1. A computed tomography (CT) imaging apparatus to improve registration and reduce artifacts in digital subtraction computed tomography, the apparatus comprising: processing circuitry configured to obtain first mask frames and contrast frames generated during scans of an object, each first mask frame and contrast frame being obtained at a respective projection angle of a plurality of projection angles; subtract each first mask frame from the corresponding contrast frame to generate difference frames; reconstruct a first contrast image using the difference frames; project the first contrast image at the plurality of projection angles to generate contrast re-projection frames; blend each first mask frame with the corresponding contrast re-projection frame to generate blended mask frames; register each contrast frame with the corresponding blended mask frame to align absorption features of the contrast frame with absorption features of the blended mask frame using a transformation that includes a translation; subtract each first mask frame from the corresponding contrast frame after the first mask frame and the contrast frame are aligned using the transformation to generate subtracted frames; and reconstruct a second contrast image using the subtracted frames.