A METHOD OF INVERSE CALCULATION FOR REGIONS OF SMALL SUSCEPTIBILITY VARIATIONS

The paper considers the determination, from a knowledge of the perturb ation of a constant incident field, the susceptibility of regions wher e the susceptibility is 0.01 to 10 ppm different from the majority of the space of interest, This is pertinent to Magnetic Resonance Imaging (MRI), and is an inverse problem, Two forward methods of calculating the B field are considered, First, the normal finite element method an d secondly, the method of considering the induced polarity in the regi on of enhanced susceptibility, and calculating the difference field, T he accuracies of the methods are explored and the induced polarity met hod is chosen for the inverse calculation, An iterative technique for inverse calculation is described utilising knowledge of the geometry o f the problem, which is Available from an MR plot, The numerical work is validated using experimental data from an MR imaging machine.