Beam Hardening

Beam hardening is well known problem that is described in many works, see [DEM13]. Lab based X-ray Computational Tomography (CT) machines use X-ray tubes as their source of radiation. The voltage to the tube can be varied to control the maximum X-ray energy generated, but the actual output is composed of the broad spectrum Bremsstrahlung component and characteristic peaks. The X-ray attenuation of materials (in general) decreases as the energy increases. This means that as a broad spectrum beam passes through a target the low energy X-rays will be absorbed first leaving a beam that has increasing mean energy. It is this effective hardening of the beam that means that a plot of the log of the attenuation against material thickness is not linear, as would be the case if a monochromatic X-ray source was used. Since CT reconstruction algorithms assume that this linear relationship holds artifacts are generated in the final image due to the beam hardening.




[DEM-13]  Graham R. Davis, Anthony N.Z. Evershed, and David Mills: Quantitative high contrast X-ray microtomography for dental research: Journal of Dentistry, Volume 41, Issue 5, May 2013, Pages 475–482.