Phase contrast Breast tomography with synchrotron radiation: study and optimisation of experimental and reconstruction parameters.

Breast cancer is one of the most frequently diagnosed cancer types among women and it is the second leading cause of cancer related death. Therefore, breast cancer has become a critical public health concern and its detection at an early stage is crucial. For this reason, to improve the effectiveness of breast cancer screening, there is an increasing interest in the development of novel tools and imaging techniques that may supplement or replace mammography which is known for its limited specificity. In this context, in recent years, the development of x-ray phase-contrast imaging techniques, which exploit x-rays refraction in the body, have shown promising results for refining breast cancer diagnosis. It has been demonstrated that for x-rays passing through soft tissues, phase variations can be two to three orders of magnitude larger than absorption, and therefore an increased image contrast can be achieved with phase sensitive techniques. In addition, phase-contrast techniques hold the potential to reduce the radiation dose delivered to the patient. Several approaches for phase-contrast CT techniques have been studied, however, in this work, the focus is on propagation-based phase-contrast CT (PB-CT) as it does not re quire additional optical elements which makes it very dose efficient and easy to implement. Up to now the required level of spatial coherence of the incident x-ray beam used in PB-CT has practically limited its application to synchrotron facilities, however, the optimisation scheme for PB-CT breast imaging presented here can be generalised to novel generator-based phase-contrast imaging setups (including analyser-based imaging, edge-illumination, and grating-based imaging) or compact sources. The aim of this thesis work was to optimise the experimental protocol and data analysis for in-vivo three-dimensional phase-contrast breast imaging. Results are presented of feasibility studies carried out at Elettra and Australian Synchrotron using the propagation-based phase-contrast tomography method. Several breast tissue samples, of a size corresponding to a full human breast, and a custom designed plastic phantom have been studied at different resolution scales and experimental conditions. Several computed tomography reconstruction algorithms with different pre-processing and post-processing steps have been considered. Special attention was paid to the effect of phase retrieval on the diagnostic value of the reconstructed images. The images were quantitatively evaluated using objective quality indices and subjective assessments performed by experienced radiologists and pathologists. The results of the analysis indicate that, within the investigated range of parameters, both the objective image quality characteristics and the subjective radiological scores of propagation-based phase-contrast images of breast tissues monotonically increase with the strength of phase contrast which in turn is directly proportional to the product of the radiation wavelength and the sample-to-detector distance. In addition, the application of phase retrieval processing together with use of iterative reconstruction algorithms is fundamental to increase the intrinsic quality of the reconstructed CT images. The outcomes of this study serve to define the practical imaging conditions and the CT reconstruction procedures appropriate for low-dose phase-contrast breast imaging of live patients at specially designed synchrotron beamlines.