Postprocessing technique to correct for background gradients in image-based R-2(*) measurements

Background static magnetic field gradients are a source of signal loss in g radient-echo imaging, as they typically result from discontinuity in the ma gnetic susceptibility at air-tissue boundaries. Moreover, these induced gra dients severely compromise the measurement of R-2(*), the effective transve rse relaxation rate, which is of interest in many biomedical applications o f MRI, Since the slice thickness is usually larger than the in-plane pixel dimensions, gradients parallel to the slice-select direction are of particu lar concern. In this work, a post-processing technique is introduced which attempts to correct the signal on the assumption that the background gradie nts are approximately linear across the voxel and the signal decay in the a bsence of these gradients is exponential. In this case, the time-domain sig nal is weighted by a sine function characterized by the amplitude G(b) of t he background gradient, which is typically not known a priori, The algorith m searches for the estimate of G(b) which yields the optimum fit of the cor rected experimental data to an exponential. It is shown to be effective as long as this gradient is below a critical threshold. Evaluation in a phanto m and in the human brain at 1.5 and 4 T demonstrates that this method can r estore R-2(*) in spite of the apparent rate constant exceeding the true val ue by up to 100%. Contrary to prospective correction techniques, the approa ch presented in this study does not prolong scan time. (C) 2000 Wiley-Liss, Inc.