Frequency-modulated (FM) pulses that function according to adiabatic principles are becoming increasingly popular in many areas of NMR. Often adiabatic pulses can extend experimental capabilities and minimize annoying experimental imperfections. Here, adiabatic principles and some of the current methods used to create these pulses are considered. The classical adiabatic rapid passage, which is a fundamental element upon which all adiabatic pulses and sequences are based, is analyzed using vector models in different rotating frames of reference. Two methods to optimize adiabaticity are described, and ways to tailor modulation functions to best satisfy specific experimental needs are demonstrated. Finally, adiabatic plane rotation pulses and frequency-selective multiple spin-echo sequences are considered.
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The authors gratefully acknowledge many contributions to this work by our colleagues Drs. A. Tannus, R. A. de Graaf, T.-L. Hwang, Y. Ke, A. J. Johnson, P. C. M. van Zijl, R. Gruetter, and K. Ugurbil. We also thank Mr. David Busch for valuable discussions and for producing some of the illustrations. This work was supported by NIH Grants RR08079, CA64338 and CA79732.