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 effective areas (i.e., the time integrals) of sequential gradient pulses is discussed in the context of large gradient pulsed-gradient spin-echo (PGSE) NMR diffusion measurements. Such effects result in artifactual attenuation and distortion in the spectra which, in the first instance, are similar to (and commonly mistaken for) the effects of eddy currents. Small degrees of mismatch cause "unphysical" concave downward curvature in PGSE attenuation plots of freely diffusing species. However, larger mismatches can result in artifactual diffraction peaks in the plots which could easily be confused for 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 were more tractable for the current amplifier, were more sequentially reproducible. As well as generating fewer phase-based artifacts such shaped pulses also decrease the likelihood of vibration problems.

[1]  P. Stilbs,et al.  Fourier transform pulsed-gradient spin-echo studies of molecular diffusion , 1987 .

[2]  M. Hrovat,et al.  NMR pulsed gradient diffusion measurements. II. Residual gradients and lineshape distortions , 1981 .

[3]  J. Kärger,et al.  Pitfalls in PFG NMR Self-Diffusion Measurements with Powder Samples , 1995 .

[4]  P. Callaghan.  Principles of Nuclear Magnetic Resonance Microscopy , 1991 .

[5]  W. S. Woodward,et al.  A Computer-Controlled Bipolar Magnetic-Field-Gradient Driver for NMR Electrophoretic and Self-Diffusion Measurements , 1994 .

[6]  Callaghan,et al.  High magnetic field gradient PGSE NMR in the presence of a large polarizing field , 1998, Journal of magnetic resonance.

[7]  K. Hayamizu,et al.  A model for diffusive transport through a spherical interface probed by pulsed-field gradient NMR. , 1998, Biophysical journal.

[8]  Paul T. Callaghan,et al.  PGSE-MASSEY, a sequence for overcoming phase instability in very-high-gradient spin-echo NMR , 1990 .

[9]  M. Hrovat,et al.  NMR pulsed-gradient diffusion measurements. I. Spin-echo stability and gradient calibration , 1981 .

[10]  Jörg Kärger,et al.  The propagator representation of molecular transport in microporous crystallites , 1983 .

[11]  William S. Price,et al.  Pulsed-field gradient nuclear magnetic resonance as a tool for studying translational diffusion: part II. Experimental aspects , 1998 .

[12]  E. V. Meerwall,et al.  Effect of residual field gradients on pulsed-gradient NMR diffusion measurements☆ , 1989 .

[13]  Callaghan,et al.  Evidence for reptational motion and the entanglement tube in semidilute polymer solutions. , 1992, Physical review letters.

[14]  William S. Price,et al.  Pulsed-field gradient nuclear magnetic resonance as a tool for studying translational diffusion, part 1: basic theory , 1997 .

[15]  J. E. Tanner,et al.  Spin diffusion measurements : spin echoes in the presence of a time-dependent field gradient , 1965 .

[16]  P. Kuchel,et al.  Effect of nonrectangular field gradient pulses in the stejskal and tanner ( diffusion) pulse sequence , 1991 .