I. Introduction
X-Ray computed tomography (CT) is a widely used medical imaging method that produces three-dimensional (3D) images of the inside of a body from many two-dimensional (2D) X-ray images. A 2D X-ray image captures X-ray photons that pass through a body. As different materials attenuate X-ray differently, they can be effectively differentiated by their attenuation coefficients. Using many X-ray images taken around an axis of rotation, the attenuation coefficient of each volume element (voxel) can be reconstructed, providing high-resolution imaging for medical diagnosis.