Magnetization vector signal phase

The transverse magnetization and the applied radiofrequency field will therefore periodically come in phase with one another, and then go out of phase. This causes a continuous variation of the magnetic field, which induces an alternating current in the receiver. Furthermore, the intensity of the signals does not remain constant but diminishes due to T and T2 relaxation effects. The detector therefore records both the exponential decay of the signal with time and the interference effects as the magnetization vectors and the applied radiofrequency alternately dephase and re-... 

As is evident from Fig. 5.26, we can have either a sinusoidal modulation or a cosinusoidal modulation of the signals, depending on whether the two 90° pulses have the same phase or different phases. In Fig. 5.26A, both pulses have the same phase. This means that in the first period, the magnetization vector precesses in the x 31 -plane, so if it has processed by... 

In real experiments after Fourier transformed the lineshapes are mixtures of absorptive and dispersive signals and are related to the delayed FID acquisition (first-order phase error). The delayed acquisition is a consequence of the minimum time required to change the spectrometer from transmit to receive mode, during this delay the magnetization vectors process according to their chemical shift frequencies. The zero-order phase error arises because of the phase difference between the magnetization vectors and the receiver. In NMR-SIM the delayed acquisition is not necessary because the ideal spectrometer approach does not require any switching time and the first order phase correction is normally zero if no other sources of phase deviations are present. 

This method of selecting a subset of the total signal is a simple but important example of phase cycling. Instrumentally, the subtraction is accomplished by alternating the phase of the receiver, although the phase of the 90° pulse can also be used to control along which axes (+y versus -y in this case) the various net magnetization vectors will ultimately point. 

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