Surveys the different types of microwave sensors and reviews the latest developments reported by European institutes and companies. Attention is given to resonator, transmission, reflection, radar, and radiometer sensors, and to active imaging. >

Industrial microwave sensors

To ascertain the capability of multifrequency microwave radiometry for imaging an inhomogeneous temperature distribution inside a cylindrical region of the human body, an equivalent cylindrical phantom has been constructed. Experiments have been performed using a four-channel radiometer. The physical equipment is evaluated by comparison of measured and modeled data. The effectiveness of a modified algorithm for the solution of the inverse problem of temperature retrieval from radiometric data is discussed. The agreement between measured and simulated data was fairly good, decreasing, but still satisfactory, when the distance from the outer surface was increased. The hotter tube excess temperature-ideally different from zero in circular region having sharp boundaries-has been retrieved as a smoother function, showing the maximum value in position shifted along the radial direction. >

Microwave radiometry for medical thermal imaging: theory and experiment

Microwave thermography obtains information about the temperature of internal body tissues by a spectral measurement of the intensity of the natural thermally generated radiation emitted by the body tissues. At the lower microwave frequencies radiation can penetrate through tissue for distances useful for a range of medical applications. Radiation from inside the body may be detected and measured non-invasively at the skin surface by a microwave thermography system consisting of a suitable antenna to detect the radiation and a radiometer receiver to measure its intensity. In the microwave region the radiative power emitted per unit bandwidth is proportional to the temperature of the emitting tissue and the total radiative power received from the body tissues, P, is a weighted volume average of temperature P = kB ∫w(r) T(r) dV where k is Boltzmann's constant, B is the bandwidth, T(r) is the temperature at the position r and w(r) is the weighting function. The weighting function depends on the structure and dielectric properties of the tissues being viewed, the measurement frequency and the characteristics of the antenna. The Glasgow developed microwave thermography system operates at a central frequency of 3.2 GHz, chosen to give the optimum compromise between the depth from which radiation may be received, which decreases with increasing frequency, and the lateral spatial resolution which increases with increasing frequency. A Dicke configuration radiometer receiver and a cylindrical low-impedance waveguide antenna, which operates in contact with the skin surface, are used. The output from the radiometer is calibrated to degrees Celsius to give a "microwave temperature" of the tissues being viewed. The tissue temperature distribution, T(r), reflects the vascular and metabolic state of the tissue. Diseases which affect these physiological functions will result in changes in the tissue temperature and hence in the measured microwave temperature. It is not possible, however, to solve the indirect problem of retrieval of the temperature distribution in the tissue from a single frequency measurement of microwave temperature. It is therefore necessary to model the temperature distribution in the tissue and, from this, solve the direct problem of calculation of the microwave temperature. Measured microwave temperatures may then be compared with those modelled to indicate the physiological state of the tissue. Pennes (1948) The temperature distribution in the tissue may be determined by solution of the steady-state heat transfer equation KV2T +Wbcb(Ta -T) + Q = 0 where K is the thermal conductivity of the tissue, Wb is the perfusion rate of blood through the tissue, cb is the specific heat capacity of the blood, Ta is the arterial blood temperature and Q is the rate of metabolic heat generation in the tissue. The boundary condition of heat loss at the skin surface is governed by the equation K dT/dn= h(T-Te ) where Te is the ambient temperature and h is the heat transfer coefficient due to the combined effects of heat loss by radiation, convection and evaporation. The microwave temperature may be calculated from the modelled temperature distribution and use of plane wave theory to determine the weighting function, with an increased power attenuation constant to account for the response of the antenna. The modelling of the tissue is simplified by the fact that both the tissue thermal conductivity and the microwave dielectric properties of the tissue depend primarily on the water content of the tissue. This thermal and electromagnetic modelling has been carried out to determine the expected microwave temperature profiles across the female breast. Microwave and infra-red temperature measurements were made on a group of young, normal women and a group of older, post-menopausal women with breast disease. In general the younger women will have higher water content breast tissue than that of the older women due to the higher proportion of glandular and connective tissue and the smaller proportion of low water content fat tissue.

