Beam hardening is observed when an X-ray beam comprised of polychromatic energies passes through an object and becomes ‘harder’ as the lower energy photons will be absorbed leaving only the higher energy photons 1.
The resultant artifact has two distinct appearances, streaking or dark bands and cupping artifacts.
Streaking artifact appears between two dense objects and has the distinctive appearance of multiple dark streaking bands. It is the result of the polychromatic X-ray being ‘hardened’ at different rates in the tube position.
As the polychromatic X-ray beam passes through the body, it will naturally become harder and results in the attenuation of the beam reducing, and thus the resultant expected attenuation profile being slightly different to the expected ‘no variables’ attenuation profile. This leads to a cupped shaped profile of the CT numbers and is best demonstrated when scanning phantoms 1,2.
Beam hardening reduction
Most modern CT scanners utilise filters in an attempted to overcome beam hardening. Often an attenuating substance (often metallic) is appropriated to harden the beam before it reaches the patient.
Often, CT scanners need to be calibrated with vendor specific phantoms to overcome unavoidable beam hardening artifacts such a cupping.
- 1. Pessis E, Campagna R, Sverzut JM et-al. Virtual monochromatic spectral imaging with fast kilovoltage switching: reduction of metal artifacts at CT. Radiographics. 2013;33 (2): 573-83. doi:10.1148/rg.332125124 - Pubmed citation
- 2. Barrett JF, Keat N. Artifacts in CT: recognition and avoidance. Radiographics. 2004;24 (6): 1679-91. doi:10.1148/rg.246045065 - Pubmed citation
computed tomography (CT)
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