Elimination of high-flux signals near surface coils and field gradient sample localization using depth pulses

Abstract

The previously described depth pulse sequences, 2 θ; θ[± x]; (2 θ;[± x, ± y]) n ; acquire signal (M. R. Bendall and R. E. Gordon, J. Magn. Reson. 53, 365 (1983)) for use in in vivo spectroscopy discriminate against sample regions where the sequence pulse angle, Φ, differs markedly from mπ 2 ( m an odd integer). It is shown theoretically and experimentally that high-flux signals close to the coil wire where θ; is close to 270 or 450°, may be eliminated efficiently by varying the θ; pulse length relative to the 2θ; pulse length in the depth pulses. The generality of use of depth pulses is further extended by showing experimentally that at low pulse power the depth pulses are cleanly selective with respect to chemical shift. This selectivity is improved by use of a spin-locked period subsequent to the depth pulses but prior to signal acquisition, and may be converted to spatial selectivity by applying the depth pulses in the presence of a magnetic field gradient. Application of the spin-locking field enables the field gradient to be switched off and a fully localized high-resolution spectrum may be acquired. The elimination of high-flux signals and/or the use of magnetic field gradients, in relation to various known and prospective methods for complete sample localization, is discussed.