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Beam hardening artefacts arise in tomography and microtomography with polychromatic sources. Typically, specimens
appear to be less dense in the center of reconstructions because as the path length through the specimen increases, so the
X-ray spectrum is shifted towards higher energies due to the preferential absorption of low energy photons. Various
approaches have been taken to reduce or correct for these artefacts. Pre-filtering the X-ray beam with a thin metal sheet
will reduce soft energy X-rays and thus narrow the spectrum. Correction curves can be applied to the projections prior to
reconstruction which transform measured attenuation with polychromatic radiation to predicted attenuation with
monochromatic radiation. These correction curves can be manually selected, iteratively derived from reconstructions
(this generally works where density is assumed to be constant) or derived from a priori information about the X-ray
spectrum and specimen composition. For hard tissue specimens, the latter approach works well if the composition is
reasonably homogeneous. In the case of an immersed or embedded specimen (e.g., tooth or bone) the relative
proportions of mineral and “organic” (including medium and plastic container) species varies considerably for different
ray paths and simple beam hardening correction does not give accurate results. By performing an initial reconstruction,
the total path length through the container can be determined. By modelling the X-ray properties of the specimen, a 2D
correction transform can then be created such that the predicted monochromatic attenuation can be derived as a function
of both the measured polychromatic attenuation and the container path length.
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