International Society for Industrial Process Tomography

Phase Measurement in Biomedical Magnetic Induction Tomography

S Watson1, R J Williams1, W Gough2 , A Morris2 and H Griffiths3

1 School of Electronics, University of Glamorgan, Pontypridd, CF37 1DL, UK, swatson1@glam.ac.uk

2 Department of Physics and Astronomy, Cardiff University, Cardiff CF2 3YB, UK

3 Medical Physics Directorate, University Hospital of Wales, Heath Park, Cardiff, CF4 4XW, UK

ABSTRACT

Magnetic Induction Tomography, also known as Mutual Inductance Tomography or Electromagnetic Tomography, is an imaging technique in which magnetic fields from an excitation coil induce eddy currents within a sample volume, which are then sensed by other receiver coils. The technique has attracted interest for both industrial and biomedical applications due to several potentially advantageous features, namely the non-contacting nature of measurements, speed, cost and its sensitivity to all three passive electromagnetic properties: conductivity, permittivity and permeability.

A major obstacle to be overcome in the development of practical biomedical MIT is to obtain the required measurement resolution given the low conductivities of biological tissues and the correspondingly small signal amplitudes. High frequency (3MHz - 30MHz) operation of MIT systems offers advantages in terms of the larger induced signal amplitudes compared to systems operating in the low or medium frequency ranges. Signal distribution at HF however presents difficulties, particularly in respect of isolation and phase stability. The theoretical change in receiver coil signal amplitude resulting from conduction eddy current perturbations at 10MHz is small, typically 1% with a phase lag of 90°.

Proposed measurement techniques which may provide the required resolution include the use of an additional ‘back-off’ coil together with phase sensitive detection, gradiometric approaches and the use of direct phase measurement between the reference signal at the excitation coil and the received signal. Heterodyne frequency downconversion, a process in which relative signal amplitude and phase information is in theory retained has been utilised in HF MIT systems to address the problems faced in signal distribution and measurement resolution at these frequencies.

Modelling and single channel measurements suggest that a direct phase measurement system may require a precision of the order of 0.01° to resolve a 1% variation in the B-field perturbation produced by a typical biological sample at 10MHz. The paper describes the circuits utilised in a single channel, downconverting direct phase measurement system. The results of measurements characterising the phase measurement system show that it has a precision of 0.04° based on 100-sample averaging, and had a drift of 0.015° over a 15 minute period.

Keywords Magnetic Induction Tomography, Electromagnetic Tomography