Magnetic Frequency encoding

Figure 3 describes for nine magnetization vectors the effect of the appHcation of a phase-encoding gradient, G, and a frequency-encoding gradient, G. ... 

Another issue of concern is the amplitude of magnetic field gradients required for solids imaging. For spectra up to a few kHz wide, standard frequency encoding schemes can be used to obtain reasonable spatial resolution. This will not work for broader lines, but it is well known that phase encoding is much more tolerant to line broadening and can be used for spectral widths in excess of 100 kHz with no major loss in spatial resolution. 

Caprihan, A., Davis, J.G., Altobelli, S.A., and Fukushima, E., A New Method for Flow Velocity Measurement Frequency Encoded NMR, Magnetic Resonance in Medicine, 2, 352-362 (1986). 

Figure Bl.14.12. Study of the temporal fluctuation of motion in a quail egg at the incubation times 119 h and 167 h. Spatial phase encoding of the y-dimension was omitted to increase the rapidity of the imaging experiment. Profiles were obtained by ID Fourier transformation of echoes which were acquired in the presence of a readout gradient for frequency encoding of the x-coordinate. A strong gradient pulse pair for (spatially) incoherent motion weighting was applied during the evolution period of the magnetization. A series of subsequent single-scan profiles were measured at the two different incubation times 119 h (a) and 167 h...

Figure 4 The chemical shift displacement artifact. (A) An applied linear magnetic gradient encodes spatial position in the resonant frequencies of two particular spectrum peaks (represented by the sloping lines). Peak 1 has a different chemical shift to peak 2. A selective RF pulse, centered on frequency and with bandwidth A/, will excite a slice at a different position for each peak as shown. (B) Increasing the strength of the linear magnetic gradient reduces the difference in slice position - however, to achieve slices with the same spatial width as in (A), a larger bandwidth RF pulse must be used.

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