Quantitative Absorption Tomography Using Polychromatic X-rays

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Department of Chemistry and Physics, La Trobe Molecular Sciences School, College of Science, Health and Engineering, La Trobe University, Victoria, Australia.

Thesis with publications.

X-ray computed tomography has been extensively used in many scientific and medical fields. High spatial resolution, high precision absorption x-ray tomography has been routinely conducted using monochromatized x-rays at synchrotron sources around the world. However, many practical applications of x-ray tomography have been carried out in small scale laboratories and medical clinics where the incident beams are polychromatic. Because interactions between an x-ray photon and matter depend on its energy, it is challenging to obtain quantitative results from a tomography measurement using polychromatic x-rays. One of the most significant sources of systematic error in absorption tomography using polychromatic x-rays is the well-known 'beam hardening' effect. As a polychromatic x-ray beam penetrates through an object, x-rays of lower energy are absorbed more strongly. This causes the spectrum of the x-ray beam to ‘shift’ toward the higher energy regime, resulting in an artificial reduction in the reconstructed density of the object. Overcoming this problem has attracted significant effort during the last decade or so. This work aims to develop a quantitative absorption tomography technique using polychromatic x-rays. The new technique takes into account the full spectral information of the incident polychromatic beam in the reconstruction process. The result is precise, quantitative, and free from the effects of beam hardening. Theoretical details and an experimental demonstration of the technique is described in determining the integrated column density of 2D objects and the density distribution of 3D objects. The results promise great opportunities for improving the capability of small-scale laboratories and medical clinics in conducting critical quantitative work.