Abstract: In X-ray Diffraction Enhanced Imaging (DEI), the extinction contrast in peak CT mode can intensify the boundary of the object, and effectively raise the special resolution of the tomographic image. In this article, a definite physical explanation is given to this intensifying effect. By theoretic calculating and BSRF experimental data processing, it is demonstrated that this extinction contrast leads in CT image a fictitious one-pixel wide round ring with a very high mass density, and thus intensifies the image of edges of the round object.