DIFFUSION IMAGING IN THE PRESENCE OF STATIC MAGNETIC-FIELD GRADIENTS

Problems associated with large, static magnetic-field gradients are reviewed and illustrated with pulsed-field-gradient NMR diffusion-imaging experiments on two model systems. The model systems chosen are a water-filled cylinder and a water-filled annulus between two concentric cylinders both oriented transversely to the polarizing magnetic field. The latter system exhibits large magnetic-field gradients (∼5 G cm-1), and the simplicity of the geometry permits an analytical description of the magnetic-field distribution in the sample; calculations of the field distribution are consistent with direct measurements made using the chemical-shift-imaging technique. Diffusion-imaging experiments based on either spin- or stimulated-echo sequences are reliable for the single-tube system in which static gradients are negligible. However, for the double-tube system, the large static gradients prevent reliable diffusion imaging by spin-echo methods and cause artifacts in the results of standard stimulated-echo techniques. A superior alternating-pulsed-field-gradient technique, for which echo attenuation is less sensitive to the effects of the static background magnetic-field gradients, provides reliable diffusion-coefficient images for the double-tube system. © 1993 by Academic Press, Inc.