Optimization of echo amplitudes resulting from a series of 90° pulses in an inhomogeneous static field

Journal of Magnetic Resonance
Andrew E Marble

Abstract

In an inhomogeneous static field, the pulse sequence 90°(x)-(τ-90°(y)-τ)(n) results in a train of echoes, the amplitudes of which settle to be proportional to M(0)/2 after a transient period. Analysis of the spin dynamics of the general τ-β°-τ refocusing cycle reveals that for ideal RF pulses, adding a preparation pulse followed by a delay to the above sequence can either eliminate the transient behavior, or increase the asymptotic echo intensity. This is achieved by controlling the alignment between the magnetization m and the rotation axis of the refocusing cycle. The effect of preparations pulses is demonstrated experimentally in the fringe field of a single sided magnet array. It is shown that for this instrument, transient effects in the echo train can be reduced, and asymptotic signal increased. Spin dynamics calculations indicate that the sequences are robust to finite RF pulse widths, but some discrepancy between theory and experiment is observed due to B(1) inhomogeneity. Refocusing sequences of the type studied here are useful in cases where experimental considerations, such as RF power limits, preclude the use of a 180° pulse in the refocusing cycle.

References

Jul 2, 1999·Journal of Magnetic Resonance·P M GloverP J McDonald
Jul 27, 2000·Journal of Magnetic Resonance·F BalibanuB Blumich
Sep 5, 2001·Journal of Magnetic Resonance·M D Hürlimann
Sep 27, 2003·Journal of Magnetic Resonance·M TodicaB Blümich
Feb 24, 2007·Journal of Magnetic Resonance·Andrew E MarbleBruce J Balcom

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Citations

Sep 23, 2014·Journal of Magnetic Resonance·Soumyajit MandalMartin D Hürlimann
Nov 18, 2015·Journal of Magnetic Resonance·Soumyajit MandalMartin D Hürlimann
Oct 16, 2013·Journal of Magnetic Resonance·Soumyajit MandalMartin D Hürlimann

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