Recommendations for thresholds for cortical bone geometry and density measurement by peripheral quantitative computed tomography.

Peripheral quantitative computed tomography (pQCT) is widely used for clinical and research purposes. For accurate determination of bone geometry (bone cross-sectional area, cortical thickness, and cortical area), volumetric bone mineral density (vBMD) and cortical bone mineral content (BMC), it is important to select the appropriate thresholds. A Stratec XCT-2000 scanner was used to compare current standard practice with new optimized thresholds. Currently, a single threshold of 710 mg/mL for the measurement of cortical vBMD and geometry is used. We hypothesised that this threshold may not be optimal and used the European Forearm Phantom (EFP) and patient data to test more appropriate thresholds. A single slice (1.2 mm width, 0.4 mm pixel size) was made at section 4 of the EFP (representing the diaphyseal portion of a long bone). The EFP has a known cortical thickness of 2.5 mm and, therefore, the correct threshold for geometry would be that which measures cortical thickness as 2.5 mm. Thresholds were altered at approximately the 50% value between soft tissue (60 mg/mL) and peak density (879 mg/mL), and cortical thickness versus threshold was plotted; the correct threshold for geometry was 460 mg/mL. By expressing this threshold as a percentage of the range of density values in the EFP ([460-60]/[879-60] = 49%) and then applying this percentage to in vivo data, the optimum threshold for geometry can be determined: ([1240-79] x 0.49) + 79 = 648 mg/mL. For cortical vBMD of in vivo bone measurements at the midshaft site of the radius, thresholds were varied around the peak value (1240 mg/mL), and the threshold was set to that which gave a cortical density of 1240 mg/mL; the threshold for cortical density was, therefore, 1200 mg/mL. A subset of radius scans from a population of young healthy females was analyzed using the new thresholds (648 mg/mL for bone geometry, 1200 mg/mL for cortical vBMD) versus the current threshold (710 mg/mL). For bone geometry, the mean difference between the analysis based on the new threshold and that based on the manufacturer-recommended threshold ranged between 2.1% and 14% (total area = 2.1%, cortical thickness = 14%, cortical area = 3.7%). Although there was a 10% difference between the analysis based on the new threshold and that based on the manufacturer-recommended threshold, this difference was not systematic. Thresholds will significantly affect results obtained from pQCT. The current threshold of 710 mg/mL is inadequate for accurate determination of bone geometry and cortical vBMD. New thresholds of 648 mg/mL for geometry and 1,200 mg/mL for cortical vBMD should be used.