1. A computed tomography (CT) apparatus comprising: an image processor configured to: perform back-projection on a first sinogram that is acquired by a CT scan of an object, to reconstruct a first CT image corresponding to a field of view (FOV); perform forward projection on the first CT image, to acquire a second sinogram representing a first portion of the object that is included in the FOV; sum the second sinogram and a third sinogram representing a second portion of the object that is not included in the FOV, to acquire a fourth sinogram; and correct the first CT image, based on a difference between the fourth sinogram and the first sinogram, to acquire a second CT image.