Computed tomography (CT) is a method that uses a computer to generate cross-sectional images from X-ray projection images taken at several different angles. Depending on the scanning method, there are a parallel beam optical system, a fan beam optical system, and a cone beam optical system. A computer reconstructs the projected image to recover a cross-sectional image (CT slice, tomogram). The mathematical basis of CT is the Radon, and inverse Radon transforms.