This paper reviews the field of microwave sensors. The field is broad and the applications numerous. This review will therefore only be able to present the broad outlines. Firstly the historical perspective and the physical background are briefly described, and the general advantages and disadvantages are listed. An overview of the various working principles of microwave sensors is given with a few examples of the applications mentioned. Important fields of applications and interesting examples are treated separately, starting with the measurement of moisture, which is the single most important field of applications in microwave sensors. Applications in the petroleum industry are also mentioned, because they are relatively new, they play an exceptionally important economical role, and represent the field in which the author is currently working. Followed by some trends for the future.

Industrial Microwave Sensors—A Review

Numerical modelling of the interaction between electromagnetic fields (EMFs) and the dielectrically inhomogeneous human body provides a unique way of assessing the resulting spatial distributions of internal electric fields, currents and rate of energy deposition Knowledge of these parameters is of importance in understanding such interactions and is a prerequisite when assessing EMF exposure or when assessing or optimizing therapeutic or diagnostic medical applications that employ EMFs In this review, computational methods that provide this information through full time-dependent solutions of Maxwell's equations are summarized briefly This is followed by an overview of safety- and medical-related applications where modelling has contributed significantly to development and understanding of the techniques involved In particular, applications in the areas of mobile communications, magnetic resonance imaging, hyperthermal therapy and microwave radiometry are highlighted Finally, examples of modelling the potentially new medical applications of recent technologies such as ultra-wideband microwaves are discussed

https://iopscience.iop.org/article/10.1088/0031-9155/53/16/R01/pdf

Modelling the interaction of electromagnetic fields (10 MHz–10 GHz) with the human body: methods and applications

Near-field microwave radiometry and radiometric imaging are non-invasive techniques that are able to provide temperature information at a depth of up to several centimetres in subcutaneous tissues They are based on the measurement of microwave electromagnetic thermal noise This paper describes the basic principles, measurement methods and limitations of the techniques and the results of clinical studies, and it reviews recent progress

https://iopscience.iop.org/article/10.1088/0967-3334/19/2/001/pdf

Non-invasive microwave radiometry thermometry

In the treatment of cancer, microwave hyperthermia has been established as an efficient adjunctive procedure to radiation therapy and chemotherapy. Wider acceptance of this method awaits schemes to measure volumetric temperatures noninvasively in human tissue for control of the heating process. This effort describes the design and performance of a new microstrip applicator intended for homogeneous heating of superficial tissue while at the same time monitoring temperature of the underlying tissue by noninvasive radiometric sensing of black-body radiation from the heated volume. Radiometric capabilities are assessed in terms of accuracy of up to six measured brightness temperatures applied in an inversion algorithm from which one-dimensional depth temperature profiles are generated. Based on radiometric signals recorded over the 1-4-GHz range, the temperature accuracy determined from statistical analysis of 200 realizations of the process is better than /spl plusmn/0.2/spl deg/C to a depth of 5 cm in phantom. Aperture heating uniformity is assessed with electric field scans in a homogeneous muscle phantom. As long as sufficiently thin (< 5 mm) water boli are used, SAR distributions at 1-cm depth in phantom extends effectively just outside the aperture perimeter, making this microstrip antenna an excellent building block element of larger multi-antenna array applicators.

Multifrequency radiometric determination of temperature profiles in a lossy homogeneous phantom using a dual-mode antenna with integral water bolus

Temperature detection using microwave radiometry has proven value for noninvasively measuring the absolute temperature of tissues inside the body. However, current clinical radiometers operate in the gigahertz range, which limits their depth of penetration. We have designed and built a noninvasive radiometer which operates at radio frequencies (64 MHz) with ~100-kHz bandwidth, using an external RF loop coil as a thermal detector. The core of the radiometer is an accurate impedance measurement and automatic matching circuit of 0.05 Omega accuracy to compensate for any load variations. The radiometer permits temperature measurements with accuracy of plusmn0.1degK, over a tested physiological range of 28degC-40 degC in saline phantoms whose electric properties match those of tissue. Because 1.5 T magnetic resonance imaging (MRI) scanners also operate at 64 MHz, we demonstrate the feasibility of integrating our radiometer with an MRI scanner to monitor RF power deposition and temperature dosimetry, obtaining coarse, spatially resolved, absolute thermal maps in the physiological range. We conclude that RF radiometry offers promise as a direct, noninvasive method of monitoring tissue heating during MRI studies and thereby providing an independent means of verifying patient-safe operation. Other potential applications include titration of hyper- and hypo-therapies

/pdf/absolute-temperature-monitoring-using-rf-radiometry-in-the-3a482vgbwc.pdf

Absolute Temperature Monitoring Using RF Radiometry in the MRI Scanner

A modal method is used for the determination of the near field weighting functions related to the power transmitted from a rectangular waveguide aperture towards a lossy material and, by reciprocit...

Near Field Microwave Radiometric Weighting Functions for Multilayered Materials

The measurement of the thermal noise transmitted by lossy materials in the microwave frequency range is, at the present time, the only one thermometric method able to bring information about the temperature distribution inside a material without insertion of any sensor. In this paper, we first briefly recall the principles of the method, and the present state of the experimental set-ups. This technique interest quite different domains of applications such as industrial thermometry, biomedicai engineering, domotics and microelectronics. In other words, the presentation is related to different situations:

 The paper analyses also the reasons why these method have not still much penetrated the industrial market and explains the arguments for a future extension of these kinds of applications.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400351478

Non Destructive Thermometry by Means of Microwave Radiometry

After being used extensively for remote sensing, microwave radiometry is also applied for non-invasive thermometry in medicine and industry. Several works have already shown the interest of such technique for thermological applications. This paper presents a modal method for computation of the near field weighting functions applied to multilayered structure. From this kind of data, we point out that the coupled volume is a function of the size of the radiometric probe and also that the second generation radiometric imaging is able to improve the spatial resolution. >

Near field weighting functions for microwave radiometric signals

The principle of this original thermal sensor is the same as for interferometric synthetic aperture microwave radiometry [1], presently used for remote sensing. Our aim is to apply this process for short-range thermometry. It is well known that a dissipative material creates a noise signal depending on its temperature. The correlation function of such noise signals exhibits significant values if both contributions to the correlation product are sufficiently correlated. For example, we consider both ports of a lossy transmission line to be connected to a wide-band microwave correlation radiometer. If a hot spot is located on the line, the output signal is maximal if the delay line of the correlator compensates the distance of the hot spot with respect to the centre of the line. Then, it is possible to evaluate the position and the intensity of the corresponding hot spot. Recent experimental studies made with microstrip lines and coplanar waveguides have shown that a temperature retrieval is possible in a more complicated temperature distribution by a Kalman filtering of the correlation function of the noise. The study of other passive dissipative networks is probably also promising.

Thermometry on lossy coplanar lines by microwave correlation radiometry

The present development of microwave radiometry for medical applications encourages the development of methods for computer modelling of the radiometric thermal signals transmitted by living tissues. This method, considers both homogeneous and multilayered tissues. We present here computed data for the definition of microwave radiometric imaging and for the interpretation of radiometric data.

B. Bocquet

Papers

Near Field Microwave Radiometric Weighting Functions for Multilayered Materials

Non Destructive Thermometry by Means of Microwave Radiometry

Near field weighting functions for microwave radiometric signals

Thermometry on lossy coplanar lines by microwave correlation radiometry

Computation of Three-dimensional Radiometric Signals Detected in Microwave Imaging