Abstract: We have a image reconstruction of the discrete model that based on the ideal small area replaces the original pixel grid in the traditional sense. The function about the distance form the ideal small area to the ray, to measure the size of the attenuation coefficient when the radiation passes through the ideal small area.In this image reconstruction of the discrete model we introduce the basis function.The discrete image is mapped into continuous image by convolution with a basis function.We discuss the conditions the basis function need to meet and give two basis functions and their firequency properties.