Measuring signal quality in low power wearable ultrasound imaging

Frequency Modulated Continuous Wave (FMCW) ultrasound is an emerging imaging method adapted from radar that can be implemented in wearable ultrasound systems due to the reduced size, power, and signal processing requirements. Acoustic reflections are encoded in the frequency domain, and thus evaluating signal quality currently requires extracting frequency information from a time-varying acquisition signal, which requires additional signal processing. Question: Can we differentiate FMCW ultrasound signal quality using an acquired FMCW time-series? We collected FMCW ultrasound images with a custom ultrasound transducer contacting and losing contact with tissue, to simulate moments of poor signal quality. We used two potential time-domain signal quality metrics: the mean absolute value (MAV) of the signal, and Sample Entropy. Sample Entropy is an information theory method for assessing complexity in a time-series. Both MAV and Sample Entropy could differentiate when the transducer was in good contact with the tissue. MAV required less processing time, but Sample Entropy had less noise when the transducer lost contact. FMCW ultrasound imaging is a promising imaging method that has application in wearable ultrasound systems, and FMCW ultrasound signal quality can be assessed in the time-domain without extracting frequency information. [Work supported by NIH 5U01EB027601-02.]