An x-ray beam is composed of individual photons with a range of energies. As the beam passes through an object, it becomes "harder," that is to say its mean energy increases, because the lower-energy photons are absorbed more rapidly than the higher-energy photons .Two types of artifact can result from this effect: so-called cupping artifacts and the appearance of dark bands or streaks between dense objects in the image.
Cupping Artifacts.
X rays passing through the middle portion of a uniform cylindrical phantom are hardened more than those passing though the edges because they are passing though more material. As the beam becomes harder, the rate at which it is attenuated decreases, so the beam is more intense when it reaches the detectors than would be expected if it had not been hardened. Therefore, the resultant attenuation profile differs from the ideal profile that would be obtained without beam hardening . A profile of the CT numbers across the phantom displays a characteristic cupped shape
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