Depth and refocusing pulses designed for multipulse NMR with surface coils

Abstract

When surface radiofrequency coils are used for irradiation and detection, the pulse angles are greatest in the plane of the coil and decrease rapidly with depth of penetration into the sample and this intrinsic spatial variation in pulse angle makes it difficult to use multipulse NMR techniques. However, it is shown that spin-echo pulse sequences may still be used provided the π refocusing pulses are cycled through all four phases as described by Bodenhausen et al. ( J. Magn. Reson. 27, 511 (1977)). This has the added advantage that signals are markedly reduced from sample regions where there is a large deviation from an ideal π pulse angle. Thus the π/2;π[±x, ±y];π- scheme also functions as a “depth” pulse sequence, and this property is preserved even when there are no τ delay periods. Additional depth discrimination is obtained by using additional phase-cycled π pulses, e.g., the “depth pulse”: π/2;π[±x, ±y];π[±x, ±y]. Analysis of the π/2 phase alternation introduced by Demco et al. ( J. Magn. Reson. 16, 467 (1974)) for inversion-recovery T 1 measurements shows that when this is combined with the above cycled π pulses, additional depth discrimination is obtained and accurate t 1 measurements may be made even when there are large divergences from the ideal pulse angles. Thus π;π/2[±x];π[±x,±y];π[±x, ±y] is a very good depth pulse, π-τ-π/2[±x]; π[±x, ±y];π[±x, ±y] may be used for inversion-recovery T 1, measurements and π;π/2[±x]; π[±x, ±y]-τ-π[±x, ±y]-τ-may be used for spin-echo sequences. Several simple spin-echo sequences are presented. These techniques help to relax the constraints on radio-frequency coil design imposed by the requirement to have good B 1, homogeneity and therefore may have more general application, especially in cases where the need for a large filling factor is at a premium.