The Delineation of Fiducial Points for Non-Contact Radar Seismocardiogram Signals Without Concurrent ECG
Zongyang Xia, Md Mobashir Hasan Shandhi, Yinghao Li, Omer T. Inan, Ying Zhang
Abstract
Objective: Non-contact sensing of seismocardiogram (SCG) signals through a microwave Doppler radar is promising for biomedical applications. However, the delineation of fiducial points for radar SCG still relies on concurrent ECG which requires a contact sensor and limits the complete non-contact detection of SCG. Methods: Instead of ECG, a new reference signal, the radar displacement signal of heartbeat (RDH), was derived through the complex Fourier transform and the band pass filtering of the radar signal. The RDH signal was used to locate each cardiac cycle and mask the systolic profile, which was further used to detect an important fiducial point, aortic valve opening (AO). The beat-to-beat interval was estimated from AO-AO interval and compared with the gold standard, ECG R-to-R interval. Results: For the 22 subjects in the study, the evaluation of the AOs detected by RDH (AO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RDH</sub> ) shows the average detection ratio can reach 90%, indicating a high ratio of the AO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RDH</sub> that are exactly the same as AO detected using the ECG R-wave (AO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ECG</sub> ). Additionally, the left ventricular ejection time (LVET) values estimated from the ensemble averaged radar waveform through AO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RDH</sub> segmentation are within 2 ms of those through AO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ECG</sub> segmentation, for all the detected subjects. Further analysis demonstrates that the beat-to-beat intervals calculated from AO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RDH</sub> have an average root-mean-square-deviation (RMSD) of 53.73 ms when compared with ECG R-to-R intervals, and have an average RMSD of 23.47 ms after removing the beats in which AO cannot be identified. Conclusions: Radar signal RDH can be used as a reference signal to delineate fiducial points for non-contact radar SCG signals. Significance: This study can be applied to develop complete non-contact sensing of SCG and monitoring of vital signs, where contact-based SCG is not feasible.