Development of a bent Laue beam-expanding double-crystal monochromator for biomedical X-ray imaging

Samadi

Bassey

et al. 2015

Self Cite

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.

Section: Introductionmentioning

confidence: 59%

Phase-preserving beam expander for biomedical X-ray imaging

Samadi

Bassey

et al. 2015

Self Cite

“…While ideal matching may be required for certain microfocusing applications of bent Laue double-crystal monochromators, it would appear that, as long as both crystals are in the upper sign geometry [i.e. the tilt angle of crystal is + B instead of the À B tilt that is now believed to be the primary cause of the beam blurring observed in our earlier work (Martinson et al, 2014)], the system will produce a suitable beam for biomedical imaging with phase contrast techniques. …”

Section: Resultsmentioning

confidence: 98%

“…The beam-expanding system was set up as shown by Martinson et al (2014) with the geometric focus of the second crystal matched to that of the first crystal. The bend radii of the first and second crystals were 0.5 m and 5 m, respectively, producing an expansion factor of approximately 10, with a crystal-to-crystal distance of approximately 2 m. Using a Hamamatsu detector [AA-60 beam monitor coupled to a C9300-124 CCD camera resulting in a field of view of 31.08 mm (H) Â 23.31 mm (V) and pixel size 8.75 mm] and object-to-detector distance of 134 cm, images of a knife-edge (tungsten bar) and phase object (Lucite rod) were captured through Bragg angles ranging AE 1 from the magic condition (see Fig.…”

Section: Methodsmentioning

confidence: 99%

“…1) has been designed for the BioMedical Imaging and Therapy (BMIT) beamlines at the Canadian Light Source. 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.…”

Section: Introductionmentioning

confidence: 99%

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

Chapman

2016

Self Cite

It has been established that for cylindrically bent crystals the optimal beam characteristics occur when the geometric and single-ray foci are matched. In the beam-expanding monochromator developed for the BioMedical Imaging and Therapy beamlines at the Canadian Light Source, it was unclear how critical this 'magic condition' was for preserving the transverse coherence of the beam. A study was conducted to determine whether misalignments away from the ideal conditions would severely affect the transverse coherence of the beam, thereby limiting phase-based imaging techniques. The results were that the magic condition has enough flexibility to accommodate deviations of about AE 1 or AE 5 keV.

“…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.…”

Section: Introductionmentioning

confidence: 99%

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

Samadi

Shi

et al. 2017

Self Cite

A bent Laue double-crystal monochromator system has been designed for vertically expanding the X-ray beam at the Canadian Light Source's BioMedical Imaging and Therapy beamlines. Expansion by a factor of 12 has been achieved without deteriorating the transverse coherence of the beam, allowing phasebased imaging techniques to be performed with high flux and a large field of view. However, preliminary studies revealed a lack of uniformity in the beam, presumed to be caused by imperfect bending of the silicon crystal wafers used in the system. Results from finite-element analysis of the system predicted that the second crystal would be most severely affected and has been shown experimentally. It has been determined that the majority of the distortion occurs in the second crystal and is likely caused by an imperfection in the surface of the bending frame. Measurements were then taken to characterize the bending of the crystal using both mechanical and diffraction techniques. In particular, two techniques commonly used to map dislocations in crystal structures have been adapted to map local curvature of the bent crystals. One of these, a variation of Berg-Berrett topography, has been used to quantify the diffraction effects caused by the distortion of the crystal wafer. This technique produces a global mapping of the deviation of the diffraction angle relative to a perfect cylinder. This information is critical for improving bending and measuring tolerances of imperfections by correlating this mapping to areas of missing intensity in the beam.