Investigation and validation of FDTD weighting function modelling for microwave radiometric temperature measurement

Smith, Marie L. (2003) Investigation and validation of FDTD weighting function modelling for microwave radiometric temperature measurement. PhD thesis, University of Glasgow.

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Abstract

Microwave radiometry can provide a non-invasive, non-destructive and inherently safe
method of temperature measurement suitable for a range of medical and industrial applications. The measured radiometric signal is formed by a convolution of the actual
material temperature distribution with a coupling spatial response, or weighting function,
over the viewed volume of material. The form of this weighting function depends on both
the electromagnetic coupling structure (either antenna or cavity) and on the geometry
and dielectric properties of the material. Through reciprocity, the weighting function
can be found by computation or measurement of the power dissipation distribution (also
known as the specific absorption rate (SAR)) when the coupling structure is actively excited.
Knowledge of the weighting function is used to interpret the measured radiometric
temperature. Chapter 1 introduces the method of microwave radiometry, its range of
applications and considers the key features of weighting function determination.
The suitability and validity of finite difference time domain (FDTD) SAR and weighting
function modelling was investigated for the largely travelling - wave fields appropriate
to surface contact antennas. An FDTD simulator, the Basic Electromagnetic Simulation
Tool [3], was used to computationally model a range of antenna configurations that
could then be compared directly with experimental results. Chapter 2 introduces several
numerical techniques and justifies the choice of FDTD modelling. An introduction
to the theory of the FDTD technique and a description of the BEST software is also given.
Simulations of systems where electromagnetic field distributions are known (or can be determined experimentally) allowed the direct comparison of simulation results with theoretical
predictions. Chapters 3 and 4 consider various validation examples; a monopole
radiator above ground plane and TEOl waveguide in chapter 3, experimental field determination
in lossy dielectrics using the non-resonant perturbation method in chapter 4. In all
cases considered, simulation and experiment agree within a reasonable magnitude of error.
With the successful validation of its microwave modeling capabilities, the BEST program
was then used to predict the weighting functions expected for practical radiometer antennas
for microwave temperature measurement. Of primary importance are the variations
of the effective coupling distance into the viewed material with dielectric changes, particularly
those due to water content, and with measurement frequency. Knowledge of this
behaviour is essential for estimating, at one extreme, relatively small but physiologically
important temperature gradients within the human body, and at the other extreme, the
large and rapidly varying temperature patterns induced during industrial processes.
By measuring the microwave temperature at different microwave frequencies, it is possible
to retrieve information on the temperature at varying depths within the material. To aid
in the interpretation of these measurements, the BEST program was used to ascertain the
form of the weighting function at two frequencies, 1.35 GHz and 3.2 GHz, for a specific
dual - frequency antenna in a range of phantom materials. The phantom materials were
composed of a mixture of water, protein and salts, with the intention being to simulate
common biological materials. To consider foodstuffs a mashed potato phantom was used.
Chapter 5 includes the design of this dual frequency antenna and its application to measuring
the radiometric temperature of non-isothermal mashed potato mixtures. The specific
manipulation of the potato mixture (through heating and cooling) to produce known
temperature profiles (quasi-linear and quasi-quadratic) is also considered in this chapter.
Further validation of the BEST weighting function determination is possible by comparison
with these experimental temperature measurements.

Chapter 6 initially covers the modelling of the dielectric properties of the mashed potato
and protein / saline mixtures. In particular, a model of the variation of the dielectric constant
and loss factor of the mashed potato material, covering a wide range of temperatures
at 1.35GHz and 3.2GHz, is presented and shown to agree with published literature. The
effects on the computed weighting function of variation of several key factors, including
measurement frequency and material temperature, are then considered for both phantom
types. Further, limitations in the computational modelling in terms of finite bounds and
the modelling of layers are investigated.
Finally, techniques for obtaining the physical temperature distribution from multi - frequency
microwave readings are considered in chapter 7 and their applicability at two
frequencies is discussed. By making use of the data collected from the dual - frequency
antenna and simulated microwave temperatures, the various methods of temperature profile
retrieval are compared.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: Q Science > QC Physics
Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
Supervisor's Name: Land, Dr. D. and Watt, Dr. A.
Date of Award: 2003
Depositing User: Ms Mary Anne Meyering
Unique ID: glathesis:2003-4846
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 17 Jan 2014 09:59
Last Modified: 17 Jan 2014 10:01
URI: http://theses.gla.ac.uk/id/eprint/4846

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