Po-topic IV-14: Imaging & localizing device using the barkhausen effect

Abstract

We have been investigating the use of amorphous magnetic materials as locator tags for medical applications. Heinrich Barkhausen discovered in 1919 that a slow, smooth increase of magnetic field applied to a piece of ferromagnetic material, such as iron, causes the material to become magnetized not continuously, but in small steps. A large Barkhausen effect (large step change) was observed in 1931 because of domain reversal at a rapid velocity along the wire. In the late 1980s, an even larger Barkhausen effect was produced in amorphous wires giving rise to an even sharper magnetic pulse because of domain or flux reversal in the core of the wire. In an AC magnetic field, the reversal of the domains emits large, sharp magnetic pulses of the opposite sign. We are taking advantage of this pulsed response to tag and image the location and orientation of implanted devices. Several factors affect the feasibility of imaging and reconstruction of embedded tags. These include the strength of the detected signals; the ability to reconstruct locations and orientations of the tags; and signal sensitivity to tissue composition, inhomogeneities, and the presence of neighboring metals. We have obtained voltage signals from wires of several diameters with an iron-core pickup coil along a distance from the center of a wire with the coil as a function of angle for a 60–80 Hz driving field. At a distance of about 8 cm., we have obtained a strong signal of typically 2V (depending on length), more than 50 times the background noise (about 20 mV). At 30 cm separation, we detected a signal of 0.5 V, 25:1 above background noise. When the wire direction is mis-oriented relative to the driving AC magnetic field, the amplitude and phase of the resultant output signal changes. This is because only the portion of the magnetic field that is parallel to the wire axis contributes to the Barkhausen effect for that wire. By using multiple positions of the coil, we have been able to determine the changes of these two parameters for a more accurate localization. We have also determined that intervening tissues have little effect on the signals, as was expected theoretically. Our preliminary results indicate that in vivo 3D reconstruction is feasible.