Sensitivity maps and system requirements for magnetic inductiontomography using a planar gradiometer

Abstract

We evaluated analytically and experimentally the performance of a planargradiometer as a sensing element in a system for magnetic inductiontomography. A system using an excitation coil and a planar gradiometer wascompared against a system with two coils. We constructed one excitationcoil, two different sensing elements and a high-resolution phase detector.The first sensor was a PCB square spiral coil with seven turns. The secondsensor was a PCB planar gradiometer with two opposite square spirals ofseven turns, with a distance between centres of 8 cm. Theoreticalsensitivity maps were derived from basic equations and compared withexperimental data obtained at 150 kHz. The experimental sensitivity mapswere obtained measuring the perturbation produced by a brass sphere of12 mm in empty space. The advantage of using a gradiometer is that itcan be adjusted to give a minimum signal for homogeneous objects, whileincreasing the sensitivity to local perturbations of the conductivity.Results show that a system using a planar gradiometer as detector has lessdemanding requirements for the electronic system than a system using simplecoils.