The effect of a variable focal spot size on the contrast channels retrieved in edge-illumination X-ray phase contrast imaging

A Astolfo,T Partridge,P R T Munro,M Endrizzi,D Bate,C K Hagen,I Buchanan,G K Kallon,A Olivo

doi:10.1038/s41598-022-07376-0

Abstract

Multi-modal X-ray imaging allows the extraction of phase and dark-field (or “Ultra-small Angle Scatter”) images alongside conventional attenuation ones. Recently, scan-based systems using conventional sources that can simultaneously output the above three images on relatively large-size objects have been developed by various groups. One limitation is the need for some degree of spatial coherence, achieved either through the use of microfocal sources, or by placing an absorption grating in front of an extended source. Both these solutions limit the amount of flux available for imaging, with the latter also leading to a more complex setup with additional alignment requirements. Edge-illumination partly overcomes this as it was proven to work with focal spots of up to 100 micron. While high-flux, 100 micron focal spot sources do exist, their comparatively large footprint and high cost can be obstacles to widespread translation. A simple solution consists in placing a single slit in front of a large focal spot source. We used a tunable slit to study the system performance at various effective focal spot sizes, by extracting transmission, phase and dark-field images of the same specimens for a range of slit widths. We show that consistent, repeatable results are obtained for varying X-ray statistics and effective focal spot sizes. As the slit width is increased, the expected reduction in the raw differential phase peaks is observed, compensated for in the retrieval process by a broadened sensitivity function. This leads to the same values being correctly retrieved, but with a slightly larger error bar i.e. a reduction in phase sensitivity. Concurrently, a slight increase in the dark-field signal is also observed.

Highlights

IntroductionMulti-modal X-ray imaging allows the extraction of phase and dark-field (or “Ultra-small Angle Scatter”) images alongside conventional attenuation ones
Multi-modal X-ray imaging allows the extraction of phase and dark-field images alongside conventional attenuation ones
The detector threshold was set at approximately 25 keV, this is subject to fluctuations across the sixteen 128 × 128 pixel modules that make up our 128 × 2024 pixel strip, which explains the slight “banding” visible in the retrieved images of Fig. 2

Summary

Introduction

Multi-modal X-ray imaging allows the extraction of phase and dark-field (or “Ultra-small Angle Scatter”) images alongside conventional attenuation ones. Disentangling them (“phase retrieval”) requires imaging an object at least twice, with the pre-sample mask placed in different positions A more flexible approach, which allows phase retrieval with any number of images (≥ 2) acquired with any displacement of the pre-sample mask, is based on the mathematical inversion of the system’s “illumination curve” (IC)[11] This is obtained by scanning the pre-sample mask (vertically in relation to Fig. 1a) while recording the detector readings: it is a bell-shaped curve with a maximum at the position where the apertures in the two masks overlap, and a minimum where they are fully misaligned. The IC links the detected intensity to the beam displacement, and to the refraction angle through knowledge of the distance between the m asks[11]

Methods

Results

Conclusion