Irradiation of an oscillating electromagnetic field Absorption Resonance condition rf frequency has to match Larmor frequency rf energy has to match energy difference between a and B level cos(at a linear oscillating field B, cos(ot) is identical B, co at+ sin(at) to the sum of two counter-rotating components, 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 B1) about the 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 ()magnetization can be easily described(keeping in mind the gyroscopic nature of spins) Polarisation(M) Coherence(M The flip angle B of the rf pulse depends on its field strength Bi and duration t =yB p10 Irradiation of an oscillating electromagnetic field Absorption Resonance condition: rf frequency has to match Larmor frequency = rf energy has to match energy difference between a and b level. a linear oscillating field B1 cos(wt) is identical to the sum of two counter-rotating components, 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 B1) 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 (!) magnetization can be easily described (keeping in mind the gyroscopic nature of spins): The flip angle b of the rf pulse depends on its field strength B1 and duration t: b = gB1 tp [2-11] Polarisation (M ) z Coherence (-M ) y