COMPOSITE 90-DEGREES AND 180-DEGREES PULSES TO COMPENSATE FOR RADIOFREQUENCY GRADIENTS IN TOROID NMR DETECTORS

Two new pulse sequences, a composite 90 degrees pulse and a composite 180 degrees pulse, have been developed to compensate for the extremely large radiofrequency gradients in toroid NMR detectors. The composite 90 degrees pulse has the ability to transfer more than 98% of the equ ilibrium magnetization phase-correlated into the x-y plane of the rota ting frame, even if the strongest B-1 held is nine times the weakest. The composite 180 degrees pulse attains 99% inversion of the equilibri um magnetization in the same B-1 gradient. Trajectory calculations tha t follow the fate of the magnetization during the new pulse sequences and during composite pulses reported in the literature compared favora bly with results derived from NMR experiments performed within a toroi d cavity probe. Also, the T-1 relaxation time of chloroform was measur ed by using the inversion-recovery procedure in a toroid cavity probe. When the standard pulses were substituted by the new composite pulses , the dynamic range in signal intensity was increased by a factor grea ter than two. (C) 1995 Academic Press, Inc.