(a) A Brainweb20 phantom, 20% inhomogeneity, (b) true gain field, (c) gain field obtained from N3, (d) smoothed gain field obtained using the patch-based algorithm, (e) difference between true field and N3 field, (f) difference between true field and the one obtained using our method. For smooth fields, patch based method works comparably with N3, where the coefficient of variation (CV) between (b) and (c) is 0.0101 and CV between (b) and (d) is 0.0110. These results are comparable to ones reported earlier.9