Experimental study of optimal selective 180° radiofrequency pulses

Abstract

By solving an optimal control problem with the conjugate gradient algorithm, we previously obtained a set of optimal selective 180° inversion RF pulses. In this article we demonstrate by computer simulation that the magnetization responses to a selective 180° pulse are the same whether the pulse is used for refocusing or inversion. An optimal selective 180° inversion RF pulse can be used as a selective refocusing RF pulse; this is verified experimentally by a spin-echo pulse sequence. We also demonstrate by both simulation and experiment that there exists a minimum ratio of the width of a slice edge to the thickness of the selected slice at a certain slice thickness. Thus, “optimal” has a dual meaning here: a desired slice profile is optimized and the corresponding RF pulse is optimized. To obtain a slice with a sharper edge at a certain pulse length, an optimal selective pulse with a higher peak amplitude is required. The Fourier coefficients of four selective RF pulses are supplied in the Appendix.