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Sub-Micrometer X-ray Tomography of Radiolarians: Computer Modeling and Skeletonization

Introduction X-ray tomography is used to visualize the surface and interior structure of solid objects on the basis of X-ray attenuation (the proportion of X rays scattered or absorbed as they pass through an object) [1]. A tomographic series is acquired by rotating an X-ray source around a specimen and collecting transmitted X-ray intensity on a series of detectors. The detectors acquire a set of images, which are X-ray projections. These are converted to a voxelated 3D file that can be visualized and analyzed with a wide variety of 3D-based imaging programs. Resolution of these image files (designated as voxel pitch) depends on X-ray intensity, focal spot size of the X-ray source, detector size, and noise reduction. Using a combination of these factors, X-Ray Engineering, Belgium XRE (www.xre.be), has developed custom-made X-ray scanning systems designed for a variety of applications. These instruments generate tomographic information well into the sub-micrometer range. Radiolarians are amoeboid protists with elaborate, radially symmetrical mineral skeletons [2]. Fossilized radiolarian skeletons (also called tests) consist of SiO2, which is X-ray attenuating. This provides the mechanism for acquisition of 3D data sets by X-ray tomography. With the development of high-resolution computerized tomography (CT), detailed 3D data sets of radiolarians can be acquired with sub-micrometer resolution. We report here the recording of X-ray tomograms (0.103 μm voxel pitch) of selected radiolarian tests and subsequent computer modeling of 3D data sets that reveal intricate details of the exterior and interior of the specimens. Also, digital skeletonization of some of the rendered models reveals their essential geometrical features, which can be measured with great accuracy based on the voxel size of the original data set.