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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 B1 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. |
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