Ward K A, Adams J E, Hangartner T N
Clinical Radiology, Imaging Science & Biomedical Engineering, Stopford Building, University of Manchester, Oxford Road, Manchester, M13 9PT, UK.
Calcif Tissue Int. 2005 Nov;77(5):275-80. doi: 10.1007/s00223-005-0031-x. Epub 2005 Nov 16.
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.
外周定量计算机断层扫描（pQCT）广泛应用于临床和研究目的。为准确测定骨几何结构（骨横截面积、皮质厚度和皮质面积）、骨体积密度（vBMD）和皮质骨矿物质含量（BMC），选择合适的阈值很重要。使用Stratec XCT - 2000扫描仪将当前标准做法与新的优化阈值进行比较。目前，测量皮质vBMD和几何结构时使用的单一阈值为710 mg/mL。我们假设该阈值可能并非最佳，并使用欧洲前臂模型（EFP）和患者数据来测试更合适的阈值。在EFP的第4节（代表长骨干骺端部分）制作了一个单层切片（宽度1.2 mm，像素大小0.4 mm）。EFP的已知皮质厚度为2.5 mm，因此，几何结构的正确阈值应是能将皮质厚度测量为2.5 mm的那个值。在软组织（60 mg/mL）和峰值密度（879 mg/mL）之间大约50%的值处改变阈值，并绘制皮质厚度与阈值的关系图；几何结构的正确阈值为460 mg/mL。通过将该阈值表示为EFP中密度值范围的百分比（[460 - 60]/[879 - 60] = 49%），然后将该百分比应用于体内数据，可确定几何结构的最佳阈值：（[1240 - 79]×0.49）+ 79 = 648 mg/mL。对于桡骨中轴部位体内骨测量的皮质vBMD，在峰值（1240 mg/mL）附近改变阈值，并将阈值设置为能给出皮质密度为1240 mg/mL的那个值；因此，皮质密度的阈值为1200 mg/mL。使用新阈值（骨几何结构为648 mg/mL，皮质vBMD为1200 mg/mL）与当前阈值（710 mg/mL）对一组年轻健康女性的桡骨扫描子集进行分析。对于骨几何结构，基于新阈值的分析与基于制造商推荐阈值的分析之间的平均差异在2.1%至14%之间（总面积 = 2.1%，皮质厚度 = 14%，皮质面积 = 3.7%）。尽管基于新阈值的分析与基于制造商推荐阈值的分析之间存在10%的差异，但这种差异并非系统性的。阈值将显著影响从pQCT获得的结果。当前710 mg/mL的阈值不足以准确测定骨几何结构和皮质vBMD。应使用几何结构的新阈值648 mg/mL和皮质vBMD的1200 mg/mL。
