) orientation is opposite (“antiparallel”) to B0. Biological tissues in general and human beings in particular are highly diamagnetic environments. Thus, in a real life clinical imaging scenario, the resting NMV will have an antiparallel orientation.

Answer #1: FAQ
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