In vitro ultrasonic characterization of human cancellous femoral bone using transmission and backscatter measurements: relationships to bone mineral density.

Thirty-eight slices of pure trabecular bone 1-cm thickness were extracted from human proximal femurs. A pair of 1-MHz central frequency transducers was used to measure quantitative ultrasound (QUS) parameters in transmission [normalized broadband ultrasound attenuation (nBUA), speed of sound (SOS)] and in backscatter [broadband ultrasound backscatter (BUB)]. Bone mineral density (BMD) was measured using clinical x-ray quantitative computed tomography. Site-matched identical region of interest (ROIs) of 7 x 7 mm2 were positioned on QUS and QCT images. This procedure resulted in 605 ROIs for all the specimens data pooled together. The short-term precision of the technique expressed in terms of CV was found to be 2.3% for nBUA, 0.3% for SOS and 4.5% for BUB. Significant linear correlation between QUS and BMD were found for all the 605 ROIs pooled, with r2 values of 0.73, 0.77, and 0.58 for nBUA, SOS, and BUB, respectively (all p < 0.05). For the BUB, the best regression was obtained with a polynomial fit of second order (r2 = 0.63). An analysis of measurements errors was developed. It showed that the residual variability of SOS is almost completely predicted by measurements errors, which is not the case for BUA and BUB, suggesting a role for micro-architecture in the determination of BUA and BUB.