requency has to match Larmor frequency = rf energy has to match energy difference between α and β level. a linear oscillating field B 1cos(ωt) is identical to the sum of two counter-ponents, one being exactly in resonance with the precessing spins. Rotating coordinate system Switching from the lab coordinate system to one rotating "on resonance" with the spins (and B 1) about the z axis results in both being static. Generally all vector descriptions, rf pulses etc. are using this rotating coordinate system! Now the effect of an rf irradiation (a pulse) on the macroscopic (!) ization can be easily described (keeping in mind the gyroscopic nature of spins): The flip angle β of the rf pulse depends on its field strength B 1 and duration τ: β= γB 1τp [2-11] Polarisation (M) Coherence (-M)
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