Introduction: X-ray imaging is a very powerful tool which can be exploited in several fields. In the last few years, its use in archaeology has grown consistently. One of the most recent and interesting applications of computed X-ray tomography (CT) is the analysis of soil blocks, coming from excavations, in order to seek for finds of different kinds and materials possibly contained therein. For this purpose, both medical and industrial CT scanners have been employed. In this paper, the application of a CT instrument specifically designed and developed for the analysis of Cultural Heritage materials is presented. We analysed a soil block extracted from a necropolis in the Italian region of Abruzzo and probably dating back to the VI-IV century B.C., which was found to contain a bronze belt. Results: Thanks to the versatility of the CT equipment we designed, a complete scan has been obtained in less than four hours and has delivered extremely useful information in a completely non-invasive way. The CT dataset and images allowed a virtual extraction of the find to be performed before the actual stratigraphic recovery that, in this case, was simplified thanks to the merging of the archaeological evidences and with information coming from scientific analyses. The information provided by the tomography consisted in: the distribution, shape and dimensions of fragments composing the artefact; indications about its general conditions; the recognition of repairs done in the past and the presence of different materials (although not precisely identified). Conclusions: The use of CT has great potential for the work of both archaeologists and restorers. The indirect extraction of an artefact from an archaeological excavation, which entailed moving a soil block to the laboratory, allowed one to reconstruct almost all its parts and to collect information about materials. CT analysis has been particularly useful for determining both its conditions and its repairs before the actual recovery, thereby facilitating the restoration process. The recovery and conservation of an historical piece like the one presented here can help archaeological and conservation studies, enrich a museum collection and contribute to the dissemination of acquired cultural information.
X-ray computed tomography (CT) is a non-destructive technique allowing the visualization of sample internal structure. A medical CT scanner consists in an X-ray source and a detector, which face each other and rotate around the sample. Bi-dimensional radiographic projections are acquired at various angles from 0° to 360° and then processed by a mathematical algorithm (i.e., image reconstruction) in order to obtain a 3D visualization of the object. Different types of image reconstructions exist, including the widely used filtered backprojection that requires little computational resources but produces less qualitatively accurate images, and iterative reconstruction, based on time consuming and numerically intensive algorithm but providing better images quality (Bushberg et al., 2012).The images are displayed in a gray scale related to the X-ray attenuation by the sample materials (Rizescu et al., 2001).Initially, this technology was developed for medical imaging, but its potential for other domains was rapidly understood (Mees et al., 2003).This paper explores a specific CT methodology, called dual-energy CT scanning (DECT), elaborated for the first time in the seventies by Alvarez and Makovski (1976). The technique consists in imaging objects with two different X-ray spectra and in combining the results to achieve various objectives, including one allowing the discrimination and identification of materials based on their density and elemental composition. From a physical perspective, DECT exploits the energy dependence of photoelectric absorption and Compton scattering
In this work related to the use of a commercial medical CT scanner for the non-destructive analysis of highly attenuating materials (mineral samples), the effect of backprojection techniques and truncation artifacts corrections were explored. For small ROIs, the CT couch interferes significantly in images of small samples (few centimeters). An iterative reconstruction algorithm (OSC-TV) was used to perform reconstructions from uncorrected raw projection data made available through a collaboration with the CT vendor, who provided binaries and methods to remove low-level, proprietary data corrections (for beam hardening). The OSC-TV algorithm is customizable, allowing for the use of different forward-projection and backprojection techniques. Reconstruction parameters were tuned by performing simulations in a virtual phantom involving highly attenuating materials. Strategies to reconstruct small ROIs were also explored, with the objective of reducing truncation artifacts. Three samples were scanned to compare a ray-driven backprojection and a voxel-driven backprojection technique based on bilinear interpolation. The voxel-driven approach led to better results in terms of noise and reconstruction artifacts. An iterative ROI reconstruction technique was used to reconstruct small ROIs. This technique allows obtaining a sinogram with the projections of the ROI only. With that, truncation artifacts were reduced, which led to images with less blurring.
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