Registering 2D and 3D imaging data of bone during healing

<i>Purpose/Aims of the study</i>: Bone’s hierarchical structure can be visualized using a variety of methods. Many techniques, such as light and electron microscopy generate two-dimensional (2D) images, while micro-computed tomography (µCT) allows a direct representation of the three-dimensional (3D) structure. In addition, different methods provide complementary structural information, such as the arrangement of organic or inorganic compounds. The overall aim of the present study is to answer bone research questions by linking information of different 2D and 3D imaging techniques. A great challenge in combining different methods arises from the fact that they usually reflect different characteristics of the real structure. <i>Materials and methods</i>: We investigated bone during healing by means of µCT and a couple of 2D methods. Backscattered electron images were used to qualitatively evaluate the tissue’s calcium content and served as a position map for other experimental data. Nanoindentation and X-ray scattering experiments were performed to visualize mechanical and structural properties. <i>Results</i>: We present an approach for the registration of 2D data in a 3D µCT reference frame, where scanning electron microscopies serve as a methodic link. Backscattered electron images are perfectly suited for registration into µCT reference frames, since both show structures based on the same physical principles. We introduce specific registration tools that have been developed to perform the registration process in a semi-automatic way. <i>Conclusions</i>: By applying this routine, we were able to exactly locate structural information (e.g. mineral particle properties) in the 3D bone volume. In bone healing studies this will help to better understand basic formation, remodeling and mineralization processes.

研究目的
骨骼的分级结构可通过多种方法实现可视化。诸多成像技术（如光学显微镜、电子显微镜）可生成二维（2D）图像，而显微计算机断层扫描（micro-computed tomography，µCT）则可直接获取三维（3D）骨骼结构的可视化结果。此外，不同成像技术可提供互补的结构信息，例如骨组织中有机与无机化合物的排布方式。本研究的核心目标，是通过整合不同二维与三维成像技术获取的信息，解答骨骼研究领域的相关科学问题。然而，整合多模态成像数据面临一项核心挑战：不同成像技术通常仅能反映真实骨骼结构的部分特性。
材料与方法
本研究采用显微CT（µCT）与若干种二维成像技术，对愈合过程中的骨组织样本开展了观测。背散射电子成像（backscattered electron images）可对骨组织的钙含量进行定性评估，并作为其他实验数据的定位图谱。此外，本研究还开展了纳米压痕（nanoindentation）与X射线散射（X-ray scattering）实验，以表征骨组织的力学与结构特性。
研究结果
本研究提出了一种将二维数据配准（registration）至三维显微CT参考坐标系的方法，其中扫描电子显微镜（scanning electron microscopy，SEM）可作为方法学上的衔接纽带。背散射电子成像非常适用于显微CT参考坐标系的配准，因其与显微CT均基于相同的物理原理呈现组织结构。本研究还开发了专用配准工具，可实现半自动化的配准流程。
研究结论
通过应用该实验流程，本研究可在三维骨组织体积内精准定位各类结构信息（例如矿物颗粒特性）。在骨骼愈合研究中，该方法将有助于更深入地解析骨骼的基本形成、重塑与矿化过程。