Iterative carr-purcell trains

Abstract

Iterative schemes have provided a new way to devise complex sequences of RF pulses. In broadband spin decoupling they offer a significant reduction in RF power deposition for a given operating bandwidth (I, 2) and much better tolerance of instrumental imperfections (3, 4). New composite pulses for broadband, narrowband, or band-pass inversion (5-8) or excitation (9, IO) have also been discovered. An iterative scheme defines an entire family of composite pulses at once, since many possibilities exist for the starting sequence and the iterative procedure can be terminated at different stages. It is, of course, necessary to terminate the procedure at some stage to obtain a composite pulse of finite duration. In this communication we show that iterative schemes can also be interleaved with data acquisition as multiple-pulse sequences and, as an example, discuss an iterative version of the Carr-Purcell train (II). Unlike the Meiboom-Gill modification (12), our sequences stabilize long-term behavior of both components of transverse magnetization under the sequence of 180” refocusing pulses. This multiple-pulse scheme defines an entire family of echo trains depending on the nature of the inversion pulse and degree of iteration; we demonstrate the technique using conventional 180” pulses. Neglecting relaxation, physical transport of the spins, and any time-dependent fluctuations in field, frequency, or amplitude, each 180” pulse in the echo train can be characterized by a unitary operator R and the pulses are identical except for the choice of RF phase. Pulses of different phase differ by a rotation about the z axis of the rotating reference frame, e.g., l? = exp(-i7rIZ)R exp(ia1,)