MAGNETIC-SUSCEPTIBILITY TOMOGRAPHY FOR 3-DIMENSIONAL IMAGING OF DIAMAGNETIC AND PARAMAGNETIC OBJECTS

A tomographic technique for reconstructing the three-dimensional distribution of magnetic susceptibility in an object is described. A SQUID magnetometer may be used to measure the perturbations imposed by the object on an applied magnetic field and these data contain information about the susceptibility distribution. To assess the technique, a model object was defined, simulated magnetic field data were generated, and a matrix inversion was carried out with singular value decomposition to yield a least-squares solution for the susceptibility distribution. Various relative geometries of the three interacting physical systems (the applied field, the object and the measurement space) were used and the algorithm's performance was investigated for each of the cases in which one of the systems was moved while keeping the other two fixed. With either strategy involving relative motion between the object and the measurement space, accurate, convergent solutions were obtained, but the algorithm failed when only the direction of the uniform applied field was varied. A suitable nonuniform applied field may make the algorithm robust. Applications for a tomographic imaging susceptometer in biomedical imaging, nondestructive evaluation, and geophysics are envisioned.