Dual-energy CT imaging of the brain will be performed using synchrotron radiation X-ray beams at about 40 and 80 keV to visualize the altered concentrations of the intermediate-Z elements that occur in brain lesions and atherosclerotic plaques. The monochromator designs considered allow rapid switching between these two energies without change in the position or direction of the X-ray beam. One design uses the fundamental and second harmonic radiations from an existing dual-Bragg-crystal Si〈2,2,0〉 monochromator. In this design the 40 and 80 keV beams would be isolated by monochromator detuning or gold filtration, respectively. The other designs utilize fundamental radiations for both energies and would require some modification of the existing monochromator mechanism to achieve adequate tuning speed. These designs use two Si 〈2,2,0〉 crystals, one with a Bragg and the other with either a Bragg or a Laue geometry.
