Imaging techniques: Extra dimension for bone analysis.

Abstract

A combination of two techniques — computed tomography and small-angle X-ray scattering — and serious computing power have enabled multi-scale, three-dimensional analysis of bone and tooth tissue. See Letters p.349 & p.353 Small-angle X-ray scattering (SAXS) can, in principle, probe structural ordering across a wide range of length scales, from nanoscale to the macroscopic. However, an experimental method and analysis scheme to obtain three-dimensional images while preserving nanostructure orientation information remained out of reach. Two papers in this issue of Nature combine different tomographic principles with SAXS to yield this information. Marianne Liebi et al. introduce a generally applicable model that can describe the SAXS data and show how taking account of the symmetries intrinsic to many samples of interest — such as the preferred orientation of collagen fibrils in the human trabecular bone that they have studied — can make the process more manageable. The procedure demonstrated by Florian Schaff et al. introduces the concept of virtual tomography axes, which allows arrangement of the vast amount of data to enable a direct independent reconstruction of each reciprocal space component. For their example, they show the orientation and scattering strength of mineralized collagen in a human tooth, spatially resolved over several millimetres.