Estimation of femoral bone density from trabecular direct wave and cortical guided wave ultrasound velocities measured at the proximal femur in vivo

Bone mineral density (BMD) of the proximal femur measured using bone densitometry is a predictor of hip fracture risk. We developed a Quantitative Ultrasound (QUS) scanner for measurements at this site with similar performance (FemUS). In this study, we tested if ultrasound velocities of direct waves through trabecular bone and of guided waves through cortical bone could be used to estimate BMD. In two centres, Kiel and Odense, we measured time-of-flight (TOF) of waves through the trabecular greater trochanter and cortical intertrochanter as well as a wave through soft tissue only. TOF was adjusted for leg width using ultrasound echoes reflected from the skin of the leg to yield apparent speed of sound (SOS) of different wave components. Data were cross-calibrated between centres and pooled. Bivariate correlation and multivariate analyses were used to compare QUS results with hip BMD. BMD correlated significantly both with trabecular and cortical SOS but not soft tissue SOS. The combination of trabecular, cortical and soft tissue SOS improved the correlation and all three variables contributed independently. In conclusion, multiwave ultrasound methods allow estimation of hip l BMD with a residual error of about 10%.