Reproducibility and concordance in quantitative ultrasound measurements between densitometers: a comparative study

Although calcaneal quantitative ultrasound (QUS) is an independent predictor of osteoporotic fracture, its role in monitoring changes in bone status remains limited because of its relatively poor precision compared to the rate of bone loss. Recently, imaging QUS has been developed that can standardize the region of interest in the calcaneus with the potential of improving precision. We assessed the concordance and precision of an imaging QUS scanner (UBIS 5000) and a nonimaging scanner (LUNAR Achilles+) in 52 subjects aged between 27 and 79 yr. Each subject had duplicate measurements on each scanner on the same day. The measurements were broad-band ultrasound attenuation (BUA), speed of sound (SOS), and stiffness index, which was derived from BUA and SOS. Precision was evaluated by the standard error of measurement (SEM) and within-subject coefficient of variation (CV). There was significant correlation between the two scanners in all QUS measurements (r > 0.8; p < 0.001); however, BUA and SOS measurements by the UBIS were significantly lower than by the Achilles+. The SEM of BUA (0.6 dB/MHz) and SOS (2.7 m/s) in the UBIS were significantly lower (p < 0.001) than the Achilles+ (1.4 dB/MHz for BUA and 6.3 m/s for SOS). When the SEM was expressed as the percentage of the mean, there were no significant differences in CVs between UBIS (0.9% for BUA and 0.2% for SOS) and Achilles+ (1.2% for BUA and 0.4% for SOS) scanners. The SEM of stiffness index derived by UBIS was not significantly different from that derived by the Achilles+. These data suggest that although there are systematic differences between the UBIS and Achilles+ scanners in QUS measurements, the precision of the two instruments is equivalent.