Fast Energy Subtraction Using Fluorescent X-Rays Excited by Synchrotron Radiation

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A fast switching dual-energy fluorescent x-ray source for K-edge subtraction imaging has been developed using a 6.5 GeV synchrotron radiation storage ring at Tsukuba (KEK). The fluorescent x-ray source is composed of a cylindrical rotating target border of 25mm in diameter on which a pair of metal target strips of 2 - 5 mm in width are attached at a distance of 1mm apart. The surface of the target is angled at 45 degrees to the incident beam at the two points where it crosses the beam axis. At each time the incident white x-ray beam from the bending magnet, collimated to 1mm x 1mm, irradiates a pair of target materials, fluorescent x-rays of two different energy photons are generated at time intervals of about 10 msec at the rotation rate of 120 rpm.Fluorescent x-rays, which range from about 20 keV to 75 keV can be generated by changing the target materials. In case of iodine contrast medium, fluorescent x-rays from Ba and De, having 32.2 and 34.7 keV K x-ray energies respectively, can be used for K-edge subtraction of iodine which has K absorption edge of 33.17 keV. In case of gadolinium, fluorescent x-rays from Ho and Yb, having 47.5 and 52.4 keV K x-ray energies respectively, can be used for gadolinium K-edge of 50.23 keV.A phantom which contains iodine contrast medium is used for K-edge energy subtraction subject. By taking subtractions between two images above and below each K absorption edge, which are obtained by using an image intensifier and TV system, enhanced contrast material images are obtained.At present, the photon intensity is not sufficient for imaging of human subjects. High intensity fluorescent x-rays generated by synchrotron radiation will be useful for diagnostic radiology such as monochromatic-ray CT, K-edge subtraction imaging, high resolution magnification radiography, and other medical applications.