Practical 3-D Mapping of the Interior of Nonmagnetic and Nonmetallic Everyday Materials With Fast and Noncontact Magnetostatic Susceptibility Tomography

Magnetostatic susceptibility tomography (MST) aims to map the weak magnetizability (diamagnetic or paramagnetic) inside practically nonmagnetic and non- or poorly conducting objects: plastics, minerals, wood, glasses, food or living beings, or liquid flows like oil and water. For the first time, fast and contactless 3-D MST mapping without contrast agents is presented here in practice. MST offers a noninvasive, linear, and safe methodology with noncontact 3-D data acquisition in less than a second. MST is also not subject to the special conditions of nuclear magnetic resonance. Challenging are the weak magnetic signals in the pico- to nanotesla range, requiring control of noise, electromagnetic, and mechanical interference. Furthermore, the spatial resolution is limited because the signal carrying magnetic fields are inherently diffuse: as with many electromagnetic “soft field” methods, an ill-posed inverse problem occurs. This practical work, based on recent preliminary results on the topic, presents an improved and enlarged setup with more independent measurements and increased signal-to-noise ratio (SNR). The six-channel system records subtle field deformations caused by rapidly passing objects. From the multiple signals, a Landweber inversion reconstructs a 3-D map of the inhomogeneous susceptibility inside the object. The results confirm for the first time the practical capability of 3-D MST for fast and internal examination of almost nonmagnetic everyday objects. MST could be promising for quality control, material sorting, and biomedical diagnostics, where rapid, harmless, and noncontact material differentiation is required.