CompanyName: University of Glamorgan
Abstract: Magnetic Induction Tomography; Phase vs Vector Voltmeter measurement techniques
S Watson1, R J Williams1, H Griffiths2, W Gough3 , and A Morris3
1 School of Electronics, University
of Glamorgan, Pontypridd, CF37 1DL, UK, firstname.lastname@example.org
2 Medical Physics Directorate, University Hospital of Wales, Heath Park, Cardiff, CF4 4XW, UK
3 Department of Physics and Astronomy, Cardiff University, Cardiff CF2 3YB, UK
The use of inductively applied currents rather than currents injected via electrodes, for the purpose of the impedance imaging of biological tissues, has attracted the interest of several investigators. Magnetic Induction Tomography (MIT) attempts to mitigate the operational difficulties found in some applications of EIT associated with the tissue / electrode interface and with physiological barriers such as the skull. The development of biological MIT systems presents a number of obstacles however, with the frequency dependency of the amplitude of induced currents a major issue.
High frequency operation improves signal amplitudes but potentially introduces phase instabilities during signal distribution and processing. Frequency downconversion has therefore been applied in high frequency MIT systems to address these concerns.
Signals detected from excited biological objects include the following components - in-phase signals such as the direct excitation signal (coil-coil magnetic coupling), capacitative coupling between the coils and the induced displacement current signal and the in-quadrature induced conduction current signal. Phase shifts due to propagation delays introduced by the permittivity of tissues may also be significant. Preliminary studies suggest that a phase precision of the order of 10m may be required for practical biomedical MIT.
This paper describes the results of measurements carried out with a 16-channel downconverting 10MHz MIT system utilising two types of data extraction techniques. In direct phase measurement, the received signal and a reference taken from the transmitter are limited and the time interval between the pulses is then measured to determine the phase shift. This is then considered to be directly proportional to the amplitude of the conduction current signal. In vector voltmeter measurements a digital lock-in amplifier is employed to measure the in-phase and in-quadrature components of the signal relative to a synchronous reference signal. The basic precision provided by each technique was 50mdegrees with thermal drift of phase in the HF circuits representing the major limiting factor. The operational characteristics of both techniques are discussed.