Evaluation of energy-subtraction CT images using diffracted monochromatic x rays

Abstract

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 cup** 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 cup** artifact appeared. Therefore, it was confirmed that it was essential for a CT image to use monochromatic x rays causing no beam hardening.