RESUMO
MIT (magnetic induction tomography) image reconstruction from data acquired with a single, small inductive sensor has unique requirements not found in other imaging modalities. During the course of scanning over a target, measured inductive loss decreases rapidly with distance from the target boundary. Since inductive loss exists even at infinite separation due to losses internal to the sensor, all other measurements made in the vicinity of the target require subtraction of the infinite-separation loss. This is accomplished naturally by treating infinite-separation loss as an unknown. Furthermore, since contributions to inductive loss decline with greater depth into a conductive target, regularization penalties must be decreased with depth. A pair of squared L2 penalty norms are combined to form a 2-term Sobolev norm, including a zero-order penalty that penalizes solution departures from a default solution and a first-order penalty that promotes smoothness. While constraining the solution to be non-negative and bounded from above, the algorithm is used to perform image reconstruction on scan data obtained over a 4.3 cm thick phantom consisting of bone-like features embedded in agarose gel, with the latter having a nominal conductivity of 1.4 S/m.
RESUMO
Recent work has shown that single-coil, magnetic induction tomography (MIT) is useful for visualizing three-dimensional electrical conductivity distributions within biological targets. Coil-induced eddy currents and the associated secondary field are detected as an inductive loss while the coil is relocated to several unique positions and orientations near a target. Image reconstruction is then accomplished by inversion of a convolution integral that quantitatively maps inductive loss with conductivity. Previously, coil position and orientation had to be established by a template, which required assignment of fixed locations for the coil to visit. Here, our existing device is modified so that coil position and orientation are optically tracked while measuring inductive loss. Optical tracking is accomplished via a set of infrared reflective spheres mounted on the same enclosure that supports the coil. The coil center can be tracked with submillimeter accuracy while orientation angle is known to within a fraction of a degree. This work illustrates the use of single-coil MIT in full, position-orientation-tracked scan mode while imaging laboratory phantoms consisting of features having biologically relevant conductivity.
RESUMO
Alternative coil designs for single coil magnetic induction tomography are considered in this work, with the intention of improving upon the standard design used previously. In particular, we note that the blind spot associated with this coil type, a portion of space along its axis where eddy current generation can be very weak, has an important effect on performance. The seven designs tested here vary considerably in the size of their blind spot. To provide the most discerning test possible, we use laboratory phantoms containing feature dimensions similar to blind spot size. Furthermore, conductivity contrasts are set higher than what would occur naturally in biological systems, which has the effect of weakening eddy current generation at coil locations that straddle the border between high and low conductivity features. Image reconstruction results for the various coils show that coils with smaller blind spots give markedly better performance, though improvements in signal-to-noise ratio could alter that conclusion.