1. An X-ray imaging apparatus comprising: an X-ray generator configured to radiate X-rays onto an object having a region of interest (ROI) and a non-ROI; a filter configured to adjust an X-ray dose of the X-rays incident on the ROI and the non-ROI; an X-ray detector configured to detect the X-rays transmitted through the object and convert the X-rays into X-ray data; and an image processing unit configured to obtain a frame image using the X-ray data, register the obtained frame image to a previous frame image, synthesize the frame image and the previous frame image, and generate a reconstructed frame image.