DETERMINATION OF BACKGROUND GRADIENTS WITH DIFFUSION MR-IMAGING

A new magnetic resonance (MR) imaging technique, opposite-polarity pul sed-field-gradient technique, with which the effects of background mag netic field gradients can be separated from the effects of diffusion, is described. it is based on the processing of two sets of diffusion-w eighted images, the acquisition parameters of which differ only in the polarity of the applied diffusion pulses. The two effects can be sepa rated because the cross term (b(c)) of the gradient factor function is antisymmetric with respect to reversal of the sign of the applied dif fusion pulses. The technique permits simultaneous measurement of the s patial distribution of both the diffusion constants and background mag netic field gradients, with the same spatial resolution as the parent images from which they were derived. The technique has been validated with a phantom in which the spatial distribution of susceptibility-ind uced background gradients is known, the results showing excellent agre ement with theory. The technique was applied to two systems in which t he spatial distribution of the background gradient is unknown. Sources of error in the measurement of background gradients and (unrestricted ) diffusion constants are analyzed, including the effects of voxel siz e, partial volumes, and interactions between background and imaging gr adients.