The energy subtraction (ES) method is a technique that eliminates one tissue with the same attenuation coefficient by subtraction between two kinds of images obtained for x rays with different energy. Recently, the development of the dual source CT has made possible to acquire simultaneously two kinds of images for different x-ray energy and to perform the ES processing. Since the attenuation coefficient of a material, however, varies with position in a body on using white x rays, even the same material shows a different value of the coefficient and is hard to be eliminated. If x rays are monochromatic, an arbitrary tissue image can be eliminated by the ES processing and be discriminated from the other tissues.
We attempted to use diffracted monochromatic x rays for CT. Single crystal wafer of Si with the surface orientation 111 was used as a monochromator for x-ray diffraction. A cylindrical phantom was made of water and iodine contrast media. X-ray projection data were acquired with an imaging plate for high- and low-energy x rays. CT images were obtained by reconstructing the projection data.
In the profile of the monochromatic x-ray CT images, pixel values of water and iodine contrast media were constant, and the influence of beam hardening (or cupping artifact) was not observed. On the other hand, in white x-ray CT images, the more a pixel approached the center region of the cylindrical phantom, the more the pixel value of water and iodine contrast media decreased, and the cupping artifact appeared. Therefore, it was confirmed that it was essential for a CT image to use monochromatic x rays causing no beam hardening.