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) determined by a position of an object of interest and a non-ROI, wherein a view of imaging the object is fixed while X-rays are radiated; 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: analyze X-ray data to obtain a frame image in which the ROI and the non-ROI are set, register the obtained frame image to a previous frame image, and synthesize the obtained frame image and the previous frame image to generate a reconstructed frame image, wherein the filter comprises: a filter sheet comprising an open area corresponding to the ROI and a closed area corresponding to the non-ROI; and a driver configured to move the open area of the filter sheet, and wherein the X-ray imaging apparatus further comprises: a controller configured to track the object of interest in the obtained frame image, and control the driver of the filter to move the filter sheet such that the open area of the filter corresponds to the ROI determined by the position of the tracked object of interest.