TY - JOUR

T1 - Use of composite refocusing pulses to form spin echoes

AU - Odedra, Smita

AU - Wimperis, Stephen

PY - 2012/1

Y1 - 2012/1

N2 - The radiofrequency pulses used in NMR are subject to a number of imperfections such as those caused by inhomogeneity of the radiofrequency (B-1) field and an offset of the transmitter frequency from precise resonance. The effect of these pulse imperfections upon a refocusing pulse in a spin-echo experiment can be severe. Many of the worst effects, those that distort the phase of the spin echo, can be removed completely by selecting the echo coherence pathway using either the "Exorcycle" phase cycle or magnetic field gradients. It is then tempting to go further and try to improve the amplitude of the spin-echo signal by replacing the simple refocusing pulse with a broadband composite 1800 pulse that compensates for the relevant pulse imperfection. We show here that all composite pulses with a symmetric or asymmetric phase shift scheme will reintroduce phase distortions into the spin echo, despite the selection of the echo coherence pathway. In contrast, all antisymmetric composite pulses yield no phase distortion whatsoever, both on and off resonance, and are therefore the correct symmetry of composite refocusing pulse to use. These conclusions are verified using simulations and P-31 MAS NMR spin-echo experiments performed on a microporous aluminophosphate. (C) 2011 Elsevier Inc. All rights reserved.

AB - The radiofrequency pulses used in NMR are subject to a number of imperfections such as those caused by inhomogeneity of the radiofrequency (B-1) field and an offset of the transmitter frequency from precise resonance. The effect of these pulse imperfections upon a refocusing pulse in a spin-echo experiment can be severe. Many of the worst effects, those that distort the phase of the spin echo, can be removed completely by selecting the echo coherence pathway using either the "Exorcycle" phase cycle or magnetic field gradients. It is then tempting to go further and try to improve the amplitude of the spin-echo signal by replacing the simple refocusing pulse with a broadband composite 1800 pulse that compensates for the relevant pulse imperfection. We show here that all composite pulses with a symmetric or asymmetric phase shift scheme will reintroduce phase distortions into the spin echo, despite the selection of the echo coherence pathway. In contrast, all antisymmetric composite pulses yield no phase distortion whatsoever, both on and off resonance, and are therefore the correct symmetry of composite refocusing pulse to use. These conclusions are verified using simulations and P-31 MAS NMR spin-echo experiments performed on a microporous aluminophosphate. (C) 2011 Elsevier Inc. All rights reserved.

KW - Composite pulses

KW - Symmetry

KW - B-1 inhomogeneity

KW - Resonance offset

KW - Phase cycling

KW - Pulse imperfections

KW - NMR POPULATION-INVERSION

KW - BROAD-BAND

KW - ITERATIVE SCHEMES

KW - NARROW-BAND

KW - COMPENSATION

KW - SEQUENCES

KW - IMPERFECTIONS

KW - SUPPRESSION

U2 - 10.1016/j.jmr.2011.10.006

DO - 10.1016/j.jmr.2011.10.006

M3 - Journal article

VL - 214

SP - 68

EP - 75

JO - Journal of Magnetic Resonance

JF - Journal of Magnetic Resonance

SN - 1090-7807

ER -