Determination of number and locations of ultrasound sensors needed to accurately track scapular motion in vivo

Abstract

ABSTRACTUltrasonic imaging is a safe, established and non-invasive method to visualise the position of tissues within the body. Coupling existing skin mounted motion capture markers to ultrasound sensors could allow for precise measurement of bone positions and orientations, without the loss in accuracy due to relative displacement between skin and bone present in standard motion capture techniques. This is especially important in motions where skin-based markers alone are impractical or inaccurate, as for tracking scapular position when shoulder abduction is greater than 80 degrees. It is currently unclear how many ultrasound-coupled skin-mounted markers would be required to provide accurate scapular tracking, and where they should be placed. In this study, ultrasound-coupled skin marker quantities and positions were optimised using Monte Carlo and simulated annealing methods. Monte Carlo simulation was used to determine the average target registration error in simulated scapular plane abduction motions for groups ranging from 1 to 40 ultrasound-coupled sensors. After the range of ideal sensor quantities was identified, a simulated annealing algorithm was used to identify optimal positions for the sensors. Using these positions, a single skin-mounted, ultrasound-coupled sensor placed over the acromion was found to reduce simulated registration errors by 40.5% compared to regular skin-mounted markers. Groups consisting of three or more sensors reduced simulated registration error at maximum scaption to sub-millimeter levels.