Non-contact Heart Sound Measurement by Defocused Speckle Imaging

Phonocardiogram (PCG) is a graphical representation of heart sounds and murmurs, playing a vital role in the early screening and prevention of cardiovascular diseases (CVDs). In this study, we propose a first camera-stethoscope that is capable of recording PCG contactlessly, enabled by the principle of defocused speckle imaging (DSI). By illuminating the chest surface adjacent to the heart with a laser beam and employing a defocused camera to capture the reflected laser speckle interference, the subtle cardiac vibrations involved in heart sound generation are magnified and measured. By adjusting the defocused level, we can achieve different amplification effects of heart sounds. Additionally, we propose an algorithm to extract PCG signals from the video, including the fast detection of S1 and S2 events. In laboratory experiments, we evaluated the robustness of laser positioning and revealed a nonlinear amplification relationship between defocused level and PCG signals. The reliability of camera-stethoscope has been validated on 21 healthy adult subjects against the reference of contact electronic stethoscope. Moreover, we conducted clinical validation in the Department of Ultrasound. The proposed camera-stethoscope achieved F1-scores of 0.94 for both S1 and S2 detection on 45 patients, demonstrating its reliability and clinical potential. This work established the fundamental optical sensing model and algorithms for novel PCG monitoring.