Depth and refocusing pulses for use with inhomogeneous radiofrequency coils in nuclear magnetic resonance spectroscopy

摘要

In a pulsed nuclear magnetic resonance method of analysis, in which a single radiofrequency pulse is applied using an irradiation coil which provides an inhomogeneous radiofrequency field across the sample volume, the inhomogeneity of this field ensures that some sample regions experience a radiofrequency pulse which is closer to a &pgr;/2 pulse angle than for other regions, resulting in a larger signal intensity from the former regions as compared to the latter. If a second pulse is applied after the first, prior to acquisition of the signal, and if the phase of this pulse is alternated through the four phases: 0°, 90° , 180° and 270°, during a series of transients, then, provided the receiver phase is changed by 180° when the second pulse phase is changed by 90°, the signal intensity will accumulate from regions where the second pulse angle is &pgr; radians and there will be discrimination against regions where the second pulse angle differs markedly from &pgr; radians. This discrimination towards certain sample regions can be improved by adding further pulses after the second pulse provided the phase of each additional pulse is alternated through all four 90° phase shifts, during a series of transients, independently of the phase alternation of the other pulses, and provided that for each individual 90° phase shift, the receiver phase is changed by 180°. In its simplest form, such a series of pulses may be represented by &thgr;;(2&thgr;[.+-.x,.+-.y]).sub.n indicating n phase alternated pulses additional to the first excitation pulse, where these additional pulses are twice as long as the first pulse.