Monitoring Variations of Biological Impedances at Microwave Frequencies

Two interferometer systems for monitoring time-varying biological impedances at 915 MHz and 913 GHz were developed. The systems compare the phase of the signal scattered from a region of biological tissue to that of a reference signal, using a phase sensitive detector. The phase changes of the scattered signal are an indication of the net changes within the test region due to various physiological processes, for example, the displacements of blood vessels during the cardiac cycle. The systems were tested with simulation models and their detection characteristics were found to be a linear function of the phase changes for return losses within the test region as high as 60 dB. Because of the interference between the signal scattered from the test region and the signal reflected at the antenna-tissue interface, the maximum range of operation of the systems was limited to a fraction of a wavelength in the test medium.