The biggest error is created if the scattered photons end up in a detector area which would count otherwise only few photons. The scattering is caused by interactions with the material, for instance by Compton scattering. The diffraction angle is random and the distribution of the scattered photons on the detector is dependent of the object. Scatter and Noiseĭuring the propagation of the radiation through the material a part of the photons are scattered and as a result diffracted from the original path. Furthermore, a dual energy CT can be used to reduce beam hardening. This problem can be addressed through an iterative approach. However high attenuation material can thus not be fully corrected because of the large difference to the mean value. Another correction method for beam hardening is performed with the calibration on the average attenuation of the material. Alternatively a flat piece of material, typically made of copper, can be used to “pre-harden” the beam and filter out the low energy radiation. The beam hardening artifacts can be reduced by using a higher voltage for producing the X-ray radiation, which leads to a harder beam and thus less beam hardening artifacts. The beam hardening can result in dark streaks along the lines of greatest attenuation. The part is a metal bar with homogeneously material, the cross-section is shown. The thickness of the rays represents the intensity of the radiation.įigure 2: Example for cupping beam hardening artifacts. In the thicker object b) the X-Rays with lower frequency are absorbed almost completely in comparison to the thinner object a). This is called cupping artifact and can be seen in Figure 2.įigure 1: Schematic illustration of the beam hardening of X-Rays with a low (green) and a high (orange) frequency. When radiating through a cylinder X-Rays passing through the middle are hardened more because they are passing through more material. Through the beam hardening object b) in Figure 1 appears thicker. In Figure 1 the beam hardening effect is displayed on two objects of different thickness. The intensity of beam hardening depends on the material and geometry of the object. This causes the frequency spectrum to shift in the direction of higher frequencies, but this does not mean that the maximum energy is increased. ĭuring the propagation of the X-Rays through the material, lower energy photons are absorbed more frequently and the beam becomes “harder”. H is the constant of proportionality with the name Planck’s constant. This is explained by the Planck-Einstein relation which states that the energy E of a photon is proportional to its frequency X-ray beams are composed of photons within a specific range of energy (see X-Ray spectrum) and therefore they are not monochromatic.
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