Wong F S, Elliott J C
Department of Paediatric Dentistry, Faculty of Clinical Dentistry, St. Bartholomew's and the Royal London School of Medicine and Dentistry, Queen Mary and Westfield College, London, U.K.
Scanning. 1997 Nov;19(8):541-6. doi: 10.1002/sca.4950190803.
X-ray absorption and backscattered electron (BSE) microscopies are two commonly used techniques for estimating mineral contents in calcified tissues. The resolution in BSE images is usually higher than in x-ray images, but due to the previous lack of good standards to quantify the grey levels in BSE images of bones and teeth, x-ray microtomography (XMT) images of the same specimens have been used for calibration. However, the physics of these two techniques is different: for a specimen with a given composition, the x-ray linear attenuation coefficient is proportional to density, but there is no such relation with the BSE coefficient. To understand the reason that this calibration appears to be valid, the behaviour of simulated bone samples was investigated. In this, the bone samples were modelled as having three phases: hydroxyapatite (Ca10(PO4)6(OH)2), protein, and void (either empty or completely filled with polymethylmethacrylate (PMMA), a resin which is usually used for embedding bones and teeth in microscopic studies). The x-ray linear attenuation coefficients (calculated using published data) and the BSE coefficients (calculated using Monte Carlo simulation) were compared for samples of various phase proportions. It was found that the BSE coefficient correlated only with the x-ray attenuation coefficient for samples with PMMA infiltration. This was attributed to the properties of PMMA (density and mean atomic number) being very similar to those of the protein; therefore, the sample behaves like a two-phase system which allows the establishment of a monotonic relation between density and BSE coefficient. With the newly developed standards (brominated and iodinated dimethacrylate esters) for BSE microscopy of bone, grey levels can be converted to absolute BSE coefficients by linear interpolation, from which equivalent densities can be determined.
X射线吸收和背散射电子（BSE）显微镜技术是估算钙化组织中矿物质含量的两种常用技术。BSE图像的分辨率通常高于X射线图像，但由于以往缺乏用于量化骨骼和牙齿BSE图像中灰度级的良好标准，同一标本的X射线显微断层扫描（XMT）图像已被用于校准。然而，这两种技术的物理原理不同：对于给定成分的标本，X射线线性衰减系数与密度成正比，但与BSE系数不存在这种关系。为了理解这种校准看似有效的原因，研究了模拟骨样本的行为。在此，将骨样本建模为具有三个相：羟基磷灰石（Ca10(PO4)6(OH)2）、蛋白质和空隙（要么为空，要么完全填充有聚甲基丙烯酸甲酯（PMMA），一种通常用于在显微镜研究中包埋骨骼和牙齿的树脂）。比较了不同相比例样本的X射线线性衰减系数（使用已发表数据计算）和BSE系数（使用蒙特卡罗模拟计算）。发现对于有PMMA浸润的样本，BSE系数仅与X射线衰减系数相关。这归因于PMMA的性质（密度和平均原子序数）与蛋白质的性质非常相似；因此，样本表现得像一个两相系统，这使得密度和BSE系数之间能够建立单调关系。有了新开发的用于骨骼BSE显微镜检查的标准（溴化和碘化二甲基丙烯酸酯），灰度级可以通过线性插值转换为绝对BSE系数，由此可以确定等效密度。
