Computed tomography with monochromatic x rays.

Recent advances in the development of synchrotron facilities that provide high-energy and high-flux x-ray beams allow the use of monochromatic x rays at a < 0.5% energy bandwidth (i.e. energy width of < 500 eV at 100 keV) for computed tomography (CT) of humans. Such CT systems will consist of a fixed, horizontal, low-divergence fan beam and a seated subject rotating about a vertical axis. Compared to conventional CT, the new system should significantly improve contrast resolution of the image and provide better image quantification because of its lack of beam-hardening effects and its efficient implementation of energy-selective imaging methods such as dual-photon absorptiometry and K-edge subtraction with high-atomic-number (high-Z) contrast-enhancement elements. Further, the nearly parallel synchrotron x-ray beam geometry minimizes patient-to-detector scattering when the detector is positioned far from the patient. The design considerations and the expected performance of monochromatic CT are described, in particular those of the CT being developed at the National Synchrotron Light Source. The potential applications of the method in neuroradiology, cardiac and vascular imaging, bone densitometry, lung imaging, and radiotherapy are discussed. The future of monochromatic CT in research and in clinical diagnostics is also reviewed, as are the prospects for development of compact sources of x rays with narrow-energy bandwidths.