Phase-preserving beam expander for biomedical X-ray imaging

Abstract

The BioMedical Imaging and Therapy beamlines at the Canadian Light Source are used by many researchers to capture phase-based imaging data. These experiments have so far been limited by the small vertical beam size, requiring vertical scanning of biological samples in order to image their full vertical extent. Previous work has been carried out to develop a bent Laue beamexpanding monochromator for use at these beamlines. However, the first attempts exhibited significant distortion in the diffraction plane, increasing the beam divergence and eliminating the usefulness of the monochromator for phase-related imaging techniques. Recent work has been carried out to more carefully match the polychromatic and geometric focal lengths in a so-called 'magic condition' that preserves the divergence of the beam and enables fullfield phase-based imaging techniques. The new experimental parameters, namely asymmetry and Bragg angles, were evaluated by analysing knife-edge and in-line phase images to determine the effect on beam divergence in both vertical and horizontal directions, using the flat Bragg double-crystal monochromator at the beamline as a baseline. The results show that by using the magic condition, the difference between the two monochromator types is less than 10% in the diffraction plane. Phase fringes visible in test images of a biological sample demonstrate that this difference is small enough to enable inline phase imaging, despite operating at a sub-optimal energy for the wafer and asymmetry angle that was used.

Measuring the criticality of the `magic condition' for a beam-expanding monochromator

“…Both vertically and horizontally oriented edges were analysed for each test object and Bragg angle using the procedure given by Martinson et al (2015). The phase peak width was measured using a pseudo-Gaussian fit to measure the distance (in pixels) between inflection points in the plot profile.…”

“…During our earlier work (Martinson et al, 2014), significant beam blurring in the vertical direction (corresponding to horizontally oriented object edges) was believed to be caused by a mismatch between the single-ray and geometric focus types. A key improvement in the design was the preservation of the transverse coherence of the beam (Martinson et al, 2015), which allows phase-sensitive imaging techniques to be performed with a large field of view. This was achieved by matching the two focus types (single-ray focus, f p , and geometric focus, f g ) in the first crystal to each other and to the geometric focus of the second crystal.…”

“…At the time it was unclear how sensitive the system was to deviations from this 'magic condition'. The single-ray focus equation (Martinson et al, 2015) is…”

Characterization of a bent Laue double-crystal beam-expanding monochromator

“…Making full use of the large animal imaging stage, a feature unique to this facility (Wysokinski et al, 2007), similarly requires a larger field of view. Previous results (Martinson et al, 2014(Martinson et al, , 2015 reported on the development of a phase-preserving bent Laue beamexpanding double-crystal monochromator: two silicon (Si) crystal wafers were cylindrically bent with the concave sides facing the X-ray beam and arranged with the geometrical foci of both crystals co-located and the diffraction planes parallel ISSN 1600-5775 between each crystal as in Fig. 1.…”

An energy dispersive bent Laue monochromator for K-edge subtraction imaging

“…where υ is the Poisson ratio for the crystal type and orientation and all other variables are as defined earlier [13]. To achieve a small focus, the requirement is that the two foci, the geometric, , and polychromatic, L, match.…”