1. A radiation diagnosis apparatus to improve registration and reduce artifacts in digital subtraction computed tomography, the apparatus comprising: processing circuitry configured to identify first regions corresponding to a metal artifact based on a plurality of first frames at a plurality of angles or a plurality of second frames at the plurality of angles, the plurality of first frames including projection data from a CT scan performed at a first time, and the plurality of second frames including projection data from a CT scan performed at a second time, which is different from the first time, correct misregistrations between the first frames and the second frames based on the first regions, generate subtraction frames between the first frames and the second frames after correcting the misregistrations, and reconstruct a first reconstructed image based on the subtraction frames respectively obtained at the plurality of angles.