Use of composite refocusing pulses to form spin echoes

Odedra, S. and Wimperis, S. (2012) Use of composite refocusing pulses to form spin echoes. Journal of Magnetic Resonance, 214, pp. 68-75. (doi: 10.1016/j.jmr.2011.10.006)

Abstract

The radiofrequency pulses used in NMR are subject to a number of imperfections such as those caused by inhomogeneity of the radiofrequency (B1) field and an offset of the transmitter frequency from precise resonance. The effect of these pulse imperfections upon a refocusingpulse in a spin-echo experiment can be severe. Many of the worst effects, those that distort the phase of the spinecho, 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 refocusingpulse with a broadband composite 180° pulse that compensates for the relevant pulse imperfection. We show here that all compositepulses with a symmetric or asymmetric phase shift scheme will reintroduce phase distortions into the spinecho, despite the selection of the echo coherence pathway. In contrast, all antisymmetric compositepulses yield no phase distortion whatsoever, both on and off resonance, and are therefore the correct symmetry of compositerefocusingpulse to use. These conclusions are verified using simulations and 31P MAS NMR spin-echo experiments performed on a microporous aluminophosphate.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Odedra, Dr Smita and Wimperis, Professor Stephen
• Authors: Odedra, S., and Wimperis, S.
• College/School: College of Science and Engineering > School of Chemistry
• Journal Name: Journal of Magnetic Resonance
• Journal Abbr.: J. Magn. Reson
• ISSN: 1090-7807
• ISSN (Online): 1096-0856
• Published Online: 18 October 2011