Accurate Mineralization Quantification in X-Ray Microtomography

Background: X-ray microtomography (XMT or micro-CT) using a conventional X-ray tube is a non-destructive three-dimensional imaging technique that can be used to measure and map mineral concentration in teeth in vitro. Since X-rays from a tube are polychromatic, however, greyscale artefacts are produced by ‘beam hardening' — the preferential absorption of low-energy photons in matter. This leads to errors in the measured X-ray linear attenuation coefficient (from which the mineral concentration is deduced). Furthermore, since this error varies spatially, it cannot be corrected simply by the use of a phantom.

Objectives: The primary objective of this project is to correct for beam hardening in the X-ray projections (prior to reconstruction) using a model of X-ray attenuation and detection whose parameters are derived from real attenuation measurements. This will minimise both the absolute and regional variation in errors caused by beam hardening, allowing for more precise mapping of mineral concentration.

Methods: A novel calibration test piece was developed in order to offer a wide range of X-ray attenuation measurements to characterise the model. The software then allows the model to be adapted according to the composition of the specimen using a priori knowledge of its composition and published X-ray attenuation data, making it unnecessary to procure physical phantoms made of the same or similar material as the specimen.

Results: The test piece calibration software produced calibration polynomials reliably and, with appropriate choices of filter material, with a significantly smaller discrepancy between the calculated and measured attenuations than the calibration step wedge previously in use.

Conclusion: This method of beam hardening correction allows for more accurate mapping of mineral concentration in teeth using X-ray microtomography.

This research was funded by the Engineering and Physical Sciences Research Council, under grant EP/G007845/1.