NMR spectral selectivity by radio-frequency field gradients

It is shown that a train of radio-frequency field (B1) gradient pulses, separated by appropriate precession intervals, enables the selection of a given frequency zone, that is the refocusing of magnetizations whose resonance frequencies belong within this zone whereas complete defocusing occurs outside. The B1 gradient pulse length must be adjusted to produce a 180-degrees flip angle at the center of the sample (in practice, a mean 180-degrees pulse); more reliable results are obtained if the pulse train is applied to transverse magnetization rather than to longitudinal magnetization.