Optimization of two-dimensional spatially selective NMR pulses by simulated annealing

Two-dimensional spatial localization can be achieved with a single NMR inverting or refocusing pulse, by applying the RF pulse in the presence of a net magnetic field gradient which reorients through two dimensions during the course of the pulse. Simple rotating ("ρ{variant}") pulses of this type, however, have the disadvantage of producing ringing outside the central selected volume of the 2D sensitivity profile. In order to eliminate this ringing, a simulated annealing technique has been used to simultaneously optimize the RF and gradient waveforms, producing pulses which leave the NMR signal localized to a well-defined cylinder. These pulses have been implemented on a whole-body NMR system to produce restricted field-of-view images of agarose disk phantoms and of the head. Applications of optimized ρ{variant} pulses include localization in ³¹P spectroscopy and elimination of aliasing artifacts in NMR imaging.