Sekhon Kiranjit, Kazakia Galateia J, Burghardt Andrew J, Hermannsson Bryan, Majumdar Sharmila
Musculoskeletal Quantitative Imaging Research Group, Department of Radiology and Biomedical Imaging, University of California, San Francisco, 185 Berry St, Suite 350, San Francisco, CA 94107, USA.
Bone. 2009 Sep;45(3):473-9. doi: 10.1016/j.bone.2009.05.023. Epub 2009 Jun 6.
Accurate bone mineral density (BMD) quantification is critical in clinical assessment of fracture risk and in the research of age-, disease-, and treatment-related musculoskeletal changes. The development of high-resolution peripheral quantitative computed tomography (HR-pQCT) imaging has made possible in vivo assessment of compartmental volumetric BMD (vBMD) and bone micro-architecture in the distal radius and tibia. HR-pQCT imaging relies on a polychromatic X-ray source and therefore is subject to beam hardening as well as scatter artifacts. In light of these limitations, we hypothesize that the accuracy of HR-pQCT vBMD measurement in the trabecular compartment (vBMD(trab)) is not independent of bone density and geometry, but rather influenced by variations in trabecular bone volume fraction and cortical thickness. The goal of this study, therefore, was to evaluate the accuracy of HR-pQCT vBMD(trab) measurement in the radius and tibia, and to determine the dependence of this measurement on geometric and densitometric parameters. Our approach was to use a series of idealized hydroxyapatite (HA) phantoms with varying densities and geometries to quantify the accuracy of HR-pQCT analysis. Two sets of custom-made HA phantoms designed to mimic the distal tibia and distal radius were manufactured. Geometric and densitometric specifications were based on a dataset of healthy volunteers and osteopenic patients. Multiple beam hardening correction (BHC) algorithms were implemented and evaluated in their ability to reduce measurement error. Substantial errors in measured vBMD(trab) were found. Overestimation of vBMD(trab) increased proportional to cortical shell thickness and decreased proportional to insert density. The most pronounced vBMD(trab) overestimation therefore occurred in the phantoms with the lowest insert densities and highest shell thickness, where error was as high as 20 mg HA/cm3 (33%) in the radius phantom and 25 mg HA/cm(3) (41%) in the tibia phantom. Error in vBMD(trab) propagates to the calculation of micro-architectural measures; 41% error in vBMD(trab) will produce 41% error in volume fraction (BV/TV) and trabecular thickness (Tb.Th), and 5% error in trabecular separation (Tb.Sp). BHC algorithms supplied by the manufacturer failed to eliminate these errors. Our results confirm that geometric and densitometric variations influence the accuracy of HR-pQCT vBMD(trab) measurements, and must be considered when interpreting data across populations or time-points.
准确的骨矿物质密度（BMD）定量对于骨折风险的临床评估以及与年龄、疾病和治疗相关的肌肉骨骼变化的研究至关重要。高分辨率外周定量计算机断层扫描（HR-pQCT）成像技术的发展使得在体内评估桡骨远端和胫骨的分区体积骨密度（vBMD）和骨微结构成为可能。HR-pQCT成像依赖于多色X射线源，因此容易受到束硬化以及散射伪影的影响。鉴于这些局限性，我们假设小梁骨区域（vBMD(trab)）的HR-pQCT vBMD测量准确性并非独立于骨密度和几何形状，而是受小梁骨体积分数和皮质厚度变化的影响。因此，本研究的目的是评估桡骨和胫骨中HR-pQCT vBMD(trab)测量的准确性，并确定该测量对几何和密度参数的依赖性。我们的方法是使用一系列具有不同密度和几何形状的理想化羟基磷灰石（HA）体模来量化HR-pQCT分析的准确性。制造了两组旨在模拟胫骨远端和桡骨远端的定制HA体模。几何和密度规格基于健康志愿者和骨质减少患者的数据集。实施并评估了多种束硬化校正（BHC）算法减少测量误差的能力。发现测量的vBMD(trab)存在大量误差。vBMD(trab)的高估与皮质壳厚度成正比增加，与插入物密度成反比降低。因此，在插入物密度最低且壳厚度最高的体模中，vBMD(trab)的高估最为明显，其中桡骨体模中的误差高达20 mg HA/cm³（33%），胫骨体模中的误差高达25 mg HA/cm³（41%）。vBMD(trab)的误差会传播到微结构测量的计算中；vBMD(trab)中41%的误差将在体积分数（BV/TV）和小梁厚度（Tb.Th）中产生41%的误差，在小梁间距（Tb.Sp）中产生5%的误差。制造商提供的BHC算法未能消除这些误差。我们的结果证实，几何和密度变化会影响HR-pQCT vBMD(trab)测量的准确性，在解释不同人群或时间点的数据时必须予以考虑。
