Correction for soft tissue in cortical bone assessment by ultrasound

Abstract

One of the key points in ultrasound measurements on cortical bone is the correction for soft tissue. We designed a new probe based on bi-directional axial transmission which automatically compensates velocity measurements for the soft tissue effect without preliminary evaluation of soft tissue properties. The probe consists in a linear arrangement of transducers with two sources placed on both sides of a unique group of receivers. The velocity of waves propagating parallel to the bone axis is deduced from a combination of the time delays derived from waves propagating in opposite directions at successive receivers separated by a known distance. This technique efficiently corrects for the major source of error on velocity encountered in clinical measurements which is caused by the variation of soft tissue thickness along the probe. The bi-directional technique was validated on test samples for which the residual precision error on velocity measurements was reduced to 0.2%. In vivo measurements yielded a value of 0.5% for the interoperator reproducibility. The clinical range of variation of the velocity measured by bi-directional technique is evaluated using clinical measurements on more than 200 subjects. Bi-directional transmission is a promising technique to minimize the variability of in vivo velocity measurements.