Composite 90° and 180° Pulses to Compensate for Radiofrequency Gradients in Toroid NMR Detectors

Two new pulse sequences, a composite 90° pulse and a composite 180° pulse, have been developed to compensate for the extremely large radiofrequency gradients in toroid NMR detectors. The composite 90° pulse has the ability to transfer more than 98% of the equilibrium magnetization phase-correlated into the x-y plane of the rotating frame, even if the strongest B ₁ held is nine times the weakest. The composite 180° pulse attains 99% inversion of the equilibrium magnetization in the same B ₁ gradient. Trajectory calculations that follow the fate of the magnetization during the new pulse sequences and during composite pulses reported in the literature compared favorably with results derived from NMR experiments performed within a toroid cavity probe. Also, the T ₁ relaxation time of chloroform was measured 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 greater than two.