Physics, Techniques and Procedures

Beam hardening

the phenomenon that a polychromatic X ray beam becomes more penetrating, or harder, as it traverses through matter. The X-ray beams used in medical imaging are polychromatic with a moderately broad energy spectrum. The attenuation processes in matter are energy dependent. Nonuniform attenuation of different energies results in the preferential depletion of X-rays in energy ranges with higher attenuation coefficients. In general, X-rays in energy ranges that are more easily attenuated are referred to as soft X-rays while those in ranges that are more penetrating are referred to as hard X-rays. Thus, beam hardening is the process of selective removal of soft X-rays from the X-ray beam. As these X-rays are removed, the beam becomes progressively harder or more penetrating. The amount of beam hardening depends on the initial X-ray spectrum as well as on the composition of the material or tissue traversed. However, for any fixed initial X-ray spectrum and tissue type, the process of beam hardening represents a monotonic increase in beam hardness as a function of tissue thickness traversed.

As a result of beam hardening, the effective attenuation coefficient of a material depends on the thickness of material traversed. This effect causes so-called beam hardening artefacts in CT images. If uncorrected, beam hardening artefacts appear as cupping, or a reduction of the reconstructed attenuation coefficient toward the centre of a large object. See also cupping artefact.

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