A Noninvasive Microwave Sensor and Signal Processing Technique for Continuous Monitoring of Vital Signs

The use of the noninvasive microwave method for measuring changes in lung water content has been previously reported and validated in animal experiments. The approach is based on measuring the transmission coefficient across the thorax and correlating results with changes in lung water content. In this letter, we extend this technique to the monitoring of multiple vital signs including heart rate and breathing as well as the changes of lung water content from a single transmission coefficient measurement. Using a short-time Fourier transform-based digital signal processing method, it is shown that these vital signs can be accurately detected and extracted from a single microwave transmission coefficient measurement. Experimental measurements were made on a thorax phantom model, and the obtained data confirmed the validity and accuracy of the proposed approach. The radiation bio-safety aspects of this approach were evaluated using a DASY4 near-field scanner, often used for certifying cell phones. Obtained results for a 30-mW input power show that the specific absorption rate (SAR) values are about one third of the FCC safety standard at the operating frequency of 915 MHz. The microwave applicator design together with the experimental procedure and the extracted vital signs results are presented, and avenues for future clinical implementation are discussed.