Raum K
Dept. of Orthopedics, Martin Luther Univ. of Halle-Wittenberg, Halle, Germany.
IEEE Trans Ultrason Ferroelectr Freq Control. 2008 Jul;55(7):1417-31. doi: 10.1109/TUFFC.2008.817.
Several high-frequency ultrasound techniques have been developed during the last decade with the intention of assessing elastic properties of bone at the tissue level. The basic measurement principles can be divided into: 1) measurement of the compressional wave velocity in thin tissue sections; 2) measurement of surface acoustic wave velocities in thick sections; and 3) derivation of the acoustic impedance from the confocal reflection amplitude in thick sections. In this paper, the 3 principles are described with example measurements given in the frequency range from 50 MHz to 1.2 GHz. The measurements were made with 2 microscopes operating in the pulse-echo mode, either with frequencies up to 200 MHz and time-resolved detection or between 100 MHz and 2 GHz and amplitude detection. The methods are compared and their application potentials and limitations are discussed with respect to the hierarchical structure of cortical bone. Mapping of the confocal reflection amplitude has superior capabilities for deriving quantitative elastic and structural parameters in the heterogeneous bone material. Even at low frequencies (50 MHz), the mineralized tissue matrix can be separated from the larger pores (Haversian canals), and the elastic coefficient in the probing direction can be measured in 2 dimensions. Depending on the type of sample surface preparation (flat or cylindrically shaped), local distribution of a single elastic coefficient or the average transverse isotropic stiffness tensor can be derived. With frequencies in the GHz range, the lamellar bone structure can be analyzed. However, at one GHz, the acoustic wavelength is still one order of magnitude larger than the individual mineralized collagen fibrils. Although the thickness of a lamellar unit can easily be assessed from the acoustic image, the derivation of the anisotropic elastic properties of the mineralized collagen fibrils as well as the detailed structure of a lamella can only be accomplished with further model assumptions.
在过去十年中，已经开发出几种高频超声技术，旨在评估组织水平上骨骼的弹性特性。基本测量原理可分为：1）测量薄组织切片中的压缩波速度；2）测量厚切片中的表面声波速度；3）从厚切片中的共焦反射幅度推导声阻抗。在本文中，描述了这三种原理，并给出了在50 MHz至1.2 GHz频率范围内的示例测量。测量是使用两台以脉冲回波模式运行的显微镜进行的，一台频率高达200 MHz并采用时间分辨检测，另一台频率在100 MHz至2 GHz之间并采用幅度检测。对这些方法进行了比较，并就皮质骨的层次结构讨论了它们的应用潜力和局限性。共焦反射幅度映射在推导异质骨材料中的定量弹性和结构参数方面具有卓越能力。即使在低频（50 MHz）下，矿化组织基质也可以与较大的孔隙（哈弗斯管）分离，并且可以在二维中测量探测方向上的弹性系数。根据样品表面制备的类型（平面或圆柱形），可以推导出单个弹性系数的局部分布或平均横向各向同性刚度张量。在GHz范围内的频率下，可以分析板层骨结构。然而，在1 GHz时，声波波长仍比单个矿化胶原纤维大一个数量级。尽管可以从声学图像轻松评估板层单元的厚度，但只有通过进一步的模型假设才能完成矿化胶原纤维各向异性弹性特性以及板层详细结构的推导。
