Sensing Depth of Microwave Radiation for Internal Body Temperature Measurement

This paper investigates modeling of the interaction of human tissues with a probe antenna situated on the surface of the skin. The goal is to differentiate temperature changes at different depths under the skin for passive radiometric measurements of internal body temperature. An improved metric is defined in order to differentiate thermal power radiated from a specific part of the tissue relative to the total thermal power received by the probe antenna. The frequency range between 0.5 and 3 GHz is investigated due to the sensing depth that can be achieved for a given receiver sensitivity. The general approach is to validate full-wave finite element method (FEM) EM simulations with a spectral-domain analysis for layered lossy dielectrics with a dipole at the interface between two of the layers. This was successfully done for a dipole on a water half-space and a stack of three tissues (skin, fat, and muscle). A new type of multifrequency probe was designed for 1.4, 2.7, and 4.9 GHz operation, and the predicted impedance validated experimentally. To the authors´ knowledge, this is the first time that the near-field received power of a dipole probe placed in contact with lossy layered media is examined as a function of frequency. This is shown to be significantly different than the plane wave case, and is necessary knowledge for the design of a portable radiometer for microwave internal body thermometry.