Absolute phase effects on CPMG-type pulse sequences

Soumyajit Mandal; Sangwon Oh; Martin D Hürlimann

doi:10.1016/j.jmr.2015.10.014

Absolute phase effects on CPMG-type pulse sequences

J Magn Reson. 2015 Dec:261:121-32. doi: 10.1016/j.jmr.2015.10.014. Epub 2015 Oct 31.

Authors

Soumyajit Mandal¹, Sangwon Oh², Martin D Hürlimann³

Affiliations

¹ Case Western Reserve University, Cleveland, OH 44106, USA; Schlumberger-Doll Research, Cambridge, MA 02139, USA. Electronic address: sxm833@case.edu.
² Yale University, New Haven, CT 06520, USA; Schlumberger-Doll Research, Cambridge, MA 02139, USA. Electronic address: sangwon.oh@yale.edu.
³ Schlumberger-Doll Research, Cambridge, MA 02139, USA. Electronic address: hurlimann@slb.com.

PMID: 26575106
DOI: 10.1016/j.jmr.2015.10.014

Abstract

We describe and analyze the effects of transients within radio-frequency (RF) pulses on multiple-pulse NMR measurements such as the well-known Carr-Purcell-Meiboom-Gill (CPMG) sequence. These transients are functions of the absolute RF phases at the beginning and end of the pulse, and are thus affected by the timing of the pulse sequence with respect to the period of the RF waveform. Changes in transients between refocusing pulses in CPMG-type sequences can result in signal decay, persistent oscillations, changes in echo shape, and other effects. We have explored such effects by performing experiments in two different low-frequency NMR systems. The first uses a conventional tuned-and-matched probe circuit, while the second uses an ultra-broadband un-tuned or non-resonant probe circuit. We show that there are distinct differences between the absolute phase effects in these two systems, and present simple models that explain these differences.

Keywords: Absolute phase; CPMG; Phase transients; Pulse synchronization.