Beam hardening

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

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 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.