Sigmund E E, Cho H, Song Y-Q
Department of Radiology, New York University, 660 First Avenue, New York, New York 10016, USA.
NMR Biomed. 2009 May;22(4):436-48. doi: 10.1002/nbm.1354.
Trabecular bone structure is known to play a crucial role in the overall strength, and thus fracture risk, of such areas of the skeleton as the vertebrae, spine, femur, tibiae, or radius. Several MR methods devoted to probing this structure depend upon the susceptibility difference between the solid bone matrix and the intervening fluid/marrow/fat, usually in the context of a linewidth (1/T(2)') measurement or mapping technique. A recently demonstrated new approach to this system involves using internal gradients to encode diffusion weighting, and extracting structural information (e.g., surface-to-volume ratio) from the resulting signal decay. This contrast method has been demonstrated in bulk measurements on cleaned, water-saturated bovine trabecular bone samples. In the present work, microscopic imaging (0.156 mm in-plane resolution) is performed in order to spatially resolve this contrast on the trabecular level, and confirm its interpretation for the bulk measurements. It is found that the local rate of decay due to diffusion in the internal field (DDIF) is maximal close to the trabecular surfaces. The overall decay rate in a lower resolution scan probes the abundance of these surfaces, and provides contrast beyond that found in conventional proton density weighted or T(1)-weighted imaging. Furthermore, a microscopic calculation of internal field distributions shows a qualitative distinction between the structural sensitivities of DDIF and T(2)'. DDIF contrast is highly localized around trabecular walls than is the internal field itself, making it a less sensitive but more specific measure of such important properties as trabecular number.
已知小梁骨结构在骨骼某些部位（如椎骨、脊柱、股骨、胫骨或桡骨）的整体强度以及骨折风险中起着关键作用。几种专门用于探测这种结构的磁共振方法依赖于实心骨基质与其间的液体/骨髓/脂肪之间的磁化率差异，通常是在测量线宽（1/T(2)'）或成像技术的背景下。最近展示的一种针对该系统的新方法涉及使用内部梯度对扩散加权进行编码，并从所得信号衰减中提取结构信息（例如表面积与体积比）。这种对比方法已在对清洁的、水饱和的牛小梁骨样本的整体测量中得到验证。在本研究中，进行了微观成像（平面分辨率为0.156毫米），以便在小梁水平上从空间上分辨这种对比，并确认其对整体测量结果的解释。研究发现，内部场中由于扩散导致的局部衰减率（DDIF）在小梁表面附近最大。较低分辨率扫描中的整体衰减率探测这些表面的丰度，并提供了传统质子密度加权成像或T(1)加权成像中未发现的对比度。此外，内部场分布的微观计算显示了DDIF和T(2)'在结构敏感性方面的定性差异。与内部场本身相比，DDIF对比度在小梁壁周围高度局部化，使其成为小梁数量等重要特性的不太敏感但更具特异性的度量。
