IMPROVEMENT OF Z FILTERS AND PURGING PULSES BY THE USE OF ZERO-QUANTUM DEPHASING IN INHOMOGENEOUS B1 OR B0 FIELDS

Important elements of many multiple-pulse NMR experiments are z filters and purging pulses. Such pulse sequence elements are used to select a single component of magnetization and hence ensure that the resulting spectra have both pure-phase lineshapes and undistorted multiplet structures. However, certain kinds of antiphase magnetization are not eliminated by conventional z filters and purging pulses, resulting in phase-distorted multiplets. Some solutions to this problem have been proposed, but none of these are entirely satisfactory. In this paper it is shown that these deficiencies of z filters and purging pulses have the same fundamental origin. It is then shown that a period of spin locking in an inhomogeneous B0 or B1 field can be used to form near-perfect purging pulses and, by extension, z filters. These sequences are examined theoretically, and predictions as to the optimum parameters which should be used are given. For dephasing in a B1 gradient, special composite pulse sequences are given which give more rapid dephasing than continuous-wave spin locking. Examples of how to incorporate these modified purging pulses and z filters into commonly encountered experiments are given and experimental results shown. © 1993 by Academic Press, Inc.