Respiratory System Monitoring Based on Low-frequency Transmission Ultrasound Using Wearable Transducers.

Ultrasound's application in respiratory system monitoring has been constrained by factors such as reflections at the lung-pleural interface and scattering within the lungs. Recent studies have shown that ultrasound from 10 kHz to 750 kHz can penetrate the human thorax during expiration. However, challenges persist in signal reception during inspiration due to low signal amplitude. In this study, we introduce a waveform matching method designed to ensure reliable transmission signal reception throughout the entire respiratory cycle by tracking waveform variations. The feasibility of this approach is demonstrated through a low-frequency ultrasound thorax transmission experiment using wearable transducers operating at 60 kHz. We observe that both the traveltime and energy of the first-arrival signal undergo significant changes that correlate with the breathing cycle. Furthermore, our findings indicate that variations in traveltime are predominantly due to thorax deformation, rather than changes in acoustic velocity distribution within the thorax as previously suggested by ex vivo studies. Besides, the energy variation curve features a heart rate frequency component. The experiment result affirms that low-frequency transmission ultrasound can detect variations in thorax state and holds promise for respiratory system monitoring.