Infographics / CT Physics

Beam hardening, cupping, ring & geometric artefacts

A polychromatic x-ray beam enters a uniform absorber; low-energy photons are preferentially absorbed so the beam's mean photon energy rises with depth (the beam hardens), shown by a shifting energy spectrum. This causes the CT cupping artefact, where a uniform object is measured as less dense in the centre than at the edge, plus dark streaks between dense structures.

Beam hardening: mechanism and the CT cupping artefact A polychromatic x-ray beam enters a uniform absorber; low-energy photons are preferentially absorbed so the beam's mean photon energy rises with depth (the beam hardens), shown by a shifting energy spectrum. This causes the CT cupping artefact, where a uniform object is measured as less dense in the centre than at the edge, plus dark streaks between dense structures. Beam hardening: mechanism and the CT cupping artefact Mechanism mean photon energy rises with depth source uniform absorber low-energy photons preferentially absorbed polychromatic beam in hardened beam out photon energy photons mean E rises before after (harder) leads to Result: cupping artefact in CT uniform object (axial CT) centre reads less dense (darker) edge reads denser measured CT number position across object true (uniform) measured (cupped) cupping Beam hardening is greater along longer or denser paths, so it varies across the field of view. It also causes dark streaks between dense structures such as bone and iodinated contrast.
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