| The distinct advantages of monochromatic x rays in medical imaging, especially in computed tomography (CT) and radiography with contrast agents (e.g. angiography), provide a great incentive for developing compact sources of monochromatic, or nearly monochromatic, x-ray sources for clinical use. A second incentive comes from the potential use of gadolinium-based contrast agents for radiography, which benefit from monochromatic x rays much more than the iodine-based ones commonly used. The low yield of flat-crystal monochromators prohibit their use in conjunction with conventional x-ray tubes for this purpose.; This thesis suggests two ways to overcome the problem. First, it suggests to use x-ray tubes at a very high kVp setting, such as 300 kVp, to differentially increase (for the same heat-loading effect in the tube) the yield of the fluorescence flux of the tube, which is negligible compared to the bremsstrahlung yield at the 80–160 kVp settings commonly used. Second, it proposes to use a multilayered bent-crystal monochromator which, compared to flat crystals, has a much larger yield, and smaller sensitivity to angular misalignments. The monochromator will be tuned to transmit both the Kα lines of the tube.; The absolute flux of both components of the spectrum from the tube was calculated analytically and by Monte Carlo simulations as a function of the tubes high voltage. They showed a 6 to 7 fold increase in the yield of the Kα lines in the range 300–400 kVp.; The suggested monochromator uses a technique of aligning the rocking curves of the individual wafers to overlap (instead of being added side by side) to match the narrow width of the Kα fluorescence peaks. A model of the monochromator using this alignment method was successfully tested at the National Synchrotron Light Source (NSLS) at Brookhaven National Lab.; The Monte Carlo simulations used the code EGS4 (a electron- and photon-transport code) to simulate the X-ray tube's spectra. The code was modified to include a semi-empirical formula derived in this thesis to calculate bremsstrahlung cross-sections over a broad range of electron energies.; CT-reconstructed images of a phantom, which included compartments filled with different concentrations of iodine- and gadolinium-based contrast agents, were produced using different monochromatic and non-monochromatic beams. The “CT4” computed tomography simulation software package was written specifically for this purpose. The figure-of-merit in comparing different beams was the image contrast-to-noise ratio. Among the spectra used were also those of conventional kVp settings narrowed not by a monochromator but by beam filtration with a thick (1–3 mm) copper filter.; The code was used to optimize the kVp setting of the tube for use in conjunction with both the monochromator and copper beam filtering. The optimization was carried out separately for iodine- and gadolinium-based contrast agents used in the phantom. |
