Adiabatic radiofrequency pulse forms in biomedical nuclear magnetic resonance

Adiabatic radio frequency (RF) pulses are in widespread use in biomedical magnetic resonance imaging and spectroscopy. The primary advantage of adiabatic pulses is that provided the condition for adiabaticity is satisfied they can be made insensitive to inhomogeneities in the RF field. In this pedagogical article the principles of adiabatic fast passage (AFP) are explained, and the use of AFP to invert both stationary and flowing spin systems is examined. The hyperbolic secant pulse is presented as a pulse capable of performing slice selective adiabatic inversion. Lower power alternatives to this pulse are described, and the principle of offset independent constant adiabaticity is elucidated. Instantaneous reversal of the orientation of the effective RF field as a means of producing excitation and refocusing pulses is presented, as are methods of obtaining slice selective excitation with these pulses.