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Heart Sounds Obtained With Non-Contact Continuous-Wave Echo Doppler

Heart sounds can be obtained in various ways. A recent modality is airborne ultrasound echo Doppler, which has been used either in a pulsed wave mode or a continuous wave mode. In the present work we analyse the continuous wave ultrasound echo from the chest. In 10 subjects a piezo ultrasound transceiver was used to transmit a 40 kHz acoustic sine wave directed to the chest wall. The reflected signal (echo) was recorded with a microphone and sampled at 1 MHz. The FM modulated echo was demodulated in MatLab using a quadrature signal. The signal thus obtained represented the chest displacement, which was subdivided into three frequency bands: respiration (below 0.5 Hz), movement of the myocardium (>1 Hz) and valve sounds (30–100 Hz). Above 100 Hz the signal decayed by 20 dB/decade with a noise floor above 1–2 kHz. Interestingly, signals obtained directly from the skin and signals obtained through a thin layer of clothes were similar. Non-contact continuous-wave echo Doppler thus is a promising new modality for the recording of heart sounds, although its true potential has to be further investigated.