Fixed-exit monochromator for computed tomography with synchrotron radiation at energies 18-90 keV

A fixed-exit monochromator has been constructed for computed tomography (CT ) studies at the Medical Beamline of the European Synchrotron Radiation Fac ility. A non-dispersive pair of bent Laue-type crystals is used, and the fi rst crystal is water-cooled. The monochromator operates at energies from 18 to 90 keV, and the maximum width of the beam is 150 mm. The performance of the monochromator is studied with respect to the beam intensity and energy distributions, and a close agreement is found between the calculated and e xperimental results. The intensity is between 10(9) and 10(10) photons s(-1 ) mm(-2) under typical operating conditions. The harmonic content of a 25 k eV beam is about 30% at the minimum wiggler gap of 25 mm (field 1.57 T) and decreases by an order of magnitude when the gap is increased to 60 mm (fie ld 0.62 T). The experimental set-up for CT studies includes dose monitors, goniometers and translation stages for positioning and scanning the object, and a 432-element linear-array Ge detector. Examples from phantom studies and in vivo animal experiments are shown to illustrate the spatial resoluti on and contrast of the reconstructed images.