Inhomogeneity in local magnetic field due to susceptibility contrast

A method for computing local magnetic field for the case of small permeability or susceptibility contrast is described. The method is illustrated by using an example of a periodic array of cylinders which affords the simplest yet a representative case for visualizing the local fields. The cylinders represent material of one susceptibility such as solid grains in a rock matrix (or tissues in biological systems) and the space outside represents material of another susceptibility such as fluid which fills the pore space (or fluids outside tissues). We calculate the position dependent correction from the continuum to the local field inhomogeneity and go beyond the standard uniform field term. A simple (separate) illustration when the local field diverges is given. (C) 1999 American Institute of Physics. [S0021-8979(99)08420-0].