Polychromatic phase contrast imaging as a basic step towards a widespread application of the technique

Abstract

Phase contrast imaging (PCI) is probably the most exciting amongst emerging X-ray imaging techniques, as it has the potential to remove some of the main limitations of conventional radiology. As a consequence, significant effort is currently directed towards developing the technique for the first clinical implementations. In recent years, PCI has been widely experimented, but its use has been mainly restricted to synchrotron radiation (SR) facilities. Source-related limitations are in fact the most relevant in this context, and the fact that most phase techniques require monochromatic radiation makes these limitations even more severe. Amongst the different techniques, free-space propagation is the most suited to a polychromatic implementation. A detailed simulation, based on Fresnel/Kirchoff diffraction integrals, was devised to describe this imaging modality. This simulation accounts for source dimensions, beam spectrum and divergence and detector point spread function, and can thus be applied to any X-ray imaging system. In particular, by accepting these parameters as input, along with ones describing the sample, the model can be used to optimize the geometry of the set-up, i.e. to assess the source-to-sample and sample-to-detector distances that maximize feature detection. The simulation was validated experimentally by acquiring a range of images of different samples with a laboratory X-ray source. Good agreement was found between simulated and experimental data in all cases. In order to maximize the generality of the results, all acquisitions were carried out using a polychromatic source and an energy-resolving detector. This effectively allowed the recording of a range of monochromatic and polychromatic images in a single acquisition, as an assortment of the former can be created by integrating different parts of the acquired spectra. The most notable result obtained in this study is that in most practical cases polychromatic PCI can provide the same image quality as its monochromatic counterpart, which provides an important step in the wider application of phase contrast using conventional sources.