Beam hardening is the phenomenon that occurs 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 the 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
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
Beam hardening will cause the middle of the image to decrease in value, not increase edge value, as the lower energy photons preferentially get attenuated over longer path lengths. As the beam becomes harder and passes a higher mean beam energy, the lower attenuation coefficient means the CT number goes down for longer paths.
If uncorrected during CT reconstruction, these differences in the expected attenuation profile lead to a perceived peripheral 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 as 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.
