Strategies for diagnosing and alleviating artifactual attenuation associated with large gradient pulses in PGSE NMR diffusion measurements

The generation of phase-based artifacts resulting from mismatch in the effe ctive areas (i.e,, the time integrals) of sequential gradient pulses is dis cussed in the context of large gradient pulsed-gradient spin-echo (PGSE) NM R diffusion measurements. Such effects result in artifactual attenuation an d distortion in the spectra which, in the first instance, are similar to (a nd commonly mistaken for) the effects of eddy currents, Small degrees of mi smatch cause "unphysical" concave downward curvature in POSE attenuation pl ots of freely diffusing species. However, larger mismatches can result in a rtifactual diffraction peaks in the plots which could easily be confused fo r true restricted diffusion effects, Although "rectangular" gradient pulses are preferable from a theoretical viewpoint, we found that shaped gradient (e.g,, half-sine) pulses, which due to their slower rise and fall times we re more tractable for the current amplifier, were more sequentially reprodu cible, As well as generating fewer phase-based artifacts such shaped pulses also decrease the likelihood of vibration problems. (C) 1999 Academic Pres s.