A Microwave Radio Thermometer for Measuring the Depth Temperatures of Biological Objects by a Noninvasive Method

Abstract

A new area of medical diagnostics is being actively developed at the present time, based on the complex measurement of physical fi elds and the radiations of electromagnetic waves by the human body in the microwave and infrared ranges of the spectrum during vital activity [1]. By using methods of microwave radiometry, one can carry out noninvasive radio-thermal mapping of biological systems in different spectral bands, from which one can establish the depth temperature distribution profi le. One can determine the temperature distribution of a biological medium as a whole, which is an important characteristic of the function of an organism. To measure the natural radiation of a biological medium in the microwave band, radiometers with applicator-type antennas (radio thermometers) are employed [2], where the unpredictable value of the refl ection coeffi cient at the boundary of the antenna and the biological system has the main effect on the measurement accuracy. In instruments, the effect of the refl ection coeffi cient, like the main error of the measurements, is minimized by different methods: by compensation, by additional temperature stabilization of the antenna system, by processing the signals using complex algorithms, etc. Structural solutions in constructing the input receiver units are used for the same purpose in radiometer systems, the basis of which is the method of producing additional noise on the object being investigated using a reference radiometer signal [3–5]. It is assumed here that part of the radio-wave signal from the object, with an effective noise temperature T at the boundary with the antenna, is refl ected back into the object and has a value TR for a power refl ection coeffi cient R. An adaptive determination by the radiometer of the part of the useful electromagnetic signal, returned back into the object, ensures the property of symmetry and identity of the refl ection coeffi cient both from the side of the object being investigated and from the receiving antenna of the radiometer. Radio thermometers, using the method described above, are incorporated into systems where the additional noise signal is regulated by the change in the temperature of a matched load [6]. The noise signal, generated by the matched load, is equal to its physical temperature in units of the Kelvin scale (in the thermodynamic scheme the matched load is an analog of an absolutely black body). Hence, a heated or cooled matched load may change the noise signal acting on the boundary of the antenna and the medium being investigated from the side of the input unit of the radiometer. This regulation principle uses the analog method.