Development of two-dimensional beam hardening correction for X-ray micro-CT.

BACKGROUND: Beam-hardening in tomography with polychromatic X-ray sources results from the nonlinear relationship between the amount of substance in the X-ray beam and attenuation. Simple linearisation curves can be derived with the use of an appropriate step wedge, however, this does not yield good results when different materials are present whose relationships between X-ray attenuation and energy are very different. OBJECTIVE: To develop a more accurate method of beam-hardening correction for two-phase samples, particularly immersed or embedded biological hard tissue. METHODS: Use of a two-dimensional step wedge is proposed in this study. This is not created physically but is derived from published X-ray attenuation coefficients in conjunction with a modelled X-ray spectrum, optimised from X-ray attenuation measurements of a calibration carousel. To test this method, a hydroxyapatite disk was scanned twice; first dry, and then immersed in 70% ethanol solution (commonly used to preserve biological specimens). RESULTS: With simple linearisation the immersed disk reconstruction exhibited considerable residual beam hardening, with edges appearing approximately 10% more attenuating. With 2-dimensional correction, the attenuation coefficient showed only around 0.5% deviation from the dry case. CONCLUSION: Two-dimensional beam-hardening correction yielded accurate results and does not require segmentation of the two phases individually.