VOLUME-SELECTIVE NMR-SPECTROSCOPY WITH SELF-REFOCUSING PULSES

If the 90° selective pulses that are normally used for volume localization are replaced with self-refocusing 270° pulses having a Gaussian time-domain amplitude profile, various pulse sequences can be made shorter, thus reducing the effects of transverse relaxation, flow, or diffusion. The sensitivity advantage is ascertained quantitatively for some of the commonly used sequences. The volume of interest defined in these experiments is determined by the offset dependence (spatial profile) of the conversion of transverse xy magnetization into z magnetization and vice versa. It is demonstrated that the gradients can be switched on the off within the intervals where the radiofrequency pulses are applied without affecting the spatial profile. We compare the shapes of the volumes of interest defined by (i) 90° Gaussian pulses, (ii) 90° sinc pulses, and (iii) self-refocusing 270° Gaussian pulses. In the first two cases, refocusing or predefocusing is necessary, while in the latter case it is not. The comparisonslead us to propose three new methods, (i) self-refocusing spatial and chemical-shift-encoded excitation (SR-SPACE), (ii) self-refocusing spatially resolved spectroscopy (SR-SPARS), and (iii) self-refocusing selected-volume excitation using stimulated echoes (SR-VEST). © 1990.