BeamHardening

Beam hardening is a phenomenon in X-ray imaging in which a polychromatic beam becomes harder as it passes through matter. Low-energy photons are absorbed more readily than high-energy photons, so the transmitted spectrum shifts toward higher energies. Because attenuation coefficients depend on photon energy, this spectral hardening makes the effective attenuation nonuniform along different paths and challenges the assumption of a simple linear Beer-Lambert model used in standard CT reconstruction.

In computed tomography, beam hardening can produce artifacts such as cupping, where the reconstructed attenuation is lower at the center of a uniform object than at the edges, and streaks or dark bands between dense structures such as bone or metal implants. The artifacts degrade contrast, bias CT numbers, and can compromise material differentiation.

Mitigation strategies include physical measures such as filtration to harden the beam before it reaches the patient and bow-tie filters to shape the spectrum, along with anti-scatter grids. In reconstruction, beam-hardening correction algorithms, calibration with phantoms, and iterative methods that model the polychromatic spectrum are used. Dual-energy or spectral CT provides energy-resolved data that enables material decomposition and reduces beam-hardening effects. Metal artifact reduction techniques often incorporate explicit modeling of beam hardening.