Development of Computer Modelling Techniques for Microwave Thermography

Microwave thermography obtains information about the subcutaneous body temperature by a spectral measurement of the intensity of the natural thermally generated radiation emitted by the body tissues. At lower microwave frequencies the thermal radiation can penetrate through biological tissue for significant distances. The microwave thermal radiation from inside the body can be detected and measured non-invasively at the skin surface by the microwave thermography technique, which uses a radiometer to measure the radiation which is received from an antenna on the skin. In the microwave region the radiative power received from a volume of material has a dependence on viewed tissue temperature T(r) of the form, where k is the Boltzmann's constant, B the measurement bandwidth, c(r) is the relative contribution from a volume element dv (the antenna weighting function). The weighting function, c(r), depends on the structure and the dielectric properties of the tissue being viewed, the measurement frequency and the characteristics of the antenna. In any practical radiometer system the body microwave thermal signal has to be measured along with a similar noise signal generated in the radiometer circuits. The work described in this thesis is intended to lead to improvement in the performance of microwave thermography equipment through investigations of antenna weighting functions and radiometer circuit noise sources. All work has been carried out at 3.2 GHz, the central operating frequency of the existing Glasgow developed microwave thermography system. The effects of input circuit losses on the operation of the form of Dicke radiometer used for the Glasgow equipment have been investigated using a computational model and compared with measurements made on test circuits. Very good agreement has been obtained for modelled and measured behaviour. The losses contributed by the microstrip circuit structure, that must be used in the radiometer at 3.2 GHz, have been investigated in detail. Microwave correlation radiometry, by "add and square" method, has been applied to the received signals from a crossed-pair antenna arrangement, the antennas being arranged to view a common region at a certain depth. The antenna response has been investigated using a noise source and by the nonresonant perturbation technique. The received pattern formed by the product of the individual antenna patterns gives a maximum depth in phantom dielectric material. The depth can be adjusted by changing the spacing of the antennas and the phase in an antenna path. However, the pattern is modulated by a set of positive and negative interference fringes so that the complete receive pattern has a complicated form. On uniform temperature distributions the total radiometric signal is zero with the positive and negative contributions cancelling each other out. The fringe modulation can be removed by placing the antennas close enough together, The pattern is then simple and gives a modest maximum response at a known depth in a known material. The radiometer system remains sensitive to the temperature gradients only and the wide range of dielectric properties and tissue structures in the region being investigated usually makes the system response difficult to interpret. For crossed-pair antennas in phase the effective penetration depth in high-and medium-water content tissues is about 2.5 cm at a frequency of 3.2 GHz. The field pattern observed was of the form expected from the measurements of the individual antenna behaviour with the appropriate interference pattern superimposed. The nonresonant perturbation technique has been developed and applied to assist the development of the medical application of both microwave thermographic temperature measurement and microwave hyperthermia induction. These techniques require the electromagnetic field patterns of the special antennas used to be known. These antennas are often formed by short lengths of rectangular or cylindrical waveguide loaded with a low-loss dielectric material to achieve good coupling to body tissues. The high microwave attenuation in biological materials requires the field configurations to be measured close to the antenna aperture in the near-field wave. The nonresonant perturbation is a simple technique which can be used to measure electromagnetic fields in lossy material close to the antenna. It has been applied here to measure accurately the antenna weighting function and the effective penetration depth in tissue simulating dielectric phantom materials. (Abstract shortened by ProQuest.).

An Investigation of Radiometer and Antenna Properties for Microwave Thermography

The radiometric data, obtained in medical applications, have to be quantitatively interpreted. A necessary first step, with this aim in mind, is the definition of an accurate method of computation of the radiometric signals for three dimensionnal thermal structures. We define such a tool and demonstrate its feasibility in classical and correlation radiometry.

Modeling of Radiometric Signals for Medical Applications

This paper reports on experimental tests of a four-channel microwave radiometer for thermal pattern retrieval in biological tissues. Two-dimensional temperature distributions have been reconstructed in a liquid phantom simulating muscle.

Multispectral Microwave Radiometric System for Biological Temperature Retrieval: Experimental Tests

A numerical analysis has been carried out to test a technique for the reconstruction of two-dimensional thermal patterns induced by local hyperthermia. The method uses first a deconvolution of data obtained by displacing a multifrequency microwave sensor across the surface of the heated region to achieve transverse resolution. Deconvolved data are then processed to retrieve the subcutaneous gradient with depth. The numerical results indicate that, at least in principle, the retrieval may be feasible.

F. Bardati

Papers

Multispectral Microwave Radiometric System for Biological Temperature Retrieval: Experimental Tests

Feasibility of Radiometric Sensing of Two-Dimensional Thermal Patterns in Local Hyperthermia