Comparison of ultrasound and dual energy X-ray absorptiometry measurements in the calcaneus

Ultrasound measurements have been proposed as a means of providing structural information on bone but have also been shown to correlate with density. High correlation coefficients have been obtained in vitro, but are lower for in vivo work. The aim of this study was to investigate the relationship of broadband ultrasound attenuation (BUA), speed of sound (SOS) and stiffness, to bone mineral density (BMD) measured in the calcaneus at a closely matched region of interest (ROI). Measurements were made in 55 post-menopausal and 20 young, normal women. Calcaneus BMD measurements were made using an ROI (fixed by reference to external axes) to represent the area and location of the ultrasound transducers and an ROI based on anatomical markers, positioned in the posterior part of the calcaneus. BUA was significantly correlated with calcaneus BMD in the fixed ROI (r = 0.77, p < 0.0001) and the anatomical ROI (r = 0.78, p < 0.0001). Correlation of BUA with axial BMD was lower at r = 0.63, p < 0.0001 for the spine (L2-L4) and r = 0.59, p < 0.0001 for the femoral neck. Results for SOS and stiffness were very similar. From the calcaneus dual energy X-ray absorptiometry (DXA) scan images the region representing the ultrasound measurement area was found to contain cortical bone edges in 60% of cases. In 16% of scans a small part (4.5 +/- 4.0%) of the ROI measurement area fell partially outside the heel. However, there was no obvious discrepancy in the ultrasound results in the individual cases where this occurred. The correlation between calcaneus BMD and BUA was improved from r = 0.72 to r = 0.86 when scans with a cortical edge in the measurement ROI were excluded. The values for SOS were similar. In summary, BMD of the calcaneus, when closely matched to the site of ultrasound measurement shows significant correlation with ultrasound measurements. By excluding scans in which the ultrasound measurement appeared to include bone edges, correlations of approximately r = 0.86 were obtained. However, the BMD result still does not explain all of the variance in the ultrasound results.