Instrumentation for low-conductivity magnetic induction tomography

Abstract

Magnetic Induction Tomography (MIT) belongs to a group of techniques for the non­ invasive imaging of the passive electrical properties of objects which also includes Electrical Impedance Tomography (EIT). In MIT, arrays of coils are employed to induce eddy currents within an object and to detect the resulting secondary magnetic fields. Conductivity and permittivity distributions are then reconstructed from the collected set of four-terminal transimpedance data. MIT has several potential advantages relative to EIT including (i) errors due to variability of electrode contact impedances and positioning are avoided since the coils in an MIT system are typically rigidly attached to a chassis/screen, and (ii) the magnetic fields employed easily pass through high impedance boundary layers (e.g. the skull in brain imaging applications). Applications of MIT may be broadly divided into two categories: high-conductivity and low-conductivity. The development of MIT for low-conductivity applications (a < 30Sm -1)has been much slower than for high­ conductivity ones since the conductivities involved are lower, typically by a factor of ~1x107 resulting in much smaller signals and very challenging instrumentation design.The aim of this work was to develop MIT systems with a performance level approaching that required for low-conductivity medical and industrial applications including the detection of haemorrhagic cerebral stroke and the imaging of multi-phase flows in oil pipelines. The specification, design and performance of three novel low-conductivity MIT systems are described and are discussed in relation to the target applications. The Cardiff Mkl single-frequency MIT system was only the second multi-channel MIT system to be constructed, the first low-conductivity MIT system which allowed measurement of both real and imaginary signal components, and provided a higher measurement precision (by a factor of 6) than the previous system, with a phase noise of 17mO for 30ms time constant. A planar array system is described employing a novel coil geometry which provided a very significant reduction in phase noise and drift through the use of coil orientation. Finally, the Cardiff Mk2 multi-frequency system provided an order of magnitude improvement in measurement precision in comparison to the Cardiff Mk1 system with phase measurement precision of 1.1 m0 - 8m0 over 10MHz -0.5MHz, SNR for in vivo human head measurements estimated at 59d.B - 16dB over the same frequency range and phase drift of< 1OmO over periods of up to 12 hours