In MRI after applying 90 RF puls magnetic momentum vectors of protons flip to xy plan. I can understand this from classical mechanic point of view. But I cant understand it from quantum mechanic view. becuase in quantum mechanic any spin must always have a component on quantization direction. But after 90 puls its picture on quantization axis become zero
This is achieved by applying an r.f. (radio-frequency) field. For details, consult [1] (in particular Sections 2.6 and 2.7, and Chapters 8 and 10 herein).
[1] MH Levitt, Spin Dynamics: Basics of Nuclear Magnetic Resonance, 2nd edition (John Wiley & Sons, Chichester, West Sussex, 2008).
The operations described refer to average values, in quantum mechanics. Hence no problem.
Even more simply: it is nothing else but statistics. After the pulse one half of protons' spins are up and the other half are down.
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