A bent Laue-Laue monochromator for a synchrotron-based monochromatic x-ray computed tomography system

A bent Laue–Laue monochromator for wide, fan-shaped synchrotron x-ray beams was designed and constructed for the program Multiple Energy Computed Tomography (MECT) at the National Synchrotron Light Source (NSLS). MECT uses monochromatic x-ray beams from the NSLS's X17B beamline for CT with improved image contrast-to-noise ratio, image quantification, and the efficacy for dual-energy imaging. The new monochromator uses two Czochralski-grown Si〈111〉 crystals, which are 0.7 and 1.4 mm thick, respectively, and are cut with thick ribs on their upper and lower ends. The crystals are bent cylindrically, using 4-rod benders employing two fixed and two movable rods. The bent-crystal method was needed to solve the difficulties we had with the flat Laue–Laue monochromator previously used in MECT; these included: (a) insufficient beam intensity, (b) excessive beam-intensity fluctuations, and (c) instability of the beam's horizontal profile shape. Compared with flat Laue–Laue monochromator, the bent Laue–Laue device tested at 42 keV and 108 keV showed ∼10-fold increase in the beam flux, ∼5-fold improvement of beam-profile stability, ∼10-fold suppression of beam harmonic contamination at the bending radius of 15 m, and a smaller energy bandwidth at certain bending radii. The new monochromator should reduce MECT's image noise and artifacts, bringing its performance close to that of the ideal CT. © 1998, American Association of Physicists in Medicine. All rights reserved.