Beam hardening is the phenomenon occurring when an x-ray beam comprised of polychromatic energies passes through an object, resulting in selective attenuation of lower energy photons. The effect is conceptually similar to a high-pass filter, in that only higher energy photons are left to contribute to the beam and thus mean beam energy is increased ("hardened") 1.
This same phenomenon is exploited in radiography and CT, by use of metal filters in order to "pre-harden" the x-ray spectrum and minimize low energy photons (see filters) 2.
In CT, beam hardening from a very dense target (e.g. bone or iodinated contrast) may result in characteristic artifacts. CT beam hardening artifact has two distinct manifestations, streaking (dark bands) and cupping artifacts.
Streaking artifact appears as multiple dark streaking bands positioned between two dense objects, for example at the posterior fossa. Streaking may also occur along the long axis of a single high attenuation object. It is the result of the polychromatic x-ray being ‘hardened’ at different rates according to rotational position of the tube/detector.
Cupping artifact refers to a falsely bright appearance along the periphery of an object. Because the x-ray beam is "hardened" by passing through the target tissue, the mean photon energy will be higher near the tissue exit. Since higher energy photons are less attenuated by tissue, the beam will be less attenuated versus identical tissue near the skin entry site. If uncorrected during CT reconstruction, these differences in expected attenuation profile lead to a peripherally dense appearance.
Since simple beam hardening correction is built into modern scanners, cupping artifact is not usually encountered during clinical imaging. The characteristic "cupped shaped profile" of the CT numbers is best demonstrated when scanning phantoms 1,2.
Beam hardening reduction
Most modern CT scanners utilize filters in an attempt to overcome beam hardening. Often an attenuating substance (usually metallic) is appropriated to harden the beam before it reaches the patient.
CT scanners often need to be calibrated with vendor-specific phantoms to overcome unavoidable beam hardening artifacts such a cupping.
Streak artifacts can sometimes effectively be reduced by increasing tube voltage (better penetration of high density objects), or by using a dual-energy imaging approach. Many modern scanners are now also equipped with metal artifact reduction algorithms that utilize iterative reconstruction to limit beam hardening artifacts 3.
- 1. Pessis E, Campagna R, Sverzut JM, Bach F, Rodallec M, Guerini H, Feydy A, Drapé JL. Virtual monochromatic spectral imaging with fast kilovoltage switching: reduction of metal artifacts at CT. (2013) Radiographics : a review publication of the Radiological Society of North America, Inc. 33 (2): 573-83. doi:10.1148/rg.332125124 - Pubmed
- 2. Barrett JF, Keat N. Artifacts in CT: recognition and avoidance. (2004) Radiographics : a review publication of the Radiological Society of North America, Inc. 24 (6): 1679-91. doi:10.1148/rg.246045065 - Pubmed
- 3. Benjamin L. Triche, John T. Nelson Jr, Noah S. McGill, Kristin K. Porter, Rupan Sanyal, Franklin N. Tessler, Jonathan E. McConathy, David M. Gauntt, Michael V. Yester, Satinder P. Singh. Recognizing and Minimizing Artifacts at CT, MRI, US, and Molecular Imaging. (2019) RadioGraphics. 39 (4): 1017-1018. doi:10.1148/rg.2019180022 - Pubmed
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Physics and imaging technology: CT
computed tomography (CT)
- CT technology
- dual energy CT
- CT image reconstruction
- CT image quality
- CT dose
CT contrast medium
- iodinated contrast media
- coronary CT angiography
- patient-based artifacts
- physics-based artifacts
- hardware-based artifacts
- CT safety
- history of CT