Michael G J, Henderson C J
Centre for Medical and Health Physics, Queensland University of Technology, Brisbane, Australia.
Phys Med Biol. 1998 Sep;43(9):2583-96. doi: 10.1088/0031-9155/43/9/011.
The precision achieved in measuring bone mineral density (BMD) by commercial dual-energy x-ray absorptiometry (DXA) machines is typically better than 1%, but accuracy is considerably worse. Errors, due to inhomogeneous distributions of fat, of up to 10% have been reported. These errors arise because the DXA technique assumes a two-component model for the human body, i.e. bone mineral and soft tissue. This paper describes an extended DXA technique that uses a three-component model of human tissue and significantly reduces errors due to inhomogeneous fat distribution. In addition to two x-ray transmission measurements, a measurement of the path length of the x-ray beam within the patient is required. This provides a third equation, i.e. T = ts + tb + tf where T, ts, tb and tf are the total, lean soft tissue, bone mineral and fatty tissue thicknesses respectively. Monte Carlo modelling was undertaken to make a comparison of the standard and extended DXA techniques in the presence of inhomogeneous fat distribution. Two geometries of varying complexity were simulated. In each case the extended DXA technique produced BMD measurements that were independent of soft tissue composition whereas the standard technique produced BMD measurements that were strongly dependent on soft tissue composition. For example, in one case, the gradients of the plots of BMD versus fractional fat content were for standard DXA (-0.183+/-0.037) g cm(-2) and for extended DXA (0.027+/-0.044) g cm(-2). In all cases the extended DXA method produced more accurate but less precise results than the standard DXA technique.
商用双能X线吸收测定仪（DXA）测量骨矿物质密度（BMD）所达到的精度通常优于1%，但准确性要差得多。据报道，由于脂肪分布不均匀导致的误差高达10%。这些误差的产生是因为DXA技术假定人体为双组分模型，即骨矿物质和软组织。本文描述了一种扩展的DXA技术，该技术使用人体组织的三组分模型，显著减少了因脂肪分布不均匀而产生的误差。除了两次X线透射测量外，还需要测量患者体内X线束的路径长度。这提供了第三个方程，即T = ts + tb + tf，其中T、ts、tb和tf分别是总厚度、瘦软组织厚度、骨矿物质厚度和脂肪组织厚度。进行了蒙特卡罗建模，以比较在存在不均匀脂肪分布的情况下标准DXA技术和扩展DXA技术。模拟了两种不同复杂程度的几何形状。在每种情况下，扩展DXA技术产生的BMD测量值与软组织组成无关，而标准技术产生的BMD测量值强烈依赖于软组织组成。例如，在一种情况下，BMD与脂肪含量分数的关系图的斜率，标准DXA为（-0.183±0.037）g/cm²，扩展DXA为（0.027±0.044）g/cm²。在所有情况下，扩展DXA方法产生的结果比标准DXA技术更准确但精度更低。
