Feasibility study of in vivo bone depth measurement using high frequency ultrasound

Accurate measurement of bone motion in vivo during dynamic activities has the potential to improve our understanding of injury mechanisms and enhance our ability to design protective equipment and/or devices such as exoskeletons for rehabilitation and human augmentation. Existing technologies such as skin-mounted markers and fluoroscopy are limited either in accuracy or portability. The purpose of this study was to demonstrate a proof of concept for a wearable ultrasound (US) sensor array to measure bone positions around a human joint in vivo. A single off the shelf US sensor was tested for repeatability and accuracy in measuring soft tissue depth between a skin surface and imbedded bone with (a) a gelatinous analog for human tissue, and (b) a porcine leg specimen. In measuring the gel analog the sensor was able to measure depth with a repeatability of 0.25 mm. In measuring the porcine leg specimen, measurement accuracy was compared to a Qualisys optical motion capture system with accuracy on the order of 0.5 mm, and the US measurement error was found to be 1.1 mm. This study demonstrates that low cost off the shelf US sensors have acceptable quality to measure bone positions accurately in vivo. Future work must focus on integration of multiple sensors in an array and assessment of factors that may decrease accuracy during dynamic activities.