Methods for reproducible in vivo apparent velocity in cancellous bone

The apparent velocity of ultrasound (AVU) yields clinically useful information about bone fragility. AVU is measured at the patella using a small, hand-held probe consisting of a pair of identical, wideband transducers affixed to a digital caliper capable of measuring the width of the bone. The apparent velocity of ultrasound transmission is determined by dividing the measured bone width by the time of flight of the transmitted ultrasound signal. The resulting measurement variability is 2%. An approach for further reducing variability using the arrival time derived from the first zero crossing (ZC), instead of the leading edge, is described. This produces a significant reduction in the variability caused by noise, dispersion of the leading edge, and multipath propagation. Results from a 2-year clinical study of 273 women show a reduction of variability from 41 M/s to 31 M/s. Theory, methods, and clinical results are presented.