Phase lag in the amplitude modulated magnetic resonance Original Research Article

Response of a magnetic spin system to the amplitude modulated radiofrequency field is studied. Spin system is assumed to obey Bloch equations. For a spin system resonant at Larmor frequency ω0, calculations demonstrate how the phase lags Φ⊥ and Φz, of magnetization components Mx, My and Mz occur and how sensitive they are to the relaxation properties of the system. Relative to Larmor frequency the angular frequency of the amplitude modulation, Ω⪡ω0, is very low, at the same time it is comparable to the relaxation rates of the spins. With reference to the analytical solution of a damped harmonic oscillator a simple formula tanΦ⊥=T2Ω describes the magnitude of the phase lag Φ⊥ as a function of relaxation time T2.