X-Ray Tomography of Radiolarians
a series of X-ray projection images of internal structure are acquired through a rotation of 360 degrees [ 8 ]. Micro-CT systems have limited resolution, and sub- micrometer or Nano CT systems are required to resolve fi ne structure such as corrosion-casted blood capillary systems [ 8 ]. Micro- fossils of radiolarians and fora- miniferans [ 9 , 10 ] are ideal for Nano CT because they have X-ray attenuating skeletons with very fi ne detail, only resolved at the submicron level.
Figure 4 : Digital skeleton (medial axis transform) of a radiolarian test shown against a bisected background of the original model. This represents the essential geometry of the test, including polygonal segments and nodes (intersections of segments). Segment length is derived from the voxel dimension of the data set (inset).
of the file because each segment was a single row of voxels (inset of Figure 4 ). Figure 5 shows a 3D-printed model of a radiolarian, large enough to hold in your hand. Currently these models are printed in a strong but fl exible polymer.
Discussion
When microscopic biological structures are fluorescent or X-ray attenuating, 3D data sets can be acquired from them. In confocal fluorescence microscopy, a series of confocal fluorescent planes is used to generate a 3D file ( z -stack). However, because the exciting fluorescent beam is reduced in energy as it passes through the specimen, the thickness of the z -stack with sufficient fluorescent signal is limited [ 7 ]. This limitation is overcome by X-ray tomography, in which
Since the first detailed descriptions of radiolarians by Ernst Haeckel [ 11 ], thousands of extant and extinct species have been described [ 12 ] based on their collection sites and the geometry of their tests. Early reports relied on micro - scopy with poor capabilities in contrast, resolution, and depth of field. Scanning elec- tron microscopy has greatly
improved the ability to resolve radiolarian fi ne structure [ 13 ]. Although the SEM can be engineered to view specimens through 360 degrees, the internal structure of radiolarians such as the medullary tests [ 13 ] is obscured or hidden from view.
These limitations are overcome with sub-micrometer X-ray tomography, where highly resolved 3D models of radio larians can be rotated through any angle of viewing and digitally sliced to reveal medullar shells and their associa- tions with outer cortical shells. This greatly improves the precision of radiolarian description and taxonomic classification.
Mathematical analysis based on the geometry of the tests can also be performed because voxel dimension provides a precise measure of polygonal segments of the tests, as well as nodes and segment numbers. The rendered models consti- tute 3D files, which are amenable to scaling up and used for 3D printing of models that can be held in one’s hand.
Conclusion
Figure 5 : 3D-printed model of a radiolarian produced from a high-resolution tomographic dataset.
2015 September •
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Sub-micrometer X-ray tomography revealed the intricate structure of siliceous radiolarians at a resolution not achieved by conventional light microscopy. The ability to digitally slice rendered models of radiolarian tests provides additional internal structural details of medullary tests and the struts holding them to the outer wall. Skeletonization of radiolarian models provides a means to quantitate connecting nodes of polygonal units, as well as the number and lengths of the polygonal sides. These capabilities greatly improve structural characterization of radiolarians and their taxonomic classification.
